Justify,giving proper reasoning,whether the work done in the following case is positive or negative: Work done by friction on a body sliding down an inclined plane.

(NEGATIVE) The work done by friction is negative.
Reasoning: Work done $(W)$ is defined by the formula $W = F \cdot s \cdot \cos(\theta)$,where $F$ is the force,$s$ is the displacement,and $\theta$ is the angle between the force and displacement vectors.
When a body slides down an inclined plane,the direction of motion (displacement) is downwards along the plane.
Frictional force always acts in the direction opposite to the motion of the body.
Therefore,the angle $\theta$ between the frictional force and the displacement is $180^{\circ}$.
Since $\cos(180^{\circ}) = -1$,the work done $W = F \cdot s \cdot (-1) = -F \cdot s$,which is negative.