Predict the sign of the work done in the following cases:-
(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 gravitational force in the above case (a).
(c) work done by friction on a body sliding down an inclined plane. 
(d) work done by the resistive force of air on a vibrating pendulum in bringing it to rest.

Asked by Topperlearning User | 4th Jul, 2014, 12:53: PM

Expert Answer:

(a) Since tension in the rope is in the same direction as displacement, work done is positive.
(b) Work done is negative since gravitational force acts downwards, while displacement is upwards.
(c) Since friction opposes motion down the plane, it is directed opposite to displacement. Therefore work is negative.
(d) Work done by resistive force of air is negative because this force opposes motion.

Answered by  | 4th Jul, 2014, 02:53: PM