A conservation law means that something is conserved with time.

Usually we have a system in an initial state (for example, two blocks before the collision), then we have a motion (for example, collision between the blocks) and finally we have the system in the final state (for example, two blocks after the collision). The conservation law means that the value of something (for example, the total momentum of two blocks) in the initial state is equal to its value in the final state.

The main conservation laws are:

•  the momentum conservation law and

•  the (mechanical) energy conservation law .

Both the momentum and the energy of the system depend on the motion of the objects. The main difference between the momentum and the energy is that the momentum is a vector, but the energy is a scalar. Therefore the momentum depends on the velocity (vector) of the object, but the energy depends on the magnitude of velocity – speed. In this sense it is easier to apply the energy conservation law than the momentum conservation law.