Question

A 10,000kg railroad car is travelling at 1.5m/s when it strikes another 9,000kg railroad car that is at rest. If the cars lock together, what is the final speed of the two railroad cars?

Answer 1

The final speed can be found by momentum conservation principle:

$$Mv=(m+M)u,\\\space\\ u=\frac{Mv}{m+M}=0.79\text{ m/s}.$$