Question

A 35-gram stainless steel ball on a track is moving at a velocity of 9 m/s. On the same track, a 75-gram stainless steel ball is moving at a velocity of –7 m/s. After colliding, the 35-gram ball moves at a velocity of –15 m/s. Assume there is no net force on the system.

What is the velocity of the 75-gram ball after the collision? Round to the nearest tenth.

–4.2 m/s
–0.5 m/s
0.5 m/s
4.2 m/s

Answer 1

Use conservation of momentum: m1v1 + m2v2 = m1v1' + m2v2'.

0.035(9) + 0.075(−7) = 0.035(−15) + 0.075 v2'

0.315 − 0.525 = −0.525 + 0.075 v2' → 0.075 v2' = 0.315

v2' = 0.315 / 0.075 = 4.2 m/s

Answer: 4.2 m/s.