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Elastic Collosions- help please?

  1. Mar 30, 2008 #1
    [SOLVED]Elastic Collosions- help please?

    1. A tennis ball of mass 57g is held just above a basketball of mass 590g. With their centers aligned, both are relesed from rest at the same moment, to fall 1.2m. a)Find the magnitude of the downward velocity of the basketball as it reaches the ground. Assume an elastic collision with the ground instantaneously reverses the velocity of the basketball while the tennis ball is still moving down. next, the two balls meet in an elastic collision. b) To what height does the tennis ball rebound?

    2.vf^2=vi^2 + 2*a(xf-xi) Conservation of energy equation

    3. The attempt at a solution
    I have already solved for part a using the kinematic equation above. It comes out at 4.85m/s. Next, I tried to use the consevation of energy equation to solve for a height, but I got stuck when trying to figure out how to account for both the basketball and the tennis ball masses. The teacher gave us a hint by saying to find Vf of the tennis ball and basketball and then use the conservation of energy equation, but I can't figure out how to get a different Vf (instead of the 4.85 m/s) for either of the masses. Any help or hints on where to get started with this problem would be most appreciated.
    Thank you!
    Last edited: Mar 30, 2008
  2. jcsd
  3. Mar 30, 2008 #2
    Before the final collision, the tennisball is going down with 4.85 m/s and the basketball is going up with that speed. You know both the initial kinetic energy and the initial momentum before this collision. Both momentum and kinetic energy must be conserved.
    If the balls both have a speed of 4.85 m/s after this collision, this will conserve energy, but not momentum.
  4. Mar 30, 2008 #3
    That was a perfect hint! I've figured it out now. I just didn't have a sign in the right place so it screwed up the rest of the problem. Thank you so much!!!
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