Whenever one object exerts a force on another object, the second object exerts an equal and opposite force on the first. Forces always come in pairs and result from interactions. A jump pushes the ground down and the ground pushes the jumper up. A balloon pushes air out one way and the air pushes the balloon the other way. The two forces in a pair act on different objects, so they do not cancel. This is Newton's third law, and it is the key to every collision.