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Homework Help: Elastic Collision, Equal Masses

  1. Oct 22, 2009 #1
    Okay, i have a problem here with 2 dimensional elastic collision between 2 balls with equal mass, they are both moving in differant directions before collision

    what i know:

    vx and vy of both balls,
    the slope of their movement, which can be converted to an angle with tan-1 x

    the problem is all ive got so far is momentum conservation with

    m1v1 + m2v2 = m1v1' + m2v2'

    but of course we have 2 unknown variables here v1' and v2'

    so how do i use slope or angle to get v1 or v2 and then the other is easy to solve for.


    EDIT: could i use impulse (J = Ft) and use a really small number of time and we know that J = p2 - p1

    EDIT2: and where could i get the F in impulse from? cant really use newtons second law of F = ma here because then i would need to know the acceleration of what it hit was which is essentially what finding J does for me... so anyone know of a better method??

    Last edited: Oct 22, 2009
  2. jcsd
  3. Oct 23, 2009 #2


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    Science Advisor


    - treat it as a 1 dimesional problem along the collision axis (connects centers of the balls). nothing changes perpedicular to it

    - m1 = m2 in your case

    - use the reference frame of one ball for calculation (one initial v becomes zero)

    - use google
    Last edited: Oct 23, 2009
  4. Oct 23, 2009 #3
    i have searched google, i can't get much that helps me or i don't understand what it means, if i make it a 1d problem along the collision axis, i guess i need to find how much momentum in that direction, using triangles i assume with x and y momentum, but what if both circles are moving the same direction and slightly differant angles, and differant speeds (obviously cause if they were same speed and direction they wouldn't collide) so it comes and hits it from behind, there is no momentum acting against the other ball,
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