A railroad car of mass m and speed v collides and sticks to an identical railroad car that is initially at rest. After the collision, the kinetic energy of the system
A) is the same as before.
B) is half as much as before.
C) is one third as much as before.
D) is one fourth as much as before.
E) is one quarter as much as before.

Question

Quizplus · Accepted Answer

The initial kinetic energy is (1/2)mv^2. After the collision, the combined mass is 2m and the velocity is v/2, so the kinetic energy is (1/2)(2m)(v/2)^2 = (1/2)mv^2/2, which is half the initial kinetic energy.

A Railroad Car of Mass M and Speed V Collides