state (with reasons ) whether work done in the following cases is positive or negative:
e. Work done in moving on a circular track
f. Work done by a man in lifting a bucket out of a well with the help of a rope tied to the bucket
g. Work done by the gravitational force in case f.
h. Work done by friction on a body sliding down
an inclined plane
i. Work done by a force applied on a body moving on a rough horizontal plane with uniform velocity.
J. Work done by the resistive force of air on a vibrating pendulum to bring it to rest.
Answers
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Explanation:
e. positive
The work done on a body moving in a circular path is also zero. This is because, when a body moves in a circular path, then the centripetal force acts along the radius of the circle, and it is at right angles to the motion of the body. Thus, the work done in the case of moon moving round the earth is also zero.
f. negative
Work done by gravitational force when a man in lifting a bucket out of a well by means of a rope tied to the bucket. 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.
g. negative
If the object is lifted straight up at constant speed, then the force needed to lift it is equal to its weight mg. The work done on the mass is then W = Fd = mgh. We define this to be the gravitational potential energy (PEg) put into (or gained by) the object-Earth system.
h. negative
the frictional force acting on a body sliding down acts in direction opposite to the direction of displacement of block, the work done is negative.
i. 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.
j. positive
Work done is positive, because the bucket moves in the direction of the force applied by the man.