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ThriJ
07-18-2006, 06:54 AM
:stumped: Official BrainTeAsErS Pondering Thread :stumped:

Questions:confused:, life is full of them. In this thread we will try and answer some of the more interesting ones.

Please remember to keep it civilized and productive. Even if someone is bullheaded and completely wrong, that does not mean you have to shoot their dog and insult their mother.:argue:

Ok then…

On the initial agenda:

1: ( Looking Through The Mirror?8~ ) If the eye is focus on a object can what the object is reflecting be out of focus and visa-versa?

The source and details is here. (=http://www.newtek.com/forums/showthread.php?t=53620) The general conclusion is yes.

2: ( To Fly Or To Roll?:chicken: ) A plane is standing on runway that can move (some sort of band conveyer). This conveyer has a control system that tracks the tire speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction). So the question is will the plane take off or not?

The source and details is here on page 4 post49 of the same thread as question 1. (http://www.newtek.com/forums/showthread.php?t=53620&page=4) The general conclusion is yes.

3: ( Coffee Anyone?:caffeine: ) I have a cup of coffee. I want a bit of milk in it. The doorbell rings. Assuming everything is perfect, and the temperature of the milk is lower than that of the coffee, and the room temperature is normal, and thus lower than the coffee too. And the time to answer the door is neither infinitely short, nor infinitely long. Expect a reasonable timeframe. The question is will the coffee be cooler if I add the milk now, and then go to open the door and come back, or will it be cooler if I open the door first, and add the milk later?

May the confusion begin!:stumped:

Kurtis
07-18-2006, 06:59 AM
If you're traveling at the speed of sound, does it matter what station you have the radio set to?

If you're traveling at the speed of light, does it matter if your headlights are on?

How much wood could a woodchuck chuck if a woodchuck would chuck wood?

righteous
07-18-2006, 07:10 AM
Haha, good on you for starting the thread.

Answers based on no evidence other than I say so.

a)Yes
b)Yes
c)Open Door first

:agree:

:D

Exception
07-18-2006, 07:11 AM
How much wood could a woodchuck chuck if a woodchuck would chuck wood?

I havn't seen Chuck with any wood lately.

(thanks for the great thread ThriJ)

ThriJ
07-18-2006, 07:12 AM
Ahh..8/

One at a time!8~ :stumped: :bangwall:

Brain huurrtting:oye: :help:

Weetos
07-18-2006, 07:16 AM
If you're traveling at the speed of light, does it matter if your headlights are on?

Nope. Light velocity (celerity?) can't be added to the speed of the vehicle, so at the speed of light, nothing in front of the vehicle will be lit by the headlights

Exception
07-18-2006, 07:20 AM
If you're traveling at the speed of sound, does it matter what station you have the radio set to?

No it doesn't matter.
Because who's to say if you'll be listening?
But if the question is, if you can hear the radio, that depends on the setup. Are you in a pressurised cabin with air, then you can hear the radio since the air in the cabin is stationary, so the sound waves can reach you just fine. If you're in a vacuum in a suit, you won't hear anything as there's no sound in a vacuum.
The reception of the radio is not influenced.

Lightwolf
07-18-2006, 07:23 AM
If you're traveling at the speed of light, does it matter if your headlights are on?
But of course, it will drain your batteries ;)

Cheers,
Mike

jameswillmott
07-18-2006, 07:57 AM
Coffee question, add milk later for cooler coffee.

Exception
07-18-2006, 08:04 AM
Regarding the coffee question, it's not as easy as it looks...

Karmacop
07-18-2006, 08:26 AM
With the coffee it'll be cooler if you add the coffee before going to the door. As you're at the door both the coffee and the milk will start to cool/heat to the room temperature. So if you add the milk before going to the door it'll have a longer period of time cooling while in contact with the milk than if you add the milk when you come back (as the coffee would have almost no time of cooling because of the milk).

Exception
07-18-2006, 08:36 AM
Well, the temperature of the coffee and the milk don't influence each other that way. Once mixed, they are one, and have one temperature. This is because they mix and have pretty much the same temperature coefficient value (english?). So that explanation doesn't really work.... nice shot tho Karmacop...

(Its not that I know so much about this stuff, but I found this riddle and read up on it for hours and hours... I'm not telling tho... :) )

jameswillmott
07-18-2006, 08:44 AM
I remember from physics that heat loss is proportional to the difference in temperature ( or something )

Pure coffee will initially cool faster than coffee+milk, until they reach the same temperature, then both will cool at the same rate. ( Assuming we take our time answering the door )

Adding cold milk to the (now cooled) coffee will reduce it's temperature below that of the coffee+milk mixture.

Safe Harbor
07-18-2006, 08:58 AM
Airplane question...

Doesn't an airplane require wind beneath its wings to provide lift? The wheels turning won't make it take off - there has to be wind. So I would say no, it's not going to take off because it's not moving forward to create wind which would create lift.

MiniFireDragon
07-18-2006, 09:01 AM
How much wood could a woodchuck chuck if a woodchuck would chuck wood?

As much wood, as a woodchuck could, if a woodchuck could chuck wood.

2: ( To Fly Or To Roll? ) A plane is standing on runway that can move (some sort of band conveyer).

The answer is no. You need to have the air moving over the wings of the plane, not the ground beneath it to have it leave the ground. The forward momentum of a plane is provided by the Jets, not the wind. The runway is only there for a plane to get enough airspeed over the wings to lift off.

So in theory, if u had air blowing over the wings, u'll still need some "zone" that the plane could accelerate in so it's speed would be suficent enough that when it left the "zone" it could continue to fly, or simply excellerate the plane from 0 - needed airspeed (I am sure the G's would be a but uncomfy).

1: ( Looking Through The Mirror? ) If the eye is focus on a object can what the object is reflecting be out of focus and visa-versa?

Being "in focus" is relative to the viewer. Nothing is ever in focus or out of focus.

3: ( Coffee Anyone? ) I have a cup of coffee. I want a bit of milk in it. The doorbell rings. Assuming everything is perfect, and the temperature of the milk is lower than that of the coffee, and the room temperature is normal, and thus lower than the coffee too. And the time to answer the door is neither infinitely short, nor infinitely long. Expect a reasonable timeframe. The question is will the coffee be cooler if I add the milk now, and then go to open the door and come back, or will it be cooler if I open the door first, and add the milk later?

I would say it would be the same temperature either way. Time is the factor in this equation. Things cool relative to time, and the same amount of time would have passed either way you poured the milk.

If you're traveling at the speed of light, does it matter if your headlights are on?

The speed of light is a constant no matter how fast u are moving. So if you turned your headlights on at the speed of light, the light would never leave the filament, and therfore even if u were looking at it, it would appear to be off. As a matter of thought everything would be like a picture, until your eyes absorbed all the photons, then u'd see nothing. That is providing you could function at the speed of light.

If you're traveling at the speed of sound, does it matter what station you have the radio set to?

I am not sure if I could take that question seriously. Unless you are under the assumption that radio waves travel at the speed of sound, which they don't. Sound waves travel via vibrating air and other matter. So even if u are travling faster then the speed of sound, the air around u in the vehicle is also moving as fast as u are, so at the speed of sound, the sound in your car is traveling twice the speed of sound (relative to someone outside the car) but at the speed of sound relative to you inside the car.

Here is one for you all:

Fill a glass with ice to the point it is higher then the brim of the glass, then fill the glass with water to the brim. Now when the ice metls will it overflow?

Lightwolf
07-18-2006, 09:02 AM
So I would say no, it's not going to take off because it's not moving forward to create wind which would create lift.
I'll keep it short... the plane will move forward, the belt can not stop it from doing that, the only thing that will happen is that the wheels spin faster.
Mainly because the plane is propelled by thrust, and not by a reaction with the ground.

Cheers,
Mike

Karmacop
07-18-2006, 09:05 AM
I remember from physics that heat loss is proportional to the difference in temperature ( or something )
Exactly, the larger the difference in temperature the faster the temperature object will heat/cool.

Pure coffee will initially cool faster than coffee+milk, until they reach the same temperature, then both will cool at the same rate. ( Assuming we take our time answering the door )
Even though coffee will cool (at the start) faster than coffee+milk,
cofee+milk will start at a lower temperature and should reach a cooler temperature faster.

Adding cold milk to the (now cooled) coffee will reduce it's temperature below that of the coffee+milk mixture.
Ahh, I see where you're coming from. I thought the milk would be out (along with the coffee) and so the milk would never be able to bring the coffee below room temperature (because milk has a smaller temperature differential).

Well, the temperature of the coffee and the milk don't influence each other that way. Once mixed, they are one, and have one temperature.
Yes but coffee+milk is cooler than coffee by itself.

Exception
07-18-2006, 09:06 AM
Adding cold milk to the (now cooled) coffee will reduce it's temperature below that of the coffee+milk mixture.

VERY close!

And, what helps too, is that the mass of the coffee an milk grow, but its surface area grows less when adding milk, which means there is more of an insulative capacity with the coffee+milk mixture, making it hotter too...

However...

milk is fatty...

uh-oh... :)

gjjackson
07-18-2006, 09:45 AM
This reminds me of the Netflix radio commercials. "Now for the bonus round". At the moment I can't think of any of them.

Captain Obvious
07-18-2006, 10:01 AM
The coffee will be colder if you add the milk later, for the following reason:

The rate at which a given object's temperature drops, is a function of the object's temperature compared to the temperature and density of the environment. A 60°C cup of coffee will not drop in temperature at all if it sits in a 60°C room. A 50°C cup of coffee in a 22°C room will drop in temperature more slowly, when measured in degrees per second, than a 60°C cup sitting in the same room. Thusly, if you pour the milk first, you will decrease the temperature of the coffee and reduce the rate at which the temperature falls off.

Of course, there are plenty of ifs and buts! If the milk is sitting in the window, being hit by sunlight, it might be warmed up rather quickly, and thusly offset the whole thing. But if we assume the milk's temperature will stay about the same, then it's better to pour the milk first if you want hot coffee.

The aeroplane will explode in a massive fireball immediately. As Lightwolf said, the conveyor belt cannot stop the plane from moving forward; it will just make the wheels spin faster. Of course, as the wheels spin faster and the aeroplane accelerates, the conveyor belt will go faster, making the wheels spin faster, making the conveyor belt go faster... Basically, if there is no delay at all between the speed of the wheels and the speed of the conveyor belt, the belt and wheels will accelerate at INFINITE speed. Allow me to demonstrate:

Let's first assume there is a 1 second delay between the speed of the wheels and the speed of the conveyor belt, and the plane has a constant acceleration of 10 m/s per second. Then it goes like this:

Second 0: Plane is standing still. Conveyor belt is standing still.

Second 1: Plane starts to accelerate. It has reached a speed of 10 meters per second after the first second of acceleration. Conveyor belt is still standing still. Wheels spin at 10 meters per second.

Second 2: Plane is now moving forward at 20 meters per second. Conveyor belt accelerates in the opposite direction to 10 meters per second, making the wheels spin as if the plane was moving at 30 meters per second.

Second 3: Plane is now at 30 m/s. Conveyor belt accelerates to match the speed of the wheels: 30 m/s. Wheels spin at a speed of 60 m/s.

Second 4: Plane is at 40 m/s. Conveyor belt matches the speed of the wheels: 60 m/s. Wheels are now spinning at 100 m/s.

Second 5: Plane is at 50 m/s. Conveyor belt reaches 140 m/s, and the wheels now rotate at 190 m/s.

Now, let's instead assume there is a 0.1 second delay between the speed of the wheels and the speed of the conveyor belt. Then it goes like this:

Second 0: Plane is standing still. Conveyor belt is standing still.

Second 0.1: Plane moves at 1 m/s. Conveyor belt is still standing still. Wheels spin at 1 m/s.

Second 0.2: Plane moves at 2 m/s. Conveyor belt reacts to the movement of the wheels, and accelerates to 1 m/s. Wheels now spin at 3 m/s.

Second 0.3: Plane moves at 3 m/s. Conveyor belt accelerates to 3 m/s. Wheels spin at 6 m/s.

Let's skip a few, up to second 1: Plane moves at 10 m/s. Conveyor belt moves at 45 m/s. Wheels spin at 55 m/s.

Let's skip to second 2: Plane moves at 20 m/s. Conveyor belt moves at 190 m/s. Wheels spin at 210 m/s.

Second 3: Plane move at 30 m/s. Conveyor belt moves at 435 m/s. Wheels spin at 465 m/s.

As you can see, the wheels' acceleration, in m/s/s, is a function of the delay between the speed of the wheels and the speed of the conveyor belt. As the delay approaches zero, the acceleration of the wheels will approach infinity. When the delay gets short enough, the conveyor belt will accelerate towards the speed of light within an instant. This will generally screw things up, obviously, what with the theory of relativity and all. But even if it just accelerates to a few hundred thousand meters per second, it's still enough to instantly tear the plane apart.

Bog
07-18-2006, 10:09 AM
If you're travelling at lightspeed, what happens when the dog sticks it's head out of the window?

Exception
07-18-2006, 10:10 AM
A 60°C cup of coffee will not drop in temperature at all if it sits in a 60°C room.

Actually, strange as it may sound, it will.
You're right concerning the temperature curves, they are logarithmic... but, just to say that a 60 C fluid in a 60 C room (nice climate!) will drop below 60 C because of evaporisation, which costs energy, thus the fluid will cool off to (slightly) below the temperature of the surroundings.

The coffee thing though, although I wholehartedly agree with your piece of written physics, is not correct. The cooling law of Newton desrcibes the cooling down of objects and fluids, and although they are logarithmic, when two fluids are separated and then combined, their resulting temperature is exactly the same. Also, the complexity is increased when you experiment and find out that coffee does not cool off according to Newton's law. Which is, scary. Its curve is steeper in the beginning, and shallower in the end than what it should be. But that's jumbo and not very relevant for a puzzle...

The only complicating factors are thus the fat in the milk, which will insulate the heat, and the increase in volume while the surface stays more or less equal. I would say then that indeed adding the milk first will result in hotter coffee, but the dutch research institute TNO has done tests on this subject, (it was a national quiz), and concluded they could not measure a difference beyond statistical error...

So how's that for a brain wrench?

Captain Obvious
07-18-2006, 10:11 AM
Actually, strange as it may sound, it will.
You're right concerning the temperature curves, they are logarithmic... but, just to say that a 60 C fluid in a 60 C room (nice climate!) will drop below 60 C because of evaporisation, which costs energy, thus the fluid will cool off to (slightly) below the temperature of the surroundings.
Ah, yeah, that's true. I forgot about that.

The coffee thing though, although I wholehartedly agree with your piece of written physics, is not correct. The cooling law of Newton desrcibes the cooling down of objects and fluids, and although they are logarithmic, when two fluids are separated and then combined, their resulting temperature is exactly the same.
Isn't that just when you mix ALL of it?

Anti-Distinctly
07-18-2006, 10:55 AM
Generally speaking though the coffee will cool faster if there is a greater temperature gradient between it and its environment.
@Exception - that would be entropy would it not?

As for the aircraft. I think in the gedanken experiments (as Einstein called them) we have a completely frictionless bearing on the wheel. i.e. there is no coupling at all between the ground and the engine that is providing the thrust.
There are arguments that have raged and raged for several hundred pages about this. Maybe people trolling (I had to have that one explained to me :)) But in the end someone just had to do a practical:
http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B

Notice how the thing doesnt change speed at all?

art
07-18-2006, 11:18 AM
I'll keep it short... the plane will move forward, the belt can not stop it from doing that, the only thing that will happen is that the wheels spin faster.
Mainly because the plane is propelled by thrust, and not by a reaction with the ground.

Cheers,
Mike
Which side are you on? :) :beerchug:

Anti-Distinctly
07-18-2006, 11:28 AM
Actually, strange as it may sound, it will.
You're right concerning the temperature curves, they are logarithmic... but, just to say that a 60 C fluid in a 60 C room (nice climate!) will drop below 60 C because of evaporisation, which costs energy, thus the fluid will cool off to (slightly) below the temperature of the surroundings.

The way that I see it (the 60C room and coffee); even if the coffee did lose some energy due to evaporation, which it would you are correct, the resevior that is the room at 60C would just give the coffee back that heat.

I like physics :D

GregMalick
07-18-2006, 11:43 AM
3: ( Coffee Anyone? ) I have a cup of coffee. I want a bit of milk in it. The doorbell rings. Assuming everything is perfect, and the temperature of the milk is lower than that of the coffee, and the room temperature is normal, and thus lower than the coffee too. And the time to answer the door is neither infinitely short, nor infinitely long. Expect a reasonable timeframe. The question is will the coffee be cooler if I add the milk now, and then go to open the door and come back, or will it be cooler if I open the door first, and add the milk later?
It depends on many other non-stated variables that determine the cooling/warming of the liquids. Here are a few:

How large is the exposed surface area of the liquid (one tiny spout the other huge)
What is each container composed of (one glass another metal ?)
The total volumes of each liquid (smaller volumes of the same liquid given the exact same conditions will reach room temp faster)

Basically if the milk maintains more of it's initial temp than the coffee - then answering the door and then adding the milk will result in cooler coffee.

It's all physics.

But the more important question is: "Who is at the door?"

A reasonable amount of time with a sexy lady selling lingerie (or course she wants to model them) versus a grumpy old neighbor asking to borrow my toenail clippers results in vastly different "reasonable" timeframes.

Even Einstein would have let his coffee get cold. :hey:

kilvano
07-18-2006, 12:36 PM
Engines of planes only thrust the plane forward in order for air to pass over wings creating lower pressure above the wings to than below. It is the increase and decrease of the pressure that causes fall and lift.

So it all has to do with the speed of the air passing over the wings and NOT to do with the speed of the wheels or engines. The wheels can be spinning at 1000miles an hour but if the wind over wings is 3mph then it ain going anywhere upwards

Chris

spec24
07-18-2006, 01:37 PM
As for the aircraft. I think in the gedanken experiments (as Einstein called them) we have a completely frictionless bearing on the wheel. i.e. there is no coupling at all between the ground and the engine that is providing the thrust.
There are arguments that have raged and raged for several hundred pages about this. Maybe people trolling (I had to have that one explained to me :)) But in the end someone just had to do a practical:
http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B

Notice how the thing doesnt change speed at all?

You're mistaken, watch closely and you will see that the skateboard does in fact slow down. I think the question is a bit of a trick question anyway - depending how you read into it. No matter what the means of propulsion if the conveyer moved in the opposite direction at the same speed as the vehicle (airplane, whatever), the vehicle will not move. Regardless of what people think the plane still is using the ground to roll on to pick up speed to get enough air to pass over the wings to produce lift. If the plane does not roll along the ground the plane will not fly. The plane must have ground speed before air will flow over the wings (disregarding any outside wind or fan machine). However, in the question posed that stated that the conveyer keeps up with the speed of the airlplane you run into another problem, and that is that we don't know how speed is being measured - in relation to the earth or to the conveyer. Accepting that what I said is true than if you were measuring the speed of the plane as an earthbound observer (not on the conveyer) then you would never see the plane move (it would appear to be standing still as the conveyer passes under it). Therefore the plane's speed, as measured by the observer, is zero. If the conveyer is the same speed as the plane then the plane and the conveyer can never move (that's the trick question part). If, however, the observer were on the conveyer he could acurately measure the speed of the plane based on his observation. The observer, however (to an earthbound observer) would actually be travelling olong with the conveyer. But the conveyer observer would see the plane zip by at say, 100mph. That of course means the conveyer is moving at 100mph. Where is the air coming from to lift the plane? The plane feels no air resistance and therefore has no lift because it remains stationary - the observer on the conveyer, however, feels 100mph of wind as he whips down the conveyer past the plane.

Also, referring to the video link, look how slowly he pulls the mat out from under the skateboard. Not even close to how fast the skateboarad is moving.

Lightwolf
07-18-2006, 02:26 PM
Which side are you on? :) :beerchug:
Changed my mind, it will take off, because the belt can't keep the plane stationary to the ground, something I initially assumed is possible.

Cheers,
Mike

Bog
07-18-2006, 03:21 PM
Okiedoke. The four main forces at work in flight:

http://www.centennialofflight.gov/essay/Dictionary/four_forces/DI24G1.jpg

Weight, Thrust and Drag are pretty self-explanatory, but Lift is generated by air passing more rapidly over the curved top surface of the wing than over the flatter lower surface - it has more distance to cover in the same time, as it were, a bit like holding the racing line around a corner for a car. This generates a zone of low pressure over the wing, compared to the high pressure zone beneath it. This pressure differential pulls the wing "upwards", from the point of view of the aircraft, and when that force excees the weight of the aircraft, up you go.

It's more complex than that, thrust is a factor, but generally only fighter pilots and astronauts get to fly things with produce more thrust than the overall weight of the airplane.

If your airplane is sat on a big treadmill, as it were, then there won't be any airflow over the wing, regardless of how fast the belt is going. If I'm to be utterly, utterly picky, accelerating the belt to flight speeds will result in one of two things:

The rolling resistance of the undercarriage will result in the plane banging against the trailing edge of the conveyor belt, much like a stuck poster-tube at baggage claim, or a rogue carrot rolling down the conveyor at a cash register.

Only bigger and more likely to go "Crunch, BOOM!"

If the undercarriage is sufficiently Ideal (superconducting bearings, and perhaps a maglev instead of an Oleo strut), then the airflow generated by the belt itself may be enough to flip the aircraft, but not to make it gently saunter into the sky. Unless you're maybe talking about a microlite, but then you'd still need to keep it tethered or it's most likely just blow out of the airstream and have a graceless re-union with the land from whence it came.

In, as ever, my arrogant opinion. The only real way to get a solid answer would be to run the numbers, and it's been wayyyy too long since I've done that sort of thing...

spec24
07-18-2006, 03:27 PM
Changed my mind, it will take off, because the belt can't keep the plane stationary to the ground, something I initially assumed is possible.

Cheers,
Mike

What makes you think the plane will not be stationary (I assume you mean in reference to the real ground and not the conveyer)?

Put yourself in a real plane. Let's pretend for a second the plane has no wings so we don't have to worry about this thing ever flying. The propeller creates thrust which pushes this plane forward. If you are in the plane you could state that the ground beneath the plane is moving, and not hte airplane, because as an observer inside the airplane, to you, it looks as though the ground is moving and not the plane (as apposed to someone outside the airplane who can easily see the plane is moving). The point is that what is moving is relative to the observer, and it does not matter whether the ground is moving or if the plane is moving. So if the plane is sitting on a conveyer, the conveyer is going 100mph and the plane is going 100mph the plane remains stationary (to an observer NOT on the conveyer).

Just because the prop is pulling/pushing air does not change any of this.

If you sat in a sailboat and held a fan pointing at the sailboat would the sailboat move (all things being equal)???? The answer is no.

How about a swamp boat (you know, a big fan on the back of it) in a water tank? The water in the tank is moving at the same speed that the boat is (or better said, the speed the boat wants to go). However the fan is pushin/pulling air - does that mean that the boat will actually move forward becasue the force from the fan is not directly transferred to the water (propeller force not directly transfered to wheels)?

Lightwolf
07-18-2006, 03:41 PM
If you sat in a sailboat and held a fan pointing at the sailboat would the sailboat move (all things being equal)???? The answer is no.

How about a swamp boat (you know, a big fan on the back of it) in a water tank? The water in the tank is moving at the same speed that the boat is (or better said, the speed the boat wants to go).
Different issue, water reacts way differently and has a much higher attack surface. A boat in moving water reacts differently to, say, a skateboard on wheels when you pull the rug.
The water is a resistance to the speed of the boat, the wheels arent (or only to a miniscule part). Actually, all they do is basically reduce the friction, de-coupling the plane from the ground.
Also, what is the speed the boat wants to go, in relation to what?
The problem with the conveyor belt is that there are no opposing forces acting on the plane on the horizontal direction (we can leave the vertical out of it).

Cheers,
Mike

GregMalick
07-18-2006, 03:45 PM
2: ( To Fly Or To Roll? ) A plane is standing on runway that can move (some sort of band conveyer). This conveyer has a control system that tracks the tire speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction). So the question is will the plane take off or not?

The source and details is here on page 4 post49 of the same thread as question 1. The general conclusion is yes.

OK do this experiment:

1. Take a battery (I used a Duracell) and put it on a peice of paper on your desk (or any flat surface),
2. Roll the battery forward.
3. Conteract the roll by sliding the paper.

Notice that when the battery stops rolling it is still moving forward.
That forward motion through the atmosphere is what gives a plane lift.

Answer: The plan will take off.

spec24
07-18-2006, 03:59 PM
Different issue, water reacts way differently and has a much higher attack surface. A boat in moving water reacts differently to, say, a skateboard on wheels when you pull the rug.
The water is a resistance to the speed of the boat, the wheels arent (or only to a miniscule part). Actually, all they do is basically reduce the friction, de-coupling the plane from the ground.
Also, what is the speed the boat wants to go, in relation to what?
The problem with the conveyor belt is that there are no opposing forces acting on the plane on the horizontal direction (we can leave the vertical out of it).

Cheers,
Mike

until that plane leaves the ground the ground IS the resistance to the speed of the plane (wind resistance too if the plane is actually moving). A plane must have ground speed before it can lift into the air and that ground speed is gained by rolling on wheels. Powered by a propeller, a jet, an engine linked to the wheels, some guy pulling on a rope, whatever - without movement the plane will not and cannot fly (disregarding wind tunnels and everything else not included in the original argument). That movement is linked to the rolling of the planes wheels along the ground. Again - an observer sitting i na real plane could say the ground below him is moving and his plane is standing still. The inverse is just as valid.

There is nothing making the air pass over the wings of the plane and there is no way the plane is going to move forward on a conveyer that is matching the planes (desired) speed.

spec24
07-18-2006, 04:04 PM
OK do this experiment:

1. Take a battery (I used a Duracell) and put it on a peice of paper on your desk (or any flat surface),
2. Roll the battery forward.
3. Conteract the roll by sliding the paper.

Notice that when the battery stops rolling it is still moving forward.
That forward motion through the atmosphere is what gives a plane lift.

Answer: The plan will take off.

What? If I roll a battery on a piece of paper and slide the paper under it to counteract the battery moving (not rolling) the battery appears to stay in one place. This only shows that the plane will remain stationary (the battery being the wheels of the plane) and will not move through the air and fly. How can the battery still be moving forward if its not rolling unless you're still dragging the paper? If the battery keep rolling forever and I have an infinitely long piece of paper and I pull it to match the battery's roll then the battery will appear stationary forever.

GregMalick
07-18-2006, 04:24 PM
What? If I roll a battery on a piece of paper and slide the paper under it to counteract the battery moving (not rolling) the battery appears to stay in one place. This only shows that the plane will remain stationary (the battery being the wheels of the plane) and will not move through the air and fly.

You interpret tire speed in the original problem as being translational motion.
I interpret tire speed as being rotational.

How can the battery still be moving forward if its not rolling unless you're still dragging the paper? Yes the paper is moving and the battery stops rolling and they both are moving forward - just like the conveyor belt and the plane.

If the battery keep rolling forever and I have an infinitely long piece of paper and I pull it to match the battery's roll then the battery will appear stationary forever.Nope. It would only appear stationary if you were an ant on the paper. You didn't try it or you would see the battery does not remain stationary in relation to the rest of the world. It is only stationary in relation to the paper (or conveyor belt).

Do the experiment as described.

spec24
07-18-2006, 06:40 PM
You interpret tire speed in the original problem as being translational motion.
I interpret tire speed as being rotational.

Yes the paper is moving and the battery stops rolling and they both are moving forward - just like the conveyor belt and the plane.

Nope. It would only appear stationary if you were an ant on the paper. You didn't try it or you would see the battery does not remain stationary in relation to the rest of the world. It is only stationary in relation to the paper (or conveyor belt).

Do the experiment as described.

From what I read into your statement you have misinterpreted the original question. You are pushing the paper in the direction the battery is rolling - therefore canceling out the rotation of the battery and of course the battery moves along with the paper (conveyer). The original question has the conveyer moving in the opposite direction of the plane, just as the ground would move below a real plane on a real stationary runway (it would appear to move out behind the plane). If I roll a battery on a piece of paper away from me and pull the paper towards me, the battery will appear not to move translationally, and that is what the question asks unless you have interpreted it differently, which, from what you stated, is the case.

GregMalick
07-18-2006, 07:25 PM
So it depends on which interpretation of the problem statement you choose.
Choosing my interpretation of the problem statement -wheels not turning - the plan will take off.
Choosing your interpretation of the problem statement - plane not moving - the plane will not take off.

Why won't it take off for your interpretation? Because of Newtons 3rd law.
For the plane to stay in one place when the engines are exerting a force that is driving the plane forward, the conveyor must exert an equal and opposite force to keep it in place. There is also that fact that planes don't take-off because their engines are blowing air under their wings. If that were so, they would immediately shoot off into the air when their engines were reved up and plumit to earth when their engines were turned off (gliders and eagles couldn't fly either). Single engine jets are more proof of this - relying on Newtons 3rd law to give them the velocity to lift off.

All in the interpretation, my friend.
All in the interpretation.

:hey:

Skinner3D
07-18-2006, 07:50 PM
The plane would stay still because the rotation of the tires would be translated into the movement of the conveyor belt. This is how runners stay still on a treadmill. When the tires stopped the conveyor belt would also have to stop, but since it impossible to do this the plane would end up going backwards. This means that the plane would never get off the ground.:D

If you really want to get :screwy: then consider an aircraft carrier. To accomadate the short launch deck they use a catapault and hook system. If you concentrate on just the catapault you could say that the aircraft carrier is pushed backward as fast as the plane is pushed foward. This system results in the aircraft taking off.:stumped:

This only works because the carrier is way bigger.:D

If it could chuck wood then most people would call it a beaver, since it does not live in dams but under the ground most people would then call it a water rat. Since it does not eat fish then it is stuck with being a plain old gopher. People often confuse gophers with moles, so our woodchuck is now a large mole that doesn't eat worms, lives in holes, eats the fruits of your garden, and hibernates. Which is what they do anyway, so our woodchuck is just a groundhog:D

I have got to stop writing my brain can't tell which end is up.:foreheads

spec24
07-18-2006, 08:26 PM
Holy cow. I can't believe it myself but I've come to the conclusion that the plane can in fact take off and fly (oh, and Greg, the question states that the conveyer is moving in the opposite direction of the plane so as far as I can tell there's only one way to interpret it :) ). I said can, doesn't mean it necessarily will. If the engines are powerful enough to overcome the friction of the plane on the wheels and hte wheels on hte conveyer than yes the plane will/can achieve forward movement and take off. For those that don't understand it (myself included for a while - but I thought about it all day), imagine you have a toy plane with freely spinning wheels for landing gear. You take this plane and hold it on a conveyer belt going ten miles per hour. You can hold the plane there fairly easily because the wheels spin freely under the plane. Your hand is a force acting upon the plane just as the planes propeller would be a force acting upon the plane (you are obviously not standing on the conveyer). Now you start to move your hand and of course the plane moves with your hand because you are exerting a force that is greater than the force acting on the plane and spinning its wheels freely. No matter how fast the conveyer moves you can still push the plane forward. In the original scenario the planes engine is acting upon the air much the same way my hand was acting upon the toy plane. Because the air is not moving in relation to the conveyer the propeller (or jet engine) can "grab ahold" of the air and pull/push the plane along just as my hand can push the toy plane along on the conveyer and the conveyer could spin wildly underneath - the wheels would just spin faster. So - I stand corrected :) Goodnight everyone :)

Anti-Distinctly
07-19-2006, 01:24 AM
Holy cow. I can't believe it myself but I've come to the conclusion that the plane can in fact take off and fly...[snip]

Thank goodness :) I was about to go crazy on you :)
To summarise: With frictionless bearings in the wheels there is absolutely no coupling between the ground and the aircraft. The engines, mounted on the plane, act on the air directly. The ground and its speed are completely irrelevant.

Good example spec24, I've heard a similar one. Take a belt sander turn it upside down and put a little toy car on it. Turn on the sander and hold the car in place by placing your finger behind it. You are free to push the car forward with your finger with minimal force. The only force pushing the toy car back towards your finger is friction from the wheel bearings, which is = 0 in our case. The wheels are free to rotate.

================================================== =======

I will denote a recurring number by use of a prime ('). e.g. 9 recurring will be written as 9'. I'll occasionally write more than one of the recurring number for clarity (i.e. 0.99' = 0.9', etc.)

Proof that 0.9' = 1
-----------------

Let 0.99' = x Multiply by 10
10x = 9.9'
10x - x = 9x
=> 9.9' - 0.9' = 9x
=> 9.0 = 9x Divide out by 9
=> x = 9/9
=> x = 1

I shall leave ye to ponder...

Exception
07-19-2006, 01:45 AM
How large is the exposed surface area of the liquid (one tiny spout the other huge)
What is each container composed of (one glass another metal ?)
The total volumes of each liquid (smaller volumes of the same liquid given the exact same conditions will reach room temp faster)

1) There's only one cup, therefore, assuming we all know what an ordinary cup looks like, it doesn't really matter.
2) Ditto, there's only one, now two separate containers. you just add the milk before or after the doorbell, that's it.
3) Doesn't really matter either.

Basically if the milk maintains more of it's initial temp than the coffee - then answering the door and then adding the milk will result in cooler coffee.

I think we can assume that the temperature of the milk does not change over the period that it takes you to answer the door... that is, before it is put in the coffee.

colkai
07-19-2006, 03:58 AM
If you're travelling at lightspeed, what happens when the dog sticks it's head out of the window?
Pah!
That's an easy one, as a long time dog owner I can tell you, this is a universal constant. You get covered in drool, just at lightspeed, it happens a HECK of a lot faster :p

Exception
07-19-2006, 04:15 AM
Somehow, when dogs are involved, there is a universal physical propery, which is, that my own person will get covered in drool.
This is the fixed constant on which all other things are based.

but... whaaaaai?

Skinner3D
07-19-2006, 06:26 AM

I will denote a recurring number by use of a prime ('). e.g. 9 recurring will be written as 9'. I'll occasionally write more than one of the recurring number for clarity (i.e. 0.99' = 0.9', etc.)

Proof that 0.9' = 1
-----------------

Let 0.99' = x Multiply by 10
10x = 9.9'
10x - x = 9x
=> 9.9' - 0.9' = 9x
=> 9.0 = 9x Divide out by 9
=> x = 9/9
=> x = 1

I shall leave ye to ponder...

:stumped: I think this only works if you assume that x = 1 from the start. But if you do that than what is the point of having x?:D

Anti-Distinctly
07-19-2006, 08:01 AM
No, x is 0.99' from the start. Maybe the formatting didnt make it clear;
You start with 0.99' recurring. Lets call that x. Multiply x by 10, so we have 10x.
0.99' multiplied by 10 is 9.9'
So 10x = 9.9'
Subtract x from both sides
10x - x = 9.9' - 0.9'
9x = 9.0
x = 9.0/9 = 1

Any clearer?

Lightwolf
07-19-2006, 08:15 AM
9x = 9.0

I guess this is the culprit, 9x = 9*9.9' and that is infinitely less than 9, but not exactly nine.

Cheers,
Mike

Exception
07-19-2006, 08:22 AM
9.99' is 10 in mathematical terms anyway because the limit reaches 10 in the infinite.

But...

10*0.9' - 0.9' = not 9.0'.

Lightwolf
07-19-2006, 08:35 AM
9.99' is 10 in mathematical terms anyway because the limit reaches 10 in the infinite.

Now wouldn't that be 9.9' ~ 10 in that case?

Cheers,
Mike

Exception
07-19-2006, 08:40 AM
No that's approximation...

But 0.99' in itself is not necessarily a number... If you ask the mathematical question, what on earth is 0.99', then you'll have to use limits to find that out and using a calculation from calculus that I both partially forgot and have no keys for on my keyboard it'll turn out that the limit to infinity of 0.99' = 1.

Or so I remember :)

jameswillmott
07-19-2006, 08:50 AM
No, x is 0.99' from the start. Maybe the formatting didnt make it clear;
You start with 0.99' recurring. Lets call that x. Multiply x by 10, so we have 10x.
0.99' multiplied by 10 is 9.9'
So 10x = 9.9'
Subtract x from both sides
10x - x = 9.9' - 0.9'
9x = 9.0
x = 9.0/9 = 1

Any clearer?

Hmmmm......
1 / 9 = 0.111'
2 / 9 = 0.222'
3 / 9 = 0.333'
....
7 / 9 = 0.777'
8 / 9 = 0.888'
9 / 9 = 0.999' = 1

:)

Karmacop
07-19-2006, 09:34 AM
Now wouldn't that be 9.9' ~ 10 in that case?

No he's right it's 1.

Here's how you convert a repeating decimal ...

take the repeating number to be x
x = 0.111'

multiply the repeating number by 10 ...
10x = 0.111' * 10
10x = 1.11'

subtract the smaller repeating number from the larger repeating number
10x - x = 1.11' - 0.11'
10x - x = 1
9x = 1

If 9 * x = 1, then we know that x = 1/9
So 0.111' = 1/9

Lets do the same with 0.999'

x = 0.999'
10x = 9.99'

10x - x = 9

9x = 9

So 0.999' = 9/9 = 1

Lightwolf
07-19-2006, 09:37 AM
it'll turn out that the limit to infinity of 0.99' = 1.

Oh, the limit surely is... the limit isn't the actual value though...

Cheers,
Mike

Bog
07-19-2006, 09:54 AM
2+2=5 for sufficiently large values of 2.

jameswillmott
07-19-2006, 10:05 AM
Oh, the limit surely is... the limit isn't the actual value though...

Cheers,
Mike

Because the 9's are stipulated to be recurring, the value IS the limit, because the infinite number of 9's must be considered.

The different between 1 and 0.999' drops by 1/10^n where n is the decimal place being considered. At infinity, the difference between 0.999' and 1 is equal to 1/10^infinity = 0.

jameswillmott
07-19-2006, 10:07 AM
2+2=5 for sufficiently large values of 2.

2+2=3 after tax. :D

Lord Snarebotto
07-19-2006, 01:55 PM
A plane is standing on runway that can move (some sort of band conveyer). This conveyer has a control system that tracks the tire speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction). So the question is will the plane take off or not?

It will not. Any first year aeronautics student could tell you that.

The conclusion that it will makes the incorrect assumption that the engines provide lift, which they do not. The engines propel (hence the name propeller) the plane forward, but it is the wings which provide lift.

The engines must propel the plane forward at a high enough rate of velocity to induce lift on the lower surface the wings.

Since the hypothetical scenario specifies (the inference is indeed implicit) that the net forward velocity of this aircraft would be zero, the net lift is also equal to zero.

Also since it is not specified, implicitly or otherwise, that any propelling is indeed occurring at all, basically nothing will happen, except fish.

Lightwolf
07-19-2006, 01:59 PM
Since the hypothetical scenario specifies (the inference is indeed implicit) that the net forward velocity of this aircraft would be zero, the net lift is also equal to zero.
It doesn't specify it... I walked into the same trap ;)

Cheers,
Mike

Lord Snarebotto
07-19-2006, 02:07 PM
It doesn't specify it... I walked into the same trap ;)

Cheers,
Mike

Indeed, no other logical conclusion can be drawn from the information given.

Unless vital information has been withheld for the purpose of unduly inflating this problem to a braintease or conundrum. :D

Lightwolf
07-19-2006, 02:15 PM
Indeed, no other logical conclusion can be drawn from the information given.

Unless vital information has been withheld for the purpose of unduly inflating this problem to a braintease or conundrum. :D
Nope, the issue here is the fact that the thrust will not be counteracted by the belt (unlike a car driving the wheels if it was on the belt). The wheels are only there to reduce the friction, the belt will go faster, but that will only make the wheels spin faster, but it won't stop the plane.
In the worst case, the wheels will just burst, but the plane will move forward.

Cheers,
Mike

Lord Snarebotto
07-19-2006, 02:23 PM
Sorry, but you're wrong. If you read the question carefully, it never says that the engines are even on. In fact, all it states relating to the movement of the plane is that it is standing on a runway. A plane simply standing on a runway will never take off. ;)

Lord Snarebotto
07-19-2006, 02:32 PM
I was also pondering this: if the net speed differential between the runway and the wheels is always equal to zero, the plane will indeed never fly. Since the wheels speed differential would be the net velocity and if you do a little equation like follows:

speed differential = 0 = net velocity

it is quite clear and unequivocal that the plane will never leave the ground under the conditions specified.

mjcrawford
07-19-2006, 02:39 PM
It will not. Any first year aeronautics student could tell you that.

The conclusion that it will makes the incorrect assumption that the engines provide lift, which they do not. The engines propel (hence the name propeller) the plane forward, but it is the wings which provide lift.

The engines must propel the plane forward at a high enough rate of velocity to induce lift on the lower surface the wings.

Since the hypothetical scenario specifies (the inference is indeed implicit) that the net forward velocity of this aircraft would be zero, the net lift is also equal to zero.

Also since it is not specified, implicitly or otherwise, that any propelling is indeed occurring at all, basically nothing will happen, except fish.

I think that it can be assumed that the forward thrust has been applyed, otherwise the question is stupid, if the engines are off the plane would not move anyway.. but the simple answer is that unless the plane is somhow being held in place by some outside force it will move forward since the engines give forward thrust, not torque through a drivetrain like a car... no matter how fast the belt moves the plane will always be faster and even if the belt had no limit the plane would just burn rubber as it took off.. assuming the tires lasted long enough for the plane to tak off...

Lightwolf
07-19-2006, 02:43 PM
A plane simply standing on a runway will never take off. ;)
Lol, you're right, the way it is phrased in this thread it won't. Unless you move the runway that is (or slant it) ;)

Cheers,
Mike

art
07-19-2006, 02:47 PM
Sorry, but you're wrong. If you read the question carefully, it never says that the engines are even on. In fact, all it states relating to the movement of the plane is that it is standing on a runway. A plane simply standing on a runway will never take off. ;)
Well, if we look at it this way then the conveyor belt wont move either because there is no movement to counteract and there is no point in placing it under the plane and asking this question in the first place :)

ThriJ
07-19-2006, 03:12 PM
If you read the question carefully, it never says that the engines are even on. In fact, all it states relating to the movement of the plane is that it is standing on a runway. A plane simply standing on a runway will never take off. ;)

I was wondering when someone would point that issue out.:rolleyes: You could treat it like a trick question. I am not the originator of the question so I can’t say. I could rephrase the question and add, “When the plane gets going”.

Anyway, I think there are two things that should be assumed.

1. The plane starts up.

2. The crazed pilot attempting this will pull on the stick and try to get the plane airborne.

mjcrawford
07-19-2006, 03:13 PM
Well, if we look at it this way then the conveyor belt wont move either because there is no movement to counteract and there is no point in placing it under the plane and ask this question in the first place :)

:agree: that was my point! the question's existance implys that forward thrust from the plane is on! if not what would the point of the test be?

spec24
07-19-2006, 03:29 PM
I think we can all agree that some force is acting on the plane to make the plane move in one direction and that the conveyer moves in the opposite direction, at the same speed, that the plane does. Lord Snarebotto, the original question does not state that the conveyer cancels out hte forward motion of the plane. You only reached this assumption, as many do, by when you read the question because any vehicel - a car, bike, or a person walking - is transferring force to it's wheels or the person's feet. A plane is NOT doing this. The airplane is using it's propeller to to pull on air and push it out behind it. Because the air can be thought of as a stationary object that is not moving with the conveyer the plane can (and will) do this and move forward after it overcomes the friction of the tires (apparerntly not that hard to do). This is the same as if you were standing on a conveyer belt with roller skates on your feet and there was a railing that you could hold onto. The railing is not moving along with the conveyer. The conveyer under your feet is spinning the wheels of the rollerskates at whatever speed you like - it takes a minimal amount of force on your part to hold onto the railing and keep yourself from traveling backwards (with no friction it would take no force). Now, all you need to do is apply more force to whatever force is keeping you stationary (in relation to the earth) and you will move yourself forward - such as pulling on the railing hand over hand. This is the same as what the plane is doing when it pulls on the air.

WhiteBoy
07-19-2006, 03:40 PM
Maybe Mythbusters should tackle this one...

Anti-Distinctly
07-19-2006, 03:45 PM
@Lord Snarebotto: Read my post on page 3. but to say it even more briefly here; the motion of the plane to the ground is irrelevant. There is no coupling between the aircraft and the ground.
The wheels rotate freely on their frictionless bearings.

mjcrawford
07-19-2006, 04:17 PM
what I want to know is why is there braile on a drive up ATM? :D

Exception
07-19-2006, 04:46 PM
what I want to know is why is there braile on a drive up ATM? :D

Its for a blind michael knight in kit. Duh!

:)

Lord Snarebotto
07-19-2006, 04:54 PM
I think that it can be assumed that the forward thrust has been applyed, otherwise the question is stupid, if the engines are off the plane would not move anyway.. but the simple answer is that unless the plane is somhow being held in place by some outside force it will move forward since the engines give forward thrust, not torque through a drivetrain like a car... no matter how fast the belt moves the plane will always be faster and even if the belt had no limit the plane would just burn rubber as it took off.. assuming the tires lasted long enough for the plane to tak off...

The problem specifically presumes that the tires and runway move at the same velocity, which can only happen if there is no net thrust. No net thrust = no flight. Since this entire problem ignores the laws of physics, shall we not extrapolate that ignorance to ignoring what we think might happen, and instead equate what must happen, by way of the problems parameters?

Since many of the things we come to know as being possible or impossible due to our knowledge of the universe, can we not simply assume that the question takes place in a world where all assumed knowledge then equates to artifice, thus presenting possibilities which we would have no formal language to then describe in adequate detail for the lay person to come to an understanding of?

mjcrawford
07-19-2006, 04:57 PM
The problem specifically presumes that the tires and runway move at the same velocity, which can only happen if there is no net thrust. No net thrust = no flight. Since this entire problem ignores the laws of physics, shall we not extrapolate that ignorance to ignoring what we think might happen, and instead equate what must happen, by way of the problems parameters?

hmm... well if the question ignores the laws of physics, then what is the point as any answer would be wrong anyway..

Lord Snarebotto
07-19-2006, 04:58 PM
hmm... well if the question ignores the laws of physics, then what is the point as any answer would be wrong anyway..

Or right... :hey:

mjcrawford
07-19-2006, 05:04 PM
Its for a blind michael knight in kit. Duh!

:)

mjcrawford
07-19-2006, 05:05 PM
Or right... :hey:

:agree:
so than here is the real question... if no answer can be right, and no answer can be wrong.. why is there a question?

Skinner3D
07-19-2006, 07:52 PM
:D I think we have all come to the same conclusion that no matter which direction the plane gets going it is definately going to need a repair job afterwards, even if it is only the wheel bearings.

spec24
07-19-2006, 08:21 PM
The problem specifically presumes that the tires and runway move at the same velocity, which can only happen if there is no net thrust. No net thrust = no flight. Since this entire problem ignores the laws of physics, shall we not extrapolate that ignorance to ignoring what we think might happen, and instead equate what must happen, by way of the problems parameters?

Since many of the things we come to know as being possible or impossible due to our knowledge of the universe, can we not simply assume that the question takes place in a world where all assumed knowledge then equates to artifice, thus presenting possibilities which we would have no formal language to then describe in adequate detail for the lay person to come to an understanding of?

It's precisely the laws of physics that allow the plane to take off, the question does not ignore them. It doesn't matter how fast the wheels are turning on the plane Lord Snarebotto because the plane is not pushing against the ground to gain forward velocity - it is pushing against the air. Think of it this way. A car doing the same thing will in fact never gain forward velocity (in relation to the earth) while sitting on a conveyer that matches the car's speed. Now, think of a car that has wings on it and drive it down a regular road until you've accelerated enough to get the carplane airborn. What happens when the carplane lifts into the air??? It will immediately come back down to earth because the carplane no longer has any mode of generating forward velocity (it was generating it's velocity by pushing on the ground with its tires). A plane on the other hand is generating forward velocity by pulling/pushing the air, whether it's on the ground or in the air. Does this make it clear yet?

Digital Hermit
07-19-2006, 08:25 PM
The conveyor plane problem…

Answer: The plane would take off.

Why? The wheels are not a factor. Essentially, and relatively, to the aircraft they are frictionless because they are free rotating.

I think the wheels is what is throwing some, (because of cars maybe?)

What if we replaced the wheels with skis and have an oil-slicked conveyor causing essentially no friction. The plane would still take off because it does not matter about the speed of the conveyor (even if the conveyor was spinning 4 times the ground speed or even more, it still wouldn’t matter.) The aircraft would still reach its lifting velocity without any encumbrance from the conveyor.

So, all this means, for the wheels, is they would be spinning as fast as the conveyor would be rotating, but not be a factor in the planes forward movement.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~

Ok, I have different puzzle for you folks.

If a tank is moving at a speed of 60mph ... what is the speed/mph of the tracks contacting the ground?

Regards,

Digital Hermit

spec24
07-19-2006, 08:32 PM
Ok, I have different puzzle for you folks.

If a tank is moving at a speed of 60mph ... what is the speed/mph of the tracks contacting the ground?

Regards,

Digital Hermit

the speed of the tracks contacting the ground is zero. Just as the speed of a tire on a car at the point of contact with the ground is zero.

spec24
07-19-2006, 08:34 PM
So, all this means, for the wheels, is they would be spinning as fast as the conveyor would be rotating, but not be a factor in the planes forward movement.
Digital Hermit

actually they'd be spinning as fast as the conveyer plus the additional speed from the planes forward movement.

Digital Hermit
07-19-2006, 10:59 PM
actually they'd be spinning as fast as the conveyer plus the additional speed from the planes forward movement.

Touché! :thumbsup:

Lord Snarebotto
07-20-2006, 12:26 AM
actually they'd be spinning as fast as the conveyer plus the additional speed from the planes forward movement.

Ahh...but that would violate the conditions of the question...that the runway would would automatically match the wheel speed.

However, once again we have the conundrum that the speed of the wheels identical to the speed of the runway in reverse, yet some forward momentum (under natural circumstances) would cause the wheels to move faster, since they are in contact with the runway, and their coefficient of friction would be near to 1.0, so they essentially would be in direct contact with and directly influenced by, the runway, with gravity acting as the glue to hold them in lock-step.

Since any forward momentum by the airship would cause constant and infinite acceleration by the fictitious runway, which by decree must match the speed of the wheels, I predict nuclear fusion (some similar result already predicted by another) would occur in approximately 54 seconds, and a baby black-hole would form, a doorway to another dimension, one not only of sight and sound, but of mind...

Shall we step across the threshold?:)

jameswillmott
07-20-2006, 12:37 AM
Ahh...but that would violate the conditions of the question...that the runway would would automatically match the wheel speed.

Interesting...

Since we've confirmed earlier that the speed of the wheel at the point of contact with the conveyor is in fact 0, relative to the conveyor, the conveyor would never move because it has already matched the speed of the wheel. :D

Lord Snarebotto
07-20-2006, 12:39 AM
If a tank is moving at a speed of 60mph ... what is the speed/mph of the tracks contacting the ground?

Regards,

Digital Hermit

Well, thanks to a more well known genius, we know that the velocity of those tracks has several possible components, depending on the relative position of the observer.

Therefore, an observer on the ground would say that most of the tracks at the point of contact with the ground (the ones that were laying flat, with the ones just contacting or just leaving the ground would have a Y component velocity, since we really need to think multidimensionally here) would have zero velocity in relationship to the ground. However, an observer on the tank would say that the tracks are moving away at 60 MPH relative to his position in time and space. They would both be correct, based upon relativity...

:)

Lord Snarebotto
07-20-2006, 12:52 AM
Interesting...

Since we've confirmed earlier that the speed of the wheel at the point of contact with the conveyor is in fact 0, relative to the conveyor, the conveyor would never move because it has already matched the speed of the wheel. :D

Well, that is only true for any single instant in time, but only relative to each other, and regardless of any velocity by plane or runway.

However, since the engines would be providing thrust, we can assume that the wheels would start this irreversible process at some point in time, and since the calculation of velocity requires time, we cannot simply examine a single instant to achieve a meaningful result, since that would require division by zero.

So the condition you've described is non-existent in real terms.

One could also argue that nothing ever moves, relative to a single instant in time, should we be so foolhardy as to only consider one sample when making determinations of velocity.

Lord Snarebotto
07-20-2006, 01:04 AM
the speed of a tire on a car at the point of contact with the ground is zero.

Actually, that tire (and the ground which it contacts) is moving at a tremendous rate of speed relative to the planet Venus or Mars... :D

Let's be clear about your supposition, though. As I already explained, the velocity of the tire at the point of contact with the road is only zero for one instant in time, and that same point on the tire is actually moving once we consider other instants (consider a graph with points), spaced over a period of time (which, BTW, is the only way speed or velocity can be measured). So really, it's not the tire or the road which we are measuring thusly, it is the precise point where they meet, which moves from nanosecond to nanosecond at the same velocity as the vehicle, if we exclude all other factors (this postulate assumes a straight road, with no transverse directional components).

jameswillmott
07-20-2006, 01:26 AM
Well, that is only true for any single instant in time, but only relative to each other, and regardless of any velocity by plane or runway.

However, since the engines would be providing thrust, we can assume that the wheels would start this irreversible process at some point in time, and since the calculation of velocity requires time, we cannot simply examine a single instant to achieve a meaningful result, since that would require division by zero.

So the condition you've described is non-existent in real terms.

One could also argue that nothing ever moves, relative to a single instant in time, should we be so foolhardy as to only consider one sample when making determinations of velocity.

We're not calculating the velocity of the wheel by making a single measurement at a single instance in time, such a calculation is impossible as you pointed out, and I wasn't suggesting that it was possible to do.

But the velocity of any point on the wheel at any given instant can still be calculated, so how is that condition non existant in real terms?

Either way, the plane still takes off. Or does the Earth drop away from it???

Anti-Distinctly
07-20-2006, 02:23 AM
...and since the calculation of velocity requires time, we cannot simply examine a single instant to achieve a meaningful result, since that would require division by zero...

If you're looking at it that way, that this happens instantly, within no time at all, then there is no movement by the wheel. So you have v = s/t = 0/0 which is undefined. Usually you'd use an infinitesimals, delta_s/delta_T, which is the definition of velocity anyway.

...Since this entire problem ignores the laws of physics, shall we not extrapolate that ignorance to ignoring what we think might happen, and instead equate what must happen, by way of the problems parameters?...

You'd be supprised how many theories in physics are 'ignorant' to other concepts and rules in physics. I'm working on one myself at the moment and you have to make masses of assumptions and shortcuts. It's neccessary in the real world. But this is a gedanken experiment, so certain things can be assumed. Admittedly, I dont think the problem was specified enough in the original post (e.g. I'm not sure that frictionless bearings were mentioned, but that is how I take it otherwise the whole thing would instantly explode)

...Let's be clear about your supposition, though. As I already explained, the velocity of the tire at the point of contact with the road is only zero for one instant in time...

This is not the case relative to a person standing and watching from the sidelines, which again, I think is to be assumed to be the frame of reference for this problem.
Relative to the runway, the velocity of the point of the tyre in contact with the runway at that instant is = 0. But this is not what we're looking for, so to speak.
The obserer will not see a speed = 0 on the wheel.

Anyway, all of this is completely moot. As I've said many times, with a frictionless bearing there is no coupling between the aircraft and the ground. The wheels rotate freely at any speed.

spec24
07-20-2006, 05:09 AM
Ahh...but that would violate the conditions of the question...that the runway would would automatically match the wheel speed.

However, once again we have the conundrum that the speed of the wheels identical to the speed of the runway in reverse, yet some forward momentum (under natural circumstances) would cause the wheels to move faster, since they are in contact with the runway, and their coefficient of friction would be near to 1.0, so they essentially would be in direct contact with and directly influenced by, the runway, with gravity acting as the glue to hold them in lock-step.

Since any forward momentum by the airship would cause constant and infinite acceleration by the fictitious runway, which by decree must match the speed of the wheels, I predict nuclear fusion (some similar result already predicted by another) would occur in approximately 54 seconds, and a baby black-hole would form, a doorway to another dimension, one not only of sight and sound, but of mind...

Shall we step across the threshold?:)

it is true that the original poster has altered the original question from when it first came into being by changing the speed of the plane to the speed of the wheel. This does in fact change the properties. However, the wheel has TWO different speeds which is not stipulated by the poster (and not the original physics question as it was presented). You have rotational speed and translational speed. Translational speed is equal to the speed of the plane, and this is what is meant by the question only because when this question was original presented oh so many years ago the question was not about the rotational speed of the tire. If you look at it from the rotational speed of the tire then I suppose the conveyer would have to accelerate infinitely - this causes a myriad of problems but the plane will still take off.

ThriJ
07-20-2006, 06:24 AM
the original poster has altered the original question from when it first came into being by changing the speed of the plane to the speed of the wheel. This does in fact change the properties. However, the wheel has TWO different speeds which is not stipulated by the poster (and not the original physics question as it was presented). You have rotational speed and translational speed. Translational speed is equal to the speed of the plane, and this is what is meant by the question only because when this question was original presented oh so many years ago the question was not about the rotational speed of the tire. If you look at it from the rotational speed of the tire then I suppose the conveyer would have to accelerate infinitely - this causes a myriad of problems but the plane will still take off.

Hmmm…

Then I guess assumption number three for question two should be that the forward movement of the wheels is calculated by the conveyer and not the rotational movement.

Hey at this rate we will need to know the model of the plane, who made the conveyer, the time of day, the exact location of the event and the pilot’s name.:D

righteous
07-20-2006, 06:29 AM
"The poster" is me and thats, word for word, how it was presented to me :thumbsup:

I guess you could say its not the right question but it IS the question I asked :D so i guess you'll have to live with it. We cant just go changing questions so that they suite our answers now can we?!

ThriJ
07-20-2006, 06:38 AM
"The poster" is me and thats, word for word, how it was presented to me :thumbsup:

I guess you could say its not the right question but it IS the question I asked :D so i guess you'll have to live with it. We cant just go changing questions so that they suite our answers now can we?!

Yep,:i_agree:

BTW, I have decided that the name of the pilot is Wash. So of course the plane will fly!:D :screwy:

spec24
07-20-2006, 06:56 AM
"The poster" is me and thats, word for word, how it was presented to me :thumbsup:

I guess you could say its not the right question but it IS the question I asked :D so i guess you'll have to live with it. We cant just go changing questions so that they suite our answers now can we?!

Take it easy righteous. As I pointed out there are two ways to interpret your question (rotational speed and translational) speed, so I am living with it, it was a badly worded question. If in your question you meant that the speed of the conveyer is moving at the same rotational speed of the wheel - and you stated the plane was moving - than of course the question is not answerable because the conveyer can NEVER move as fast as the wheel. Is that what you meant when you asked the question?

spec24
07-20-2006, 07:17 AM
Well, thanks to a more well known genius, we know that the velocity of those tracks has several possible components, depending on the relative position of the observer.

Therefore, an observer on the ground would say that most of the tracks at the point of contact with the ground (the ones that were laying flat, with the ones just contacting or just leaving the ground would have a Y component velocity, since we really need to think multidimensionally here) would have zero velocity in relationship to the ground. However, an observer on the tank would say that the tracks are moving away at 60 MPH relative to his position in time and space. They would both be correct, based upon relativity...

:)

This has nothing to do with relativity.

The original question: "If a tank is moving at a speed of 60mph ... what is the speed/mph of the tracks contacting the ground?"

The tracks - and a wheel - don't move along the ground. An observer (in your relativity explanation) will still get the same measurement whether he is in the tank, on the ground, or on the wheel. Any single point on the tank tread (and a tire is just a round tank tread - also part of a tire will flatten out and contact the ground with a greater surface area than a infinite point or line) will, at the point it contacts the ground (as stated in the question), not be moving in realtion to the ground. The tire would have to be sliding across the ground for there to be some speed in relation to the ground - this is not the case. Think of the commercial for tires where it shows the tire from the underside going through the water. For an instant you see a part of the tire contacting the ground, and sure enough, it is not moving in realtion to the ground.

righteous
07-20-2006, 07:55 AM
Take it easy righteous. As I pointed out there are two ways to interpret your question (rotational speed and translational) speed, so I am living with it, it was a badly worded question. If in your question you meant that the speed of the conveyer is moving at the same rotational speed of the wheel - and you stated the plane was moving - than of course the question is not answerable because the conveyer can NEVER move as fast as the wheel. Is that what you meant when you asked the question?

Hey :D Internet is bad for this. I think you miss understand, i thought my little green smily man would have got it across. Or maybe its a bad case of Aussie humour. Im really taking it very easy, wasnt getting worked up (still not).

All I was saying was that I was given the question in a forum word for word the exact same way. We all ranted and raved for 9 odd pages and came to the same conclusions that everyone else has here. I didnt come up with the question, I just stole it from someone who stole it from someone else. Bit like a bad game of chinese whispers.

I think the "it will never move" or "it will explode instantly at the hint of anymovement" answers are good ones.

Peace out :thumbsup: ;)

spec24
07-20-2006, 08:37 AM
Hey :D Internet is bad for this. I think you miss understand, i thought my little green smily man would have got it across. Or maybe its a bad case of Aussie humour. Im really taking it very easy, wasnt getting worked up (still not).

All I was saying was that I was given the question in a forum word for word the exact same way. We all ranted and raved for 9 odd pages and came to the same conclusions that everyone else has here. I didnt come up with the question, I just stole it from someone who stole it from someone else. Bit like a bad game of chinese whispers.

I think the "it will never move" or "it will explode instantly at the hint of anymovement" answers are good ones.

Peace out :thumbsup: ;)

Sorry righteous. You're right, I didn't mean for my post to sound like it did. My appologies. But I have to disagree with the last sentence, especially since the question, as Lord stated it, is an impossibilty on its face. It would be like me asking a question that states I have a 100lb weight that I am lifting with 20lbs of force. I can't lift a 100lb weight with 20lbs of force and a conveyer can never keep speed with the planes wheels if the plane is moving on the conveyer (which it is).

righteous
07-20-2006, 08:55 AM
And thus it continues :D

spec24
07-20-2006, 10:55 AM
Well, thanks to a more well known genius, we know that the velocity of those tracks has several possible components, depending on the relative position of the observer.

Therefore, an observer on the ground would say that most of the tracks at the point of contact with the ground (the ones that were laying flat, with the ones just contacting or just leaving the ground would have a Y component velocity, since we really need to think multidimensionally here) would have zero velocity in relationship to the ground. However, an observer on the tank would say that the tracks are moving away at 60 MPH relative to his position in time and space. They would both be correct, based upon relativity...

:)

think of it this way. Imagine the tank track becomes unlinked from the tank, or the tire seperates as the vehicles continues to move forward. Does the tire or the tank move on the gorund? No, it doesn't, it lays itself out flat. This is exactly what is happening as the tire rotates around (or the tank tread). It rolls itself out onto the road. It's just at some point the tire/tank is picked back up again.

ravantra
07-20-2006, 11:13 AM
If you're traveling at the speed of light, does it matter if your headlights are on?

Nope. Light velocity (celerity?) can't be added to the speed of the vehicle, so at the speed of light, nothing in front of the vehicle will be lit by the headlights

The speed of light is always the same for all observers. So if you turn on the headlights they WILL light the way. Also only massless bodies can travel at the speed of light so the car cannot travel at the speed of light just very close to it (0.9999x speed of light).

mjcrawford
07-20-2006, 11:21 AM
If you're traveling at the speed of light, does it matter if your headlights are on?

well no becasue if you are traveling at the speed of light, according to physics you and your vehicle would be converted to pure energy (light) and therefore it would make no differance if the headlights were on or not. now of course if you were going FASTER than light that would be another story.. but then if you were going faster than light, you would be going backwards in time and therfore the headlights would be lighting where you were, not where you are going, so your tail lights would light where you are going, unless of course you were going in reverse faster than light in which case you would be going forwards and.....

Anti-Distinctly
07-20-2006, 12:09 PM
The speed of light is always the same for all observers. So if you turn on the headlights they WILL light the way. Also only massless bodies can travel at the speed of light so the car cannot travel at the speed of light just very close to it (0.9999x speed of light).

Would that be 0.9' the speed of light?
That means that we can :D

mjcrawford
07-20-2006, 12:20 PM
Would that be 0.9' the speed of light?
That means that we can :D

good one! :thumbsup:

perhaps we should rephrase the questions so that physics are not compleatly ignored?

MiniFireDragon
07-20-2006, 04:19 PM
:stumped: Official BrainTeAsErS Pondering Thread :stumped:

2: ( To Fly Or To Roll?:chicken: ) A plane is standing on runway that can move (some sort of band conveyer). This conveyer has a control system that tracks the tire speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction). So the question is will the plane take off or not?
May the confusion begin!:stumped:

This conveyer has a control system that tracks the tire speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Airplane wheels have no DRIVE SYSTEM!

Movement is caused by the THRUST of the ENGINES!

Now, I have rethought this and I would say yes it can take off (with the jet engines going). No matter what is under the aircraft, it will still move forward. The conveyor will match the rolling action of the wheels yes, but as the wheels traverse the conveyor belt, which would be the speed of the aircraft + the rolling speed of the wheel.

Think of it this way, take a toy car in your hand. Your hand is the thrust provided by the jet engines. Go find a belt sander and hold the car on it. Now throttle the belt sanders speed as u push the car forward with your hand. No matter how fast the belt sander is rolling the wheels of the toy car, you will always be pushing the car forward.

When math fails, take it out of the equation and put a picture in it's place. :)

spec24
07-20-2006, 05:05 PM
This conveyer has a control system that tracks the tire speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Airplane wheels have no DRIVE SYSTEM!

Movement is caused by the THRUST of the ENGINES!

Now, I have rethought this and I would say yes it can take off (with the jet engines going). No matter what is under the aircraft, it will still move forward. The conveyor will match the rolling action of the wheels yes, but as the wheels traverse the conveyor belt, which would be the speed of the aircraft + the rolling speed of the wheel.

Think of it this way, take a toy car in your hand. Your hand is the thrust provided by the jet engines. Go find a belt sander and hold the car on it. Now throttle the belt sanders speed as u push the car forward with your hand. No matter how fast the belt sander is rolling the wheels of the toy car, you will always be pushing the car forward.

When math fails, take it out of the equation and put a picture in it's place. :)

The conveyer cannot match the rolling speed of the wheels because the plane is moving forward on the conveyer - the wheels will always roll faster than the conveyer if the plane is moving.

art
07-20-2006, 06:08 PM
The wheels will rotate faster than the movement of the plane in respect to the surrounding/air (not the belt) would suggest but they will rotate at the speed of the belt. After all the job of the belt is to match the rotation of the wheels. If the wheels rolled faster than the belt they would have to skid and the conveyor is programmed not to allow for that.

If we try to split the hair here, by the problem definition the wheel is trying to counteract the rotation of the wheel. If that is the case the conveyor should turn in the same direction where the plane is going. Anyway, that is still irrelevant to the movement of the plane in respect to the air.

mjcrawford
07-20-2006, 06:41 PM

3: ( Coffee Anyone?:caffeine: ) I have a cup of coffee. I want a bit of milk in it. The doorbell rings. Assuming everything is perfect, and the temperature of the milk is lower than that of the coffee, and the room temperature is normal, and thus lower than the coffee too. And the time to answer the door is neither infinitely short, nor infinitely long. Expect a reasonable timeframe. The question is [I]will the coffee be cooler if I add the milk now, and then go to open the door and come back, or will it be cooler if I open the door first, and add the milk later?

I say that the coffee will be cooler if you wait to put the milk in after answering the door...

since the room is cooler thant the coffee, the coffee will loose some of its heat while at the door, thus, when the milk is added, it will reduce the temputure even further than if you did it earlier... I think... actualy I am not that sure on this one...

Digital Hermit
07-21-2006, 12:04 AM
Ok.... if you are traveling on a conveyor that is moving the speed of light in an airplane that has tank treads instead of wheels while sippin' yer coffee and you turn you head-lights on... What would your Lightwave renders smell like?

Sorry, I just have ta' know! :screwy:

Digital Hermit

Anti-Distinctly
07-21-2006, 01:34 AM
...
since the room is cooler thant the coffee, the coffee will loose some of its heat while at the door, thus, when the milk is added, it will reduce the temputure even further than if you did it earlier... I think... actualy I am not that sure on this one...

I think this has already been posted in this thread, but what the hey.

The rate of cooling is proportional to the temperature difference between one body and another. Therefore, the coffee will cool faster when it is hotter, i.e. without the milk in.
But also, if the milk is cooler than the surroundings, it will heat up.

I actually did the maths for this the other day, the breakdown goes as follows:
1. Loss of heat due to radiation, assuming a black body radiation type, was negligible.
2. Loss of heat due to the coffee becoming vapor, is negligible. I assumed that a cup of coffee will take 4 days to just evaporate on its own, I just made this figure up, but its reasonable. Based on this I estimated how much coffee will evaporate in time, t (time to get to the door and back)
3. I did the equations to work out the temperature;
3.1 When you mix them first, then cool.
3.2 When you leave them seperate for time, t, then mix.

They both came out to be exactly the same. Gah.
Problem probably was that I assumed the 'cooling' same constant, k, in Newtons law of cooling (dT/dt = k(T-T_s), T = temp, T_s = surrounding temp) for both the milk and the coffee. But that shouldn't matter. May have to think about this some more.

Anti-Distinctly
07-21-2006, 01:36 AM
Ok.... if you are traveling on a conveyor that is moving the speed of light in an airplane that has tank treads instead of wheels while sippin' yer coffee and you turn you head-lights on... What would your Lightwave renders smell like?

Sorry, I just have ta' know! :screwy:

Digital Hermit

A 1064nm laser fried chicken.

Exception
07-21-2006, 02:56 AM
They both came out to be exactly the same. Gah.

Very good calculation...
the only way where I think you are wrong is the following two important properties:

- Milk is fatty and the fat creates an insulating layer on top of the coffee, which truly matters
- By adding the milk, you increase the volume, but the cooling surface are only increases a little. The heat will bebetter maintained by a larger volume with a proportionally smaller surface.

these two work in tandem, which would lead me to say that although a basic physical model is indeed what you described, with those complexities added, the one where the milk goes in first is hotter.

Anti-Distinctly
07-21-2006, 03:41 AM
Very good calculation...
the only way where I think you are wrong is the following two important properties:

- Milk is fatty and the fat creates an insulating layer on top of the coffee, which truly matters
- By adding the milk, you increase the volume, but the cooling surface are only increases a little. The heat will bebetter maintained by a larger volume with a proportionally smaller surface.

these two work in tandem, which would lead me to say that although a basic physical model is indeed what you described, with those complexities added, the one where the milk goes in first is hotter.

I'm not concerned about the fat providing an insulating layer. Couldn't be bothered with the maths and I'm not sure of its significant, but you're right that it will keep it slightly insulated. Maybe.
The calculation I did were basically one amount of water at one temperature (coffee) and another amount of water 1/10 of the mass being added which is at another temperature (milk).
I think my calculation was too simple and your second point provides the reason why. I think the constant, k, will change depending on the area_exposed/volume ratio, which I assumed to be the same for all liquids and combinations in the calculation.
Physicists do the estimation thing all the time. I'm supprised I didn't start off by assuming the the room, coffee and milk were all spheres...

jameswillmott
07-21-2006, 03:44 AM
BTW, wouldn't you assume that the milk was kept in the fridge until it was needed?

Exception
07-21-2006, 03:48 AM
BTW, wouldn't you assume that the milk was kept in the fridge until it was needed?

No, I think that would be an unfair comparison, because you are adding energy to the milk to keep it cool. I think the milk is in a similar container on the table, but not within radiation distance of the milk, out of reach from the cat, not underneath a leaky roof, and no aliens have abducted the cow before it gave the milk. Methinks.

DuneBoy
07-21-2006, 06:11 AM
Fill a glass with ice to the point it is higher then the brim of the glass, then fill the glass with water to the brim. Now when the ice metls will it overflow?

It depends on how much higher the ice is over the brim. If the ice and water was level with the brim, the water line would drop as the ice melted.

You could, after the ice has melted, determine how much water would be needed to fill the glass back to the brim. Make an ice cube from that amount of water and that's the maximum amount of ice that could be over the brim without overflowing the glass when melted.

Skinner3D
07-21-2006, 07:10 AM
:D WRONG :D

I actually had to do this experiment for science class. The water level stays the same from the time the ice is put in, until the time the water melts.

spec24
07-21-2006, 07:30 AM
It depends on how much higher the ice is over the brim. If the ice and water was level with the brim, the water line would drop as the ice melted.

You could, after the ice has melted, determine how much water would be needed to fill the glass back to the brim. Make an ice cube from that amount of water and that's the maximum amount of ice that could be over the brim without overflowing the glass when melted.

Duneboy is right. I mean - if you were able to stack the ice a foot over the glass (somehow) and it melted then of course it would overflow. Let's say you only fill the glass half way with ice cubes and then fill it to the brim with water. The physics of water/ice tell us that the water level will drop below the brim when the ice melts because water expands when it freezes.

http://www.lucinda.net/k6science/water/w_q_a/expand.html

iconoclasty
07-21-2006, 07:38 AM
you are adding energy to the milk to keep it cool.
Technically, you'd be taking energy away from the milk to keep it cool. Wouldn't you?

Exception
07-21-2006, 08:20 AM
Technically, you'd be taking energy away from the milk to keep it cool. Wouldn't you?

Hm yes, but it depends a little what you take as a reference point. Yes, the milk has less atmic kinetic energy when cold, but when placed in a room, it has minus kinetic atomic energy. It will disippate the cold into the room, or you could say that it draws heat from the room. In the fridge, you need to use energy (fridge engine) to reduce the atomic kinetic energy... so you are adding energy to reduce energy, which is in turn something from which you can harvest energy again.
So, something that is colder has as much of an energy difference with the surrounding as something that is hotter. Like a rubber band, it doesn't matter which sign the vector has, it still has a length.

spec24
07-21-2006, 09:19 AM
Hm yes, but it depends a little what you take as a reference point. Yes, the milk has less atmic kinetic energy when cold, but when placed in a room, it has minus kinetic atomic energy. It will disippate the cold into the room, or you could say that it draws heat from the room. In the fridge, you need to use energy (fridge engine) to reduce the atomic kinetic energy... so you are adding energy to reduce energy, which is in turn something from which you can harvest energy again.
So, something that is colder has as much of an energy difference with the surrounding as something that is hotter. Like a rubber band, it doesn't matter which sign the vector has, it still has a length.

What on earth is "Atomic kinetic energy"? Since the term atomic energy is a bit outdated I can't understand what you're saying. Kinetic energy is energy as a result of an object in motion. Are you referring to kinetic energy of the air molecules?

Bog
07-21-2006, 10:07 AM
What on earth is "Atomic kinetic energy"?

The vibrational energy of heated atoms, I guess.

spec24
07-21-2006, 10:20 AM
The vibrational energy of heated atoms, I guess.

I guess. Must be talking about the Kinetic Theory of Atoms.

mjcrawford
07-21-2006, 10:37 AM
Ok.... if you are traveling on a conveyor that is moving the speed of light in an airplane that has tank treads instead of wheels while sippin' yer coffee and you turn you head-lights on... What would your Lightwave renders smell like?

Digital Hermit

Well, being a newbie, my renders would smell like crap eather way! :lol: but it would be a interesting scene to do!

lets see.. we need a conveyor, a airplane.. some tank tracks... yeah.. this could work... hehe :D

Anti-Distinctly
07-21-2006, 04:05 PM
It will disippate the cold into the room, or you could say that it draws heat from the room

Heat always flows from a hotter body to a colder body. It's the definition of heat transfer. the zeroth law of therodynamics if I recall.

I'm not sure I understood your post completely. Are you just saying that there is a relative difference in internal energy? If so, this is not correct. The environment that all your milk (or whatever) molecules are placed in doesnt matter, only the relative positions of the molecules to each other.

I think what you and iconoclasty mean is that if the milk is kept in the fridge, then you expend an external source of energy keeping it cool. Thus making the comparison unfair, as the coffee has no similar energy input to alter its temperature.

iconoclasty
07-22-2006, 09:41 AM
I wasn't really trying to say that. I was just being a jerk and pointing out a tiny mis-use of verbage.

Anti-Distinctly
07-22-2006, 11:30 AM
I wasn't really trying to say that. I was just being a jerk and pointing out a tiny mis-use of verbage.

Verlon
07-22-2006, 03:40 PM
All right, can't read all this......

So....if the plane is not moving through the air, it WILL NOT leave the ground. Wheel speed has nothing to do with flight (else retractable gear aircraft and sea planes would not work). If you flipped the conveyor ot launch the plane forward, it might take off, but it would need thrust to stay in the air. Also, the elevators are designed with prop wash in mind, and you'd need to correct for this on such planes.

On the coffee..... The coffee is getting cooler, but the milk is also getting warmer (but at a slower rate than the coffee is cooling).

How much coffee is in the cup? How much milk in the container? Real milk? Cream? Half and Half? How cold is the milk (33-40F?) How hot is the coffee (I have seen WIDELY varied advice on this....140-195F for the perfect cup). What is room temperature in this case? All of these things factor in and can lead to different results. How long are you at the door?

In the end, leave the milk in the fridge and answer the door, it could be the neighbor wanting you help her put some tanning oil on her back, and you can always reheat the coffee in the microwave if you're gone to long.

On the rocketship at the speed of light and turning on the headlights.

If you are IN the rocketship and turn on the headlights, you will see the headlights come on normally because the the speed of light is constant for all observers. To you in the space ship, light must travel at the speed of light (299978 km/sec). Now to me, sitting at the space stop sign as you come zipping by, what I see is a little different.

I obviously can't see the light from your headlights leave your ship at nearly 600,000 km/sec, because light MUST travel at the same speed for all observers, so what gives? Time does. The light WOULD leave you ship, but time had stopped for you (you also take up infinite mass now, but that's another problem....we'll assume you've licked that one with dilithium crystals or something).

If you were traveling at say 99% of the speed of light, time would be moving so slowly for you and those on your ship, that light would barely outrun the ship (it would still be travelling at 300,000km /sec, but the seconds would come slower for you than me).

iconoclasty
07-22-2006, 08:45 PM
okay, i thought the plane was put to rest but lets review real quick. The wheels have no friction with their axils whatsoever (or at least very negligable). Otherwise that variable throws everything off. So with freely spinning wheels, the conveyor doesn't stop the plane from progressing forward from the propeller just like it normally would, so the plane takes off fine. The only way the conveyor would stop it from taking off is if no one oiled the landing gear and the wheels had a lot of friction. To put it another way, because of the free spinning wheels, the plane is essentially sitting on pure ice. Even if the ice is traveling backwards really fast, the propeller can still pull the plane over the ice with ease and, hence, take off. Everyone agree?

SpaceShip headlights: First off, we all know you can't travel at the speed of light because of the whole infinite mass, infinite energy paradigm. But, Verlon, are you telling me that just because I flicked on my headlights, my spaceship suddenly turns into a time machine? That's crazy talk. I don't care how many dilithium crystals you have onboard. J/K I'm aware of the variable time theory, no need to try and explain it again Verlon.

I think this is the longest post I've ever written.

Verlon
07-22-2006, 08:59 PM
oh...sorry..I quit reading after several posts....I thought you were trying to do something a bit different with the airplane.

And the time dialation isn't a theory. We know its true because of certain particles with absurdly short lifespans being produced in the sun and actually making it to earth. :devil:

oh, and if the the conveyor spins faster and faster to try to keep the plane from taking off (and assuming everything doesn't disintigrate from any kind of frriction or just spinning to fast), the gyroscopic effect of the wheels could reach a point where the wings were unable to lift them. Not that I am dying to work the math out, but if the conveyor keeps spinning faster and faster with no speed limit, this could happen.

iconoclasty
07-22-2006, 09:44 PM
but the vector of the gyroscopic motion of the wheels would be perpendicular to the lift of the plane wouldn't it (my verbage might not be exact). I could see it effecting the plane banking or what-not, but not lifting off. The gyroscopic effect influences rotation but not translation, correct?

jameswillmott
07-22-2006, 09:49 PM
The gyroscopic effect would prevent the wheels from changing their axes of spin (bicycle wheel on rotating chair experiment anyone?) , but I wouldn't have thought they'd prevent the plane taking off.

Verlon
07-22-2006, 11:08 PM
You're right.....was thinking about rotating the plane to take off and not being able to do so.

Still, who'd want to take off in a plane that couldn't make course corrections?

ThriJ
07-23-2006, 11:00 AM
Ok, here is a new one.:D

If you have some bubble gum and you inhaled some helium, then if you blow a bubble could it float up into the air?

Yes I am aware that I am:screwy:

Verlon
07-23-2006, 01:20 PM
Well...
If you put enough helium into the bubble, it could.

BUT, the helium would leak out pretty fast, and it would fall back down.

Then, is it more likely to land in your hair just as you want to impress a girl, or stick something embarrassing to the bottom of your shoe as you go on an important job interview?

iconoclasty
07-23-2006, 02:54 PM
That's kind of a biology question and I think your lungs would filter out most of the helium. Even if you could exhale pure helium, you'd have to blow a really big bubble to overcome the weight of the gum.

Verlon
07-23-2006, 03:54 PM
no, your lungs do not do anything to the helium. Its inert (why its popular for deep diving).

Skinner3D
07-23-2006, 05:26 PM
:D This questions assume that you don't inhale the gum while inhaling the helium.

I agree with the others that you would have to blow a lot of helium into the gum, but gum is really weak and it would probably pop before you ever got enough helium into it.

Conclusion it would not float.:D

Anti-Distinctly
07-24-2006, 01:20 AM
Forces on the gum = Bouyancy of the gum/helium combo - weight of the combo.

The bubble gum will float if the average density of the gum/helium combo is less than that of the air it has displaced. I would have done the numbers, but I couldnt find the density of bubble gum.

Assuming a shell of gum, inner radius r_1 outer radius r_2, around a sphere of helium, working out the volumes and cancelling out (4/3)*PI() terms we have;
(r_2^3 - r_1^3) * density_gum + r_1^3 * density_helium < r_2^3 * density_air

Find the density of gum and solve that then you'll have your answer :)

Exception
07-24-2006, 01:30 AM
I guess. Must be talking about the Kinetic Theory of Atoms.

Yah, my first Language is not english, I was taught physics in Dutch.

Exception
07-24-2006, 01:35 AM
How much coffee is in the cup?

Doesn't matter.

How much milk in the container?

Doesn't matter.

Real milk? Cream? Half and Half?

Doesn't matter.

How cold is the milk (33-40F?)

As posted, colder than the coffee, above freezing. Otherwise doesn't matter.

How hot is the coffee (I have seen WIDELY varied advice on this....140-195F for the perfect cup).

Warmer than the milk, otherwise doesn;t matter.

What is room temperature in this case?
Doesn't really matter. but from what has been said, colder than the coffee. 'Room temperature' is an official term for 21 degrees Celcius I believe.

All of these things factor in and can lead to different results.
Not Really.

How long are you at the door?
Like I said, a reasonable time frame.

How long is this post?
Too long.
:)

When examples such as coffee, milk and door are given, one should assume normal figures for this... it makes no sense thinking the coffee would be colfer than the milk. It would have said 'ice coffee' then or if it was scorching hot 'mcdonalds coffee'. Ha.

CreepinJesus
07-24-2006, 02:37 AM
[QUOTE=ThriJ]
2: ( To Fly Or To Roll?:chicken: ) A plane is standing on runway that can move (some sort of band conveyer). This conveyer has a control system that tracks the tire speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction). So the question is will the plane take off or not?
QUOTE]

Yes it will, but only if it's propelled by a jet engine/propellor...which is fairly standard amongst planes! It will be moving because the engines 'thrust' against the air, which is not moving with the conveyor.
If the plane was putting its power down through it's wheels (like a flying car or something :)) then it would not be moving because the conveyor would be countering the movement.

Exception
07-24-2006, 02:39 AM
Ok, here is a new one.:D

If you have some bubble gum and you inhaled some helium, then if you blow a bubble could it float up into the air?

In ideal circumstances, I suppose you could, but I agree with the rest here in that the buublegum would break or be too heavy for the helium to carry.

Marvin Miller
07-24-2006, 05:14 PM

Coffe & Milk question: http://brainyplanet.com/index.php/Milk%20and%20Coffee%20Solution?PHPSESSID=d799d1707 bf6337c0547a1d6d275c326

Airplane question: My answer, watch the first part in Indiana Jones: Raiders of the Lost Ark. As Indy makes his escape from the natives, the plane takes off from the water, so there is a lot of friction slowing it down, (and in my opinion, negates most of the questions on whether or not the wheel is free of friction or not) it's the propeler pulling (or pushing, depending on your point of view) the air that moves it forward.

Doggie in the window question: Only Chuck Norris can answer that. :D

ThriJ
09-04-2006, 01:53 PM
The helium bubble gum question is solved by an extensive widely publicized scientific study done by Old Navy, and the results are quite uplifting.

Sorry, every time I see that ridiculous commercial it reminds me of this thread.:D :screwy:

And thanks for the link Marvin.