Collision Processes

Definition: If (at least) two body move towards each other and if these two bodies interact for a short time, then, in physics, we call this a collision. What bodies collide is irrelevant: billard balls, rubber balls, bumper cars, even the tiniest particles. Every form of collision can be described and explained via the law of conservation of energy and momentum.

A collision is called elastic, if the sum of kinetic energy before and after the collision is the same. This means, in an ideal elastic collision no kinetic energy is transformed into warmth during the deformation. Nearly ideal elastic collisions happen if the bodies are very hard (like billard balls or marbles) because they can hardly be deformed. That's the reason why such bodies are often used to explain these physical lasw.
An elastic collision fulfills both the law of the conservation of energy and of momentum. The velocity after the collision can be predicted and calculated exactly if we apply those two laws and if we know the mass and starting-velovity of each body.

For example:
If we throw a marble with a certain velocity onto another resting marbel of the same weight, the first marble will stop moving after the collision and the second one starts to move with the same velocity as marble number one. The marbles have exchanged their velocities, so to speak.

These results can be observed in all situations where the two bodies have the same mass. The laws of energy and momentum conservation are fulfilled.