Exercises
Question 6.1:
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 the gravitational force in the above case,
(c) work done by friction on a body sliding down an inclined plane,
(d) work is 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.
Work done, W = T.S = Fs cos θ
(a) Work done ‘positive’ because force is acting in the direction of displacement i.e., θ = 0°.
(b) Work done is negative, because the force is acting against the displacement i.e., θ = 180°.
(c) Work done is negative, because the force of friction is acting against the displacement i.e., θ= 180°.
(d) Work done is positive, because the body moves in the direction of applied force i.e., θ= 0°.
(e) Work done is negative, because the resistive force of air opposes the motion i.e., θ = 180°.
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