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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.
Asked by Abhay Last Modified
Answer 
Dharmendra Kumar
A teacher's purpose is not to create students in this own image, but to develop students who creates their own image.
A) positive
In a 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 bucket.
(b) negative
In this case, the direction of force and displacement 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 in negative.
(d) positive
Here the body is moving on a rough horizontal plane. Frictional force oppose the motion body. Therefor in order to mention a uniform velocity a uniform force must be apply to body. Since the applied force at in the direction of motion of the body the work down is positive.
(e) - Negative
The resistive force of air act in the direction opposite to the direction of motion of pendulum. Hence the work done is negative.
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