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Thread: Projectile Motion

  1. #1
    Senior Member
    Join Date
    Feb 2004

    Projectile Motion

    I'm taking general physics this semester and try to put whatever i learn
    there into flash with some succsses, but i'm not really pleased with the effect from the projectile motion.
    the formulas i'm using right now are
    where vel is v0 and a negative angle from 90 to 0;
    ball._y =(ball._y)+(vy * t) + (0.5*g*t*t);
    ball._x= (ball._x)+vx*t;
    where g=9.8 and t is an interval of time starting with 1 and incremeting by 0.2

    it makes some kind of a trajectory but well it's not perfect, it's suppost to take the same time to come down as it takes to go up and i don't think it does. Have a look on the file bellow and any idea, suggestion, correction would be great

    ps: i know there is a physic section but i like ppls opinion better in this one. I hope thats ok :P
    Last edited by AluminX; 01-30-2008 at 11:27 PM.

  2. #2
    skylogic.ca MD004's Avatar
    Join Date
    Oct 2003
    Quote Originally Posted by AluminX
    ball._y =(ball._y)+(vy * t) + (0.5*g*t*t);
    ball._x= (ball._x)+vx*t;
    where g=9.8 and t is an interval of time starting with 1 and incremeting by 0.2
    You've almost got it, the only problem is that in the formulas you're using,
    d = v0(t) + (1/2)(a)(t^2)
    d = vt
    'd' is absolute, not relative. You're adding each 'd' (ball._y/x) to the previous one, so just take away the ball._y and ball._x that you are adding in there.

    Here's a little experiment i did to make sure what i'm telling you is true:
    var v0 = -100; //initial y velocity
    var vx = 20; //constant x velocity
    var a = 9.8;
    var t = 0;
    this.onEnterFrame = function () {
    	ball._y = Stage.height + v0*t + 0.5*a*t*t; //Stage.height makes it start at bottom instead of at top (0)
    	ball._x = vx*t;
    Your first example, with the vx and vy, is correct if you want to convert a velocity into its x and y components, but changing the angle every frame would probably be just plain weird

    btw, I'm taking Physics right now as well.


  3. #3
    Senior Member webgeek's Avatar
    Join Date
    Sep 2000
    This is a little time based animation example I created and it includes full and accurate gravity as well:


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