Question

A 5,000 kg train is traveling at a velocity of 100 m/s and hits another train. The two trains stick together, and the new velocity is 50 m/s. What is the mass of the second train?

Answer 1

We assume the second train was standing still and that momentum is conserved.

Then the product of mass and velocity before the collision is

(5000 kg)·(100 m/s) = 500,000 kg·m/s.

After the collision, where M is the mass of the second train, the momentum is

((5000+M) kg)·(50 m/s) = 500,000 kg·m/s

Dividing by 50 m/s and subtracting 5000 kg, we have

(5000 +M) kg = 10,000 kg

M kg = 5000 kg

The mass of the second train is 5000 kg