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:
$(a)$ 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,
$(c)$ work done by friction on a body sliding down an inclined plane,
$(d)$ work done by an applied force 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.
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:
$(a)$ 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,
$(c)$ work done by friction on a body sliding down an inclined plane,
$(d)$ work done by an applied force 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.
$(a)$ Positive : In the given case, force and displacement are in the same direction. Hence, the sign of work done is positive. In this case, the work is done on the bucket.
$(b)$ Negative : In the given case, the direction of force (vertically downward) and displacement (vertically upward) are opposite to each other. Hence, the sign of work done is negative.
$(c)$ Negative : Since the direction of frictional force is opposite to the direction of motion, the work done by frictional force is negative in this case.
$(d)$ Positive : Here the body is moving on a rough horizontal plane. Frictional force opposes the motion of the body. Therefore, in order to maintain a uniform velocity, a uniform force must be applied to the body. Since the applied force acts in the direction of motion of the body, the work done is positive.
$(e)$ Negative : The resistive force of air acts in the direction opposite to the direction of motion of the pendulum. Hence, the work done is negative in this case.
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