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Homework Help: Pool ball collision, kinetic energy

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data
    I cooked up an example of a collision between 2 pool balls: one comes in from the right at 2.5 m/s and hits a stationary one. The original goes off at 30° up, 1m/s. What does the lower one do? I calculated the lower one to go down -17°, 1.71 m/s using conservation of momentum/ simultaneous equations. Then I calculated kinetic energies to be .531 before and .334 after (mass of pool balls assumed at .17kg)

    My question is about elasticity. I didn't make any assumptions about it, so I expected to get a perfectly elastic collision (kinetic energy preserved). I've also read that pool balls are mostly elastic. Does this mean that my problem setup assumes an inelastic collision? Would it be impossible to get a pool ball collision like this in real life?

    2. Relevant equations
    pstart = pfinal
    KE = 1/2MV2

    3. The attempt at a solution
  2. jcsd
  3. Oct 14, 2008 #2


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    Homework Helper

    Hi krausr79,

    A perfectly elastic collision is a special case, so you would need to constrain your problem for it to be elastic.


    If these identical balls were assumed to be particles colliding elastically (with one initially at rest), the angle between the final velocities would be ninety degrees; this is very different from the 47 degrees that you found in your problem.
  4. Oct 14, 2008 #3
    Ok, thanks.
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