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Homework Help: Double Ball Bounce question!

  1. May 18, 2012 #1
    1. The problem statement, all variables and given/known data

    4. The double-ball bounce
    Consider 2 balls on top of another that are being dropped from a height h = 2 m,
    see the Figure below. They will hit the ground and bounce back and we want to
    compute the velocity of the upper ball after the bounce.
    (a) (b)
    We can model this as a sequence of two collisions. Let us assume that the upper ball
    has a mass of 100 g, and that the lower ball has a mass of 10 kg. We rst consider
    the collision between the lower ball and the ground.
    (a) What is the velocity of the balls just before the lower ball hits the ground?

    (b) Assume that the collision between the ball and the ground is perfectly elastic,
    what is the velocity of the lower ball just after it hits the ground?
    Now let us consider the collision of the upper and the lower ball.
    (c) Assume that the collision is elastic with a coecient of restitution e = 0:9.
    Compute the velocity of the upper ball after the collision.
    (d) What is the maximum height the upper ball reaches after the collision?

    3. The attempt at a solution

    For this question for part a where two balls are falling and you work out the velocity, because its on top to i count it as one system? or do i calculate the velocities individually?
    Just need to clarify this so i can do it correctly :)
  2. jcsd
  3. May 18, 2012 #2
    Just realised they will both have same velocity.. lol
    and velocity will be about 2.8 m/s i believe
  4. May 18, 2012 #3
    Ok so far i have velocity of balls before hitting the ground using d = 1/2at^2 + Vit
    this works out as 2= 5t^2
    therefor t = 0.28 seconds
    And then using V = a.t i get V at 2.8 seconds (g = 10m/s^2) for this example

    Then for velocity after collision i find the Ep (potential energy) = m.g.h = 200
    And then use that for Ek = 1/2mv^2
    which means 200 = 1/2 10. V^2
    so V^2 = 200/5
    V = Sqaure root of 40
    = 6.325 m/s
    Is that correct so far?
  5. May 18, 2012 #4


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    Homework Helper

    This problem is more complicated in the real world. One issue is if the upper ball bounces off the lower ball before, at the same time, or after the lower ball bounces off the ground. You're probably supposed to assume that the upper ball bounces off the lower ball at the same time the lower ball bounces off the ground. The upper ball is going to end up with much more velocity than the lower ball, and may end up bouncing higher than the initial height it was dropped from, since some of the energy is transfered from the lower ball to the upper ball during a bounce.
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