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Question
The sign of work done by a force on a body is important to understand. State carefully if the following quantity is positive or negative:
Work done by gravitational force on a man in lifting a bucket out of a well by means of a rope tied to the bucket.
Solution
The work done by gravitational force on a man lifting a bucket out of a well by means of a rope tied to the bucket is negative. This results from the bucket moving upward, opposite to the gravitational force, which acts downward. Since the direction of the gravitational force and the displacement of the bucket form an angle of 180°, the work done is calculated as:
`W = F.dcosθ = F.dcos180° = −Fd`, making it negative.
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Work done by the resistive force of air on a vibrating pendulum in bringing it to rest.
