The work done on a particle by the resultant force is equal to the change in its kinetic energy.
Principle of conservation of energy
The total mechanical energy of a system remains constant if the internal forces are conservative and the external forces do not work.
(The sum of the kinetic energy and the potential energy is called the total mechanical energy.)
Generalised law of conservation of energy
Energy can never be created or destroyed, it can only be changed from one form to another.
(Chapter: Work, power, energy)