Law of Conservation of Energy states that, if a body or system of bodies is in motion under a conservative system of forces, the sum of its kinetic energy and potential energy is constant.

**Explanation**

From the principle of work and energy;

Work done = change in the kinetic energy

**W _{1→2} = E_{K2} – E_{K1} … … (i)**

If a body moves under the action of a conservative force, work done is stored as potential energy.

**W _{1→2} = – (E_{P2} – E_{P1}) … … (ii)**

Work done is equal to negative change of potential energy. Combining the equation (i) and (ii),

**E _{K2} – E_{K1} = – (E_{P2} – E_{P1})**

**Or, (E _{P1} + E_{K1} = E_{P2} + E_{K2}**

Which means that the sum of the potential energy and kinetic energy of a system of particles remains constant during the motion under the action of the conservative forces.