Question

Ball 1, with a mass of 10kg, is moving to the right at 20 m/s. Ball 2 is moving to the left at 20m/s. Upon collision, ball 2 comes to a standstill, and ball 1 moves to the left at twice its original speed. What is the mass of ball 2? The mass of ball 2 is ?????…..

Answer 1

Use Conservation of Momentum. Write p1 for the momentum of the first ball, p2 for the second; write p1' and p2' for the momentum afterwards. The by Conservation of Momentum:

p1 + p2 = p1' + p2'

We know that p2' = 0, because p2' = m2.v2' and v2' = 0 because the ball is still.

We also know that
p1 = m1.v1
= 10(20)
= 200

and

p2 = m2.v2
= 20.m2

Further, because the first ball goes off at twice its original speed, v2' = 40 m/s and hence
p2' = 10(40) = 400

Hence, bringing all of that together and using Conservation of Momentum,

p1 + p2 = p1' + p2'
200 + 20.m2 = 400 + 0

Now solve for m2.