The sign of work done by a force is important to understant. State carefully if the following quantities are positive or negative. (a) Work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket. (b) Work done by the gravitational force in the above case. (c ) Work done by friction on a body sliding down an inclined plane. (d) Work done by an applied froce on a body moving on a rough horizontal plane with uniform velocity. (e ) Work done by the resistive force of air on a vibrating pendulum in bringing it to rest.

Explanation

Work done, W=T.S = `Fscostheta`
a) work done 'positive' because force is acting in the direction of displacement i.e. `theta=0^(@)`. lt brgt b) Work done is negative because force is acting against the displacement i.e. `theta=180^(@)`.
c) Work done is negative, because force of friction is acting against the displacement i.e. `theta=180^(@)`.
d) Work done is positive, because body moves in the direction of applied force. i.e. `theta=0^(@)`.
e) Work done is negative because the resistive of air opposes the motion i.e. `theta=180^(@)`.