A railway car of mass m moving at a speed v collides with a stationary car of mass 2 m

A railway car of mass m moving at a speed v collides with a stationary car of mass 2 m and is coupled with it. What is the total momentum of two cars after a collision?

Given:
m is the mass of the first car;
v is the speed of the first car;
2m is the mass of the second car.
Decision:
Based on the condition, we conclude that no external forces act on the carriage system in the horizontal direction.
In this system, momentum is preserved. This means that the total impulse of the two cars after the collision will be equal to the total impulse of this system before the collision. This is expressed by multiplying the mass of the first car by the speed, that is, mv.
Answer: mv