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Homework Help: An off center collision

  1. Oct 31, 2004 #1
    A ball moving at 11 m/s makes an off-center elastic collision with another ball of equal mass that is initially at rest. The incoming ball is deflected at an angle of 30° from its original direction of motion. Find the velocity of each ball after the collision.

    Ball moving initally at 11 m/s
    (a) magnitude m/s (moving ball)?
    (b) direction ° (from the original direction of motion)?

    Ball initally at rest
    (c) magnitude m/s?
    (d) direction ° (from the original direction of motion)?

    How do I start this problem? Someone in my recitation had a problem similar to this and used three equations with three unknowns. I think it was conservation of KE, conservation of momentum, and something else but i dont remember.

    My book briefly talks about this, but there are no sample problems to work with.
  2. jcsd
  3. Oct 31, 2004 #2
    You have three equations: One conservation of energy and two for conservation of momentum (1 for the x direction and 1 for the y direction). Use the components of the velocity in the x and y directions for the conservation of momentum in the x and y direction, respectively. For the conservation of energy you just use the magnitude of the velocities.
  4. Nov 2, 2004 #3
    I'm having trouble setting up these equations. For conservation of energy, I have the initial KE, but final KE I don't know what the final kinetic energies of each ball is. I think the incoming ball is (1/2)m(11 m/s)^2*sin (30) but the final kinetic energy of the ball initially at rest I don't know what that is.
    For the conservation of momentum, the initial momentum's are easy to calculate, but the final momentums are what I don't know (in both the x and y components) I have a feeling these type of questions will be on the exam coming up on Thurs. so I want to be able to understand how to do them.
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