In the previous example, we've seen that the kinetic energy has transformed into potential energy and the work done on the object is the change in energy.

Therefore,

The sum of potential energy and kinetic energy in a system is the total mechanical energy E and it has a constant value.

This is called "The Law of Conservation of Mechanical Energy".

The Law of Conservation of Energy states that energy cannot be neither destroyed nor created and that it is the same in a closed system. Energy that is absorbed into a system must always equal to the energy released.