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.
(i) Work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket.
(ii) Work done by gravitational force in the above case.
(iii) Work done by friction on a body sliding down on inclined plane.
(iv) Work done by an applied force on a body moving on a rough horizontal plane with uniform velocity.
(v) Work done by the resistive force of air on a vibrating pendulum in bringing it to rest,
(i) Work done by the man against the force of gravity while
lifting a bucket out of a well by means of a rope tied to the bucket is + ve, because force and displacement are in same direction.
(ii) Work done by gravitational force in the above case is —ve
because direction of gravitational force is opposite to the displacement.
(iii) Work done is -ve because (direction of friction force is
opposite to sliding motion…
(iv) Work done by an applied force on a body moving on a rough horizontal plane is positive because force is being applied in the direction of motion so as to overcome
friction.
(v) Work done is negative because the resistive force of air acts in a direction opposite to the direction of motion of the vibrating pendulum.