# Law of Conservation of Energy and Momentum

## Law of Conservation of Energy:

It states that “The energy can neither be created nor destroyed, though it can be transformed from one form into any of the forms, in which the energy can exist”.

The loss of energy in any one form is always accompanied by an equivalent increase in another form. When work is done on a rigid body, the work is converted into kinetic or potential energy. If the body is elastic, some of the work will also be stored as strain energy. Thus, we say that the total energy possessed by a system of moving bodies is constant at every instant, provided that no energy is rejected or received from an external source to the system.

## Law of Conservation of Momentum:

It states that “The total momentum of a system of masses in any one direction remains constant unless acted upon by an external force in that direction”.

This principle is applied to problems on impact, i.e, the collision of two bodies. In other words, if two bodies of masses m1 and m2 with linear velocities v1 and v2 are moving in the same straight line and they collide and begin to move together with a common velocity v, then

Momentum before impact = Momentum after impact

m1v1 ± m2v2 = (m1 + m2)v