If the displacement is in a direction opposite to the force,
i.e., θ = 180°, then cos 180° = -1
W = - F * S
The work done is negative. This is usually the case when the force
opposes the motion or it tries to stop a moving body.
When a body moves on a surface , the force of friction between the body
and the surface is in a direction opposite to the motion of the body. Therefore,
the work done by the force of friction is negative.