Momentum is a physical, mechanical size, which contains the speed and mass of the moving body, thus creating one single quantity. The quantity thus formed does not change unless there are some other effects that will change the momentum, which is not the case in the given example. This is called preservation of momentum and is useful in collision analysis. When a collision occurs, the momentum affects the resting body by giving it the same quantity of momentum. For example, if you have one apple in your pocket, you want to get another apple from a tree that is high, so you can not reach it by hand. If you take an apple from your pocket, throw it, and hit an apple on the tree, it will fall because of the momentum that has the apple you have thrown, giving this momentum apple on the tree, which is why the apple from the tree will fall.
Thus, the total momentum of both balls after the collision is 50 kgm/s.
