Newton's Laws of Motion.
The second law is the sum of the forces equal to the mass and acceleration of the system.
The third law is for every action there is an opposite or equal reaction. Usually refers to colliding objects but still uses the 2nd law. The first law is that an object at rest tends to stay at rest and an object in motion stays in motion unless you apply a force to change its motion. This is the law of inertia.
When you bounce a ball against the floor, the floor applies a force, about the same amount when you hit it against the floor, this is a Normal force. When you collide with the floor, the action gets an opposite reaction against, making an elastic collision.