Block A, with a mass of 2.0 kg, moves along the x axis with a velocity of 5.0 m/s in the positive x direction. It suffers an elastic collision with block B, initially at rest, and the blocks leave the collision along the x axis. If B is much more massive than A, the velocity of A after the collision is:
A) 0
B) +5.0 m/s
C) -5.0 m/s
D) +10 m/s
E) -10 m/s

Question

Quizplus · Accepted Answer

Since the collision is elastic, momentum is conserved. Before the collision, the momentum of block A is 2.0 kg * 5.0 m/s = 10 kg m/s in the positive x direction. After the collision, if block A comes to rest, the momentum of the system would be zero. Therefore, block B must move with a momentum of 10 kg m/s in the negative x direction. Since block B is much more massive than block A, its velocity will be much smaller than 5.0 m/s, and block A will move in the negative x direction with a velocity of -5.0 m/s. Thus, the correct answer is C) -5.0 m/s.

Block A, with a Mass of 2