Arailroad diesel engine weighs 4 times as much as a flatcar. if the engine coasts
at 5 km/h into a flatcar that is initially at rest, how fast do the two coast after
they couple together?

Explanation:

Linear Momentum

The total linear momentum of an isolated system (with no external forces) is conserved regardless of the internal mutual interactions of its parts. Recall the momentum is obtained by multiplying the speed by the object's mass. We'll refer to the railroad diesel engine as mass 2 and the flatcar as mass 1.

We have the following data

Since both objects remain coupled after their encounter, the final speed is common to both:

Let's sketch the principle of conservation of linear momentum as follows

Using the mentioned conditions for the speeds

Solving for v'

the answer would be   dignified - apex