Question

in my book, work done by frictional force is given as it may be positive,negative or zero.How come??

Answer 1

It all depends on the direction of the force.
Friction opposite to the direction of motion - negative
Friction in the direction of motion - positive.
And zero when friction doesn't contribute to the motion of the object. (Can be perpendicular)

Answer 2

@Ankh but, itis sure that friction always opposes motion...then,how can friction be positive??

Answer 3

Not entirely true, here's an example: