Issac Newton was the first to state the concepts that are necessary to understanding the physics of collisions. His three laws are used again and again in all the fields of physics:
Newton's 1st Law
In the absence of external forces, an object at rest remains at rest and an object in motion remains in motion with a constant velocity.
This law can be best observed in space, far from the gravity of a star or planet, where there is no friction or air resistance. If, in the middle of deep space, you give a rock a little push, it will continue with the direction and velocity you gave it forever. The only way to stop it is to apply a force in the opposite direction. This law is not intuitive because we are surrounded by air and gravity - if we give a rock a little push on the surface of the earth, it won't travel far.
Newton's 2nd Law
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
This boils down to force equals mass times acceleration, F = ma. This little equation turns out to be immensely useful, again and again. If you add together all the forces acting on an object, they equal the mass of the object (in kg) times the acceleration of the object (in m/sec^2). Force is measured in newtons. One newton is the force required to accelerate a 1-kg mass to 1 m/sec^2.
Newton’s 3rd Law
The force exerted by object 1 onto object 2 is equal in magnitude and opposite in direction to the force exerted by object 2 onto object 1.
This law appears to make little sense and can be difficult to grasp. If you push on a brick wall, the wall is pushing back on you with an equal force. If the wall was not pushing back, then your hand would be moving away from you. However, the wall is firmly attached to the ground, so it can match the force you are applying to it. Every single action has an equal and opposite reaction. If you were standing in the middle of a perfectly slick frozen lake, and needed to get off, you could throw an object away from you. When you applied force to the object you threw, it also applied an equal force pushing you back. You would travel in the opposite direction from the throw, although slower, because you probably have mass more than the object you threw.