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Question -

 The sign of work done by a force on a body isimportant to understand. State carefully if the following quantities arepositive or negative:
(a) Work done by a man in liftinga bucket out of a well by means of a rope tied to the bucket,
(b) Work done by gravitationalforce in the above case,
(c) Work done by friction on abody sliding down an inclined plane,
(d) Work done by an applied forceon a body moving on a rough horizontal plane with uniform velocity,
(e) Work done by the resistiveforce of air on a vibrating pendulum in bringing it to rest.



Answer -

 Work done, W = T.S = Fs cos θ
(a) Work done ‘positive’, because force is acting in the direction ofdisplacement i.e., θ = 0°.
(b) Work done is negative, because force is acting against the displacementi.e., θ = 180°.
(c) Work done is negative, because force of friction is acting against thedisplacement i.e., θ= 180°.
(d) Work done is positive, because body moves in the direction of applied forcei.e., θ= 0°.
(e) Work done is negative, because the resistive force of air opposes themotion i.e., θ = 180°.

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