The sign of work done by a force on a body is important to understand. State carefully if the following quantities are positive or negative :
Work done by friction on a body sliding down an inclined plane

Explanation

Workdone `W = vec(F)* vec(S) = FS cos theta " where " theta ` smaller angle between force `vec(F) ` and displace- ment `vec(S)`
Friction always opposite the relative motion
`:. theta = 180^(@) , W = FS cos 180^(@) = - FS.` It is negative