An 800-N box is pushed up an inclined plane. The plane is 4.0 m long and rises 2.0 m. It requires 3200 J of work to get the box to the top of the plane. What was the magnitude of the average friction force on the box?

The work-energy principle:  The net work done on an object is equal to the change in the object's kinetic energy.

The principle of conservation of mechanical energy for conservative forces: If only conservative forces are acting, the total mechanical energy of a system neither increases nor decreases in any process. It stays constantπit is conserved.
The law of conservation of energy: The total energy is neither increased nor decreased in any process. Energy can be transformed from one form to another, and transferred from one object to another, but the total amount remains constant.