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**1. Homework Statement**

A 0.25kg tennis ball is placed right on top of a 1kg volleyball and dropped. Both balls hit the ground at a speed of 3 m/s simultaneously. Find the (upward) velocity of the tennis ball right after it bounces up from the volleyball. Assume elastic collisions. (HINT: the tennis ball will move faster than 3m/s)

**2. Homework Equations**

mass of tennis ball=0.25kg

mass of volleyball = 1kg

1) Va'=Va(m1-m2)/(m1+m2)

2) Vb'=Va(2m1/(m1+m2)

**3. The Attempt at a Solution**

Someone told me that when the two balls hit the ground, the volleyball reverses direction...so -3m/s? Plus it's an elastic collision, so kinetic energy and momentum are both conserved. But initial velocity of volleyball is not zero, so i can't use eq'ns 1 or 2 above? If i set the velocities of the balls to the reference frame of the tennis so that the velocity of the volleyball is zero will that work out?

Also, i'm not really sure why only the volleyball reverses direction, if both of them are simulataneously released then shouldn't they hit the ground at the same time? and why does the volleyball rebound when the collision occurs? shouldn't all the kinetic energy of the volleyball be transferred to the tennis ball since the volleyball is sandwiched between the earth and tennisball so that it's velocity is zero?