A 10,000 kg railroad car, A, traveling at a speed of 24.0 ms⁻¹ strikes an identical car, B, at rest. If the cars lock together as a result of the…

The cars lock together, so this is a perfectly inelastic collision. Momentum is conserved, but kinetic energy is not.

Momentum before = 10,000 × 24.0 + 10,000 × 0 = 240,000 kg m/s.

After the collision both cars (20,000 kg) move together: 20,000 × v = 240,000.

v = 12.0 ms⁻¹. The mass doubles, so the speed halves.