Work links the concepts of energy and force. The net force is the sum of all the forces on an object. Similarly, the work done by the net force is the sum of the work done by each of the forces acting.

W = F · Dr = F Dr cos(q)

Work can be positive or negative, positive when the force (or some component of it) is in the same direction as the displacement and negative when these are in opposite directions. Work is zero when the force is perpendicular to the displacement, and is maximized when the force is parallel to the displacement.

For a constant force:

W = F · Dr = F_x Dx + F_y Dy + F_z Dz

If a force is a conservative force (one that conserves mechanical energy) we can handle it by looking at the work done by the force or by using potential energy. In this case:

W = -DU

Friction is a good example of a non-conservative force. Friction acts to oppose motion, and for kinetic friction the force of friction is opposite to the displacement Dr. In that case:

W = -f_k Dr