Question

A runaway train car that has a mass of 15,000 kg travels at a speed of 5.4 m/s down a track. Compute the time required for a force of 1500 N to bring the car to rest.

Answer 1

The 1500 N force would need to act in the direction opposite the train car's movement, so if we take the current direction it's moving in to be positive, we have by Newton's second law

-1500 N = (15,000 kg) a → a = -0.10 m/s²

If t is the time it takes for the train to come to rest, then

-0.10 m/s² = (0 - 5.4 m/s) / t

t = (5.4 m/s) / (0.10 m/s²) → t = 54 s