A Ball Bearing Rolls Along a Smooth Horizontal Tabletop and Crashes

Multiple Choice

A ball bearing rolls along a smooth horizontal tabletop and crashes head-on into a second, stationary ball bearing of half its mass. If both ball bearings roll in the x-direction only (either positive or negative) and the initially stationary ball is observed to have a velocity of +2.4 m/s after the collision, what was the initial speed of the first ball bearing? Assume the collision is elastic.

A) More information is required
B) 7.2 m/s
C) 3.6 m/s
D) 1.8 m/s