A railroad tank car contains milk and rolls at a constant speed along a level track. The milk begins to leak out the bottom. The car then

(a) slows down.

(b) speeds up.

(c) maintains a constant speed.

(d) Need more information about the rate of the leak.

The railroad tank has milk stored in it. It rolls on the level track and the milk starts leaking. The railroad tank car's momentum is the product of the car's mass and the velocity of the car. The milk drains from the car, so its mass decreases.

Thus, the car has to make some changes in its speed to conserve momentum as the momentum remains conserved since no external force acts in the horizontal direction.

The milk starts draining from the car, and it falls in the downward direction vertically. This means no force acts in the horizontal direction of the car. The railroad tank car rolls at constant speed.

From the relation of momentum, the car's mass starts decreasing; thus, the car’s momentum tries to decrease in the same way.

The momentum of the railroad tank car has to be conserved as no external force acts in the horizontal direction; so the railroad tank car’s speed should be increased.

Thus, the railroad tank car should speed up. Therefore, option (b) is correct.