View Full Version : Hmmm...I'm ignorant, please help

harlan

07-30-2004, 05:46 PM

I'm trying to launch a rocket based on math, but my calculations on paper yield far different results than those generated with Lightwave.

Mathematically speaking, the rocket should reach roughly 400' at five seconds into it's flight, but it never works out quite right.

Anyone have an idea? Here's the math I'm using:

distance: y = (1/2)gt**2 ; g = 9.8meters/sec**2

y = (1/2)(9.8meters/sec**2)(5sec)**2

y = (4.9meters/sec**2)(25sec**2) = (4.9meters)(25)

y = 122.5 meters = 401.9 feet 6

velocity: v = gt = (9.8meters/sec**2)(5 sec) = 49meters/sec = 160.8 feet/sec = 109.6miles/hour

God, I knew I should've friggin went to college!! ;)

harlan

07-30-2004, 05:58 PM

Oh, this is based on a 10 second cycle (launch - rte) if that helps at all.

kmscottmoore

07-30-2004, 06:46 PM

Harlan,

You've got me totally confused. I think that you're trying to figure escape velocity. Here's a link that explains it:

http://www.physlink.com/Education/AskExperts/ae158.cfm

What do your variables represent?

I got that gravity = 9.8m/s(2)

(I'm guessing that **2 means squared)

The rest, I just don't understand where you're going.

V= Velocity

V=Distance/Time

Gravity will be a negative force, at the rate of -9.8m/s(2); so it squares every second. After 5 seconds, the cumulatve force of gravity is -90392 meters. Your rocket would have to be traveling at least 90392m/second to escape the earth's gravitational pull.

Dodgy

07-30-2004, 07:19 PM

Do you want it to reach 400' after five seconds? Are you accelerating constantly?

What exactly are you trying to achieve?

if it's launch then return to earth in 10 seconds, surely at 5 seconds the velocity should be 0 as it hits its peak?

harlan

07-30-2004, 08:36 PM

Exactly, it should be at a velocity of 0, 5 seconds into its flight, and it should also be at roughly 400'. Which is what you get as a result from that calculation I posted earlier. The problem is, that when I try to replicate that data via an expression in LW it dramatically overshoots its mark.

What I'm trying to say is that, if I plug in the data in LW to achieve a launch through RTE over the course of 10 seconds, it always ends up dramatically higher in altitude than 400' @ peak.

This is as close as I've gotten, but it's not 100% accurate when compared to the other math.

-9.871*Time*Time/2 + 49.000*Time + 0.000

So my question is: What would you guys use as an expression in lightwave to replicate this flight. I'm curious to know how you reach the figure to plug into the inital velocity channel in the expression.

I'm not sure if I'm making any sense here at all, but it's really not that important...I'm just screwing around.

mattclary

07-30-2004, 09:24 PM

OK, non-scientific, non-physics guy responding here, so take it for what it's worth.

In reality, wouldn't the rocket engine accelerate it more than earth's gravity? Meaning it should reach apogee BEFORE 5 seconds and take longer to fall back to earth if there is a total of 10 seconds from liftoff to return?

I know this isn't "necessarily" true, but is "probably" true.

kmscottmoore

07-31-2004, 12:15 AM

Matt,

Actually, technically, the answer to your question is no. Strictly mathematically speaking:

The force of gravity is constant, so by the time the rocket runs out of energy, i.e. velocity=0, due to the constant negative force of gravity, the distance that it has to travel back is exactly the same as the distance that it traveled up. The speed that it hits the ground will be the same speed that it took off with.

For you sticklers out there, reality is a bit more complicated than that, because of air and the resistance that it causes. So, if you measured it, the trip down probably would take longer than the trip up. Plus, if the rocket encountered any turbulance on the way down, it would spin in crazy directions, and make the trip down longer than the trip up.

kmscottmoore

07-31-2004, 12:19 AM

Harlan,

Not to be a smart ***, I know you're trying to figure out expressions, but if I were under a deadline, my answer would be:

Y=0; Keyframe 0

Y=400'; Keyframe 150

Y=0; Keyframe 300

:cool:

harlan

07-31-2004, 12:48 AM

LOL!!! I know I could do that, but that would sure take the fun out of it now wouldn't it? ;)

The basis of this stupid thing, is just an exercise to screw my head back on. Sometimes I get stuck in a zone, and the only way out is to challenge myself with something. This is actually one very tiny fragment of a much larger experiment, but the math just doesn't seem to be working out quite right. I've gotten it close enough for govt. work, but I'd like to learn a way to calculate it out exactly and have LW mimic it precisely, ya know. ;)

The goal was just to make a little game where you shoot bottle rockets at the "opposing side" in LW powered solely via expressions, but I got sidetracked.

omeone

07-31-2004, 03:33 AM

sounds like the makings of a very cool L-script :)

adrian

07-31-2004, 12:32 PM

Originally posted by harlan

[B]LOL!!! I know I could do that, but that would sure take the fun out of it now wouldn't it? ;)

What the hell is fun about maths??!!

You guys might as well be talking Martian as far as I'm concerned. I was lost by the end of the first sentence... ;-)

Good luck with your anim tho....

Adrian.

John Fornasar

07-31-2004, 10:00 PM

Harlan,

It's been a long time, but if you dig out a copy of MS-DOS 4 or 5, the Gorilla game should have the math you need (it was the game where gorillas threw exploding bananas at each other).

You can read the code in Notepad, or better yet, step through it in the supplied Basic editor.

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