Question

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: 

1. work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket. 
2. work done by gravitational force in the above case, 
3. work done by friction on a body sliding down an inclined plane, 
4. work done by an applied force on a body moving on a rough horizontal plane with uniform velocity, 
5. work done by the resistive force of air on a vibrating pendulum in bringing it to rest.

Answer 1

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.

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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.

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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.

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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.

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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.