A 0.4-kg toy train car moving forward at 3 ms collides with and sticks to a 0.8 and ndashkg toy car that is traveling in the opposite direction at and ndash2 ms. What is the size and direction of the?

The total momentum before the collision is 0.4 kg * 3 m/s - 0.8 kg * (-2 m/s) = 2.4 kgm/s - (-1.6 kgm/s) = 4 kgm/s. After the collision, the two cars stick together, so their combined mass is 0.4 kg + 0.8 kg = 1.2 kg. Therefore, the final velocity is 4 kgm/s / 1.2 kg = 3.3 m/s. The direction is forward.