Mechanical energy was defined as the capacity of the forces acting on a body to do work. Now the link between energy and work done can be expressed more precisely.

The work done by external forces acting on a body is equal to the change in the mechanical energy of the body.

This is the principle of work and energy

If the external forces act in a direction that helps promote the motion of the body the mechanical energy increases, whereas opposing external forces cause a decrease in mechanical energy.

The weight of an object is not counted as an external force in this context because work done by weight is Potential Energy and is already accounted for.

In solving problems involving this principle it is wise to use it in the form

Final ME - Initial ME = Work Done

This avoids any confusion in cases where one type of energy increases and the other decreases.