An empty railroad car traveling at a constant speed strikes a second car at rest.The second car,being full with passengers,has twice the mass of the first car.If the cars lock together as a result of the collision,the ratio of the initial to the final kinetic energies of the system is
A)1.
B)1/3.
C)3.
D)1/9.
E)9.

Question

Quizplus · Accepted Answer

The initial kinetic energy is due to the empty car only. After the collision, the combined mass is three times the initial mass (since the second car is twice the mass of the first). By conservation of momentum, the final velocity is one-third of the initial velocity, leading to a final kinetic energy that is 1/3 of the initial kinetic energy. Thus, the ratio of initial to final kinetic energy is 3.

An Empty Railroad Car Traveling at a Constant Speed Strikes