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

(a) Positive as the force applied by the man in lifting the bucket and the displacement of the bucket are in the same direction.
(b) Neagitve as gravitational force and displacement are oppositely directed.
(c ) Negative as friction is acting opposite to the displacement.
(d) Positive as the applied force and the displacement are in the same direction.
(e ) Negative as resistive force of air on a vibrating pendulum always acts in a direction opposite to the direction of motion of the pendulum.