Question

A freight car moves along a frictionless level railroad track at constant speed. The freight car is open on the top. A large load of sand is suddenly dumped into the freight car. What happens to the speed of the freight car?
A) The speed of the freight car remains the same.
B) The speed of the freight car increases.
C) The speed of the freight car decreases.
D) The speed of the freight car cannot be determined from the information given.

Answer 1

Answer:

C) The speed of the freight car decreases.

Explanation:

The speed of the freight car decreases, because the sand load increases the mass of the system.

Using the conservation of linear momentum

$m_{1}v_{1}=m_{2}v_{2}$

The linear momentum m*v is an amount that must be preserved, so this equality must be satisfied before and after the sand is added to the freight car.

Subindices 1 are for before the sand is added, and subindices 2 for after the sand is added.

In this case the mass in the second moment $m_{2}$ is greater due to the mass added by the sand than the original mass $m_{1}$, so to maintain equality the speed in this situation must decrease.