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Homework Help: Elastic Collision or Inelastic Collision? Two balls colliding.

  1. Mar 21, 2012 #1
    1. The problem statement, all variables and given/known data
    A ball of mass 4.5 kg is moving 16 m/s east when it collides with a stationary 6.2kg ball, which moves at a velocity 10 m/s after the collision. Was this an elastic or inelastic collision? Explain your choice.

    Va = 16 m/s
    Ma = 4.5 kg
    Mb = 6.2 kg
    Vb' = 10 m/s

    2. Relevant equations
    MaVa + MbVb = MaVa' + MbVb'

    3. The attempt at a solution
    Let east be positive.

    MaVa + MbVb = MaVa' + MbVb'
    4.5(16) + 0 = 4.5Va' + 6.2(10)
    72 = 4.5Va' + 62
    10 = 4.5Va'
    2.22 = Va'

    From doing the above I would say it is an elastic collision. However, from what the answers the teacher gave us it is supposed to be an inelastic collision, can anyone tell me how this is so?
  2. jcsd
  3. Mar 21, 2012 #2


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    Staff: Mentor

    What quantities are conserved through a perfectly elastic collision? How about inelastic collisions? Have you confirmed each property?
  4. Mar 21, 2012 #3
    Remember, coefficient of restitution is what tells you if the collision is elastic or completely in elastic or midway :-)
    Use it.

    Its given by e=-(v2-v1)/(u2-u1)

    Where v1 and v2 are respectively the final velocity vector (along the line of collision) of objects 1 and 2 and u1 and u2 are respectively the initial velocity vectors(along the line of impact or collision) of objects 1 and 2.

    Remember, the velocity vectors in this equation are taken along the line of collision.

    e=1 signifies perfectly elastic collision and e=0 signifies perfectly inelastic collision.



    Hope this helps :-)

    Note :Don't know why the link is not working. You can google for the wiki page on coefficient of restitution.
    Last edited: Mar 21, 2012
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