Explain conservation of energy ?

Hint:When some of the forces involved are non-conservative, a part of mechanical energy may be changed into other forms of energy such as heat light and sound energy but if we account for all forms of energy, the total energy of an isolated system does not change.

Complete answer:
Energy may be transformed from one form to another, but the total energy of an isolated system remains constant.For example the universe as a whole can be treated as an isolated system, the sum total energy of all kinds in the universe remains constant at all times. Thus, energy can neither be created nor be destroyed. If one part of the universe loses energy, some other part of the universe would gain an equal amount of energy.

Another example is when the pendulum swings back down, the potential energy is converted into kinetic energy but at all times, the sum of potential energy and kinetic energy remains constant. Thus, ${{K}_{1}}+{{U}_{1}}={{K}_{2}}+{{U}_{2}}$
where ${{K}_{1}}=$initial kinetic energy, ${{U}_{1}}=$initial potential energy, ${{K}_{2}}=$final kinetic energy and ${{U}_{2}}=$final potential energy.

Note:The principle of conservation of energy cannot be proved as such. However, no violation of this principle has ever been observed. The concept of transformation of energy into various forms and its ultimate conservation is of vital importance in all branches of science, including engineering and industry.