The Law of Conservation of Momentum
When a moving object hits another object, some or all of the momentum of the of the first object if transferred to the object that is the that got hit. If only some of the momentum is transferred, the rest of the momentum stays with the first object. A example is that a cue ball hits a billiard ball so that the billiard ball starts moving and the cue ball stops, because the momentum was transferred. This is an example of the Law of Conservation of Momentum. This law applies whether the objects stick together or bounce off each other after they collide. The billiard ball and the cue ball are also an example of an action and reaction force.
Objects Sticking Together
Sometimes after a collision the objects stick together. Some examples of this is a dog leaping or catching a ball and a teen jumping on a skateboard or when football players tackle another player. After two objects stick together, they move as one object. The mass of the combined objects is equal to the masses of the two objects added together. In a head on collision, the combined objects move in the direction of the object that had the greater momentum. When the objects have a different velocity because momentum is conserved and depends on mass and velocity. So, when mass changes, the velocity must change, too.
Objects Bouncing off Each Other
In some collisions, the objects bounce off each other. The bowling ball and bowling pins are an example of objects bouncing off each other after the collision. During this type of collision, the momentum is usually transferred from one object to another object. The transfer of momentum causes the objects to move in different directions at different speeds.