Work

A force does work on an object when the object moves while the force acts. Only the part of the force along the motion counts:

$$W = F d \cos\theta$$

$\theta$ is the angle between the force and the displacement. Work is in joules.

Only the part of F along the displacement d does work.

A force perpendicular to the motion does no work: $\cos 90^\circ = 0$.

A force against the motion, like friction, does negative work: $\cos 180^\circ = -1$.