Laws of Motion – Solutions

1) 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:

(a) The work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket.

(b) The work done by gravitational force in the above case.

(c) The work done by friction on a body sliding down an inclined plane.

Solution:

(a) To lift the bucket out of the wall, the man has to apply a minimum force, F= mg in the upward direction. So, the displacement of the bucket and the force applied by the man are in the same direction.

Work done, W=.

Since q =0⁰

Work done by the man to lift the bucket is positive.