A table tennis ball moving forward with 5 units of momentum strikes and bounces backward off a heavy bowling ball that is initially at rest and free to move. The bowling ball is set in motion with a momentum of
A. less than 5 units
B. 5 units
C. more than 5 units
D. not enough information
The quantity that is conserved is not the absolute momentum but the vectorial momentum. Initially, the total momentum of the system is +5. After the interaction, the table tennis ball bounces back, so it's momentum is negative (speed in the negative x direction). Hence, for the total momentum to be 5, the momentum of the bowling ball has to be greater than unity.