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Homework Help: Figuring out formula from graph

  1. Jul 24, 2007 #1
    Hi, i'm not sure if this is the best forum to post this question but i'm gonna give it a try.
    I would like to be able to figure out a formula from a given graph( http://img46.imageshack.us/img46/8172/graphun4.jpg [Broken] ). basically that would be the motion of a bouncing ball. Ultimately what i would like to do is figure out how to simulate those kind of motions into flash via action script to build bouncing windows, drop down menues etc.
    I guess i can go and do some research on snippets and copy past but i'd rather try to understand it how it works :P.
    thank you in advance.
     
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jul 24, 2007 #2
    MOST functions can't be easily defined in a formula.

    I'd make it a piecewise function. The first "hump" will be a kinematic equation i'm sure, then after you analyze the collision with the ground you can keep using kinematics.
     
  4. Jul 25, 2007 #3

    mgb_phys

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    You can always fit a curve with N 'humps' with a polynomial with N terms
    ( if you're an astronomer you can fit 1 hump with N terms ;-)

    For 'real world' objects you could look at Fourier series.

    For a simple bouncing simulation I would just make each bounce x% smaller than the previous, adjust x to get the degree of damping you want. This prettry much models the real energy loss in a bouncing ball.
     
  5. Jul 25, 2007 #4
    ohh the graph doesn't represent the trajectory of a bouncing ball, it represents height vs time. how high would the ball be at x seconds. sorry :P.
    basically what i need is a simple formula, for example if i want to make a point a de-acceleration, i'll just do:
    1.dist= final_x-current_x;
    2.acel = .7;
    3.next_x = current_x+(dist*acel)
    4.repeat step 3 until dist<=0;

    basically the object will move "dist" many units every tick(second).

    I'm looking for a similar formula to simulate bouncing. The formula should (lets say y=100 and a tick of 5 seconds) make y=0 after 5 seconds, y=60 after 5 more seconds; than back to 0 after 5 more secs, y=40,y=0,y=20,y=0, y=7,y=0,y=1,y=0,y=0 and than y stays constant at 0.
    Now i understand the concept is just that i don't have the knowledge to come up with a formula. I'm not sure if i'm making myself clear.
     
  6. Jul 25, 2007 #5

    mgb_phys

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    The simplest way to get something that looks like a bouncing ball would be to use y=sin(x)^2.
    Not quite the correct shape ( a parabola ) but unless you are writing physics simulations it will look close enough. You can reduce Y on each round by a factor if you want to loose energy.
     
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