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,

(e) Work done by the resistive force of air on a vibrating pendulum in bringing it to rest.


Negative Quantity.

The resistive force of air acts in the direction opposite to the direction of motion of the pendulum. Hence, negative work is done in this case.
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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:

(c) work done by friction on a body sliding down an inclined plane


Negative quantity

The direction of the frictional force is opposite to the direction of motion. Therefore, the work done by the frictional force is negative in this case.
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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:
b) 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.


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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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.


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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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:

(d) work done by an applied force on a body moving on a rough horizontal plane with uniform velocity


Positive quantity.

In this case, 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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