InterviewSolution
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InterviewSolution
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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 poisitve 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 gravitational force in the above case. (c ) Work done by friction on a body sliding down an iniclined plane. (d) Work 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. |
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Answer» Work done `W=vecF. vecs=Fs cos theta` where `theta` is smaller angle between force `vec F` and displacement `vec s`. (a) To lift the bucket, force equal to weight of bucket has to be applied vertically upwards and the bucket moves along the same direction i.e., `theta=0^(@)` therefore `W=F s cos 0^(@)=F s.` It si positive. (b) The bucket moves in a direction opposite to the gravitational force which acts vertically downwards. `:. theta=180^(@), W=F s cos 180^(@)=-Fs.` It is negative. (c) Friction always opposes the relative motion `:. theta =180^(@), W=F s cos 180^(@)=-Fs`. It is negative. (d) As the body moves along the direction of applied force `theta=0^(@), :. W=F s cos 0^(@)=Fs.`. It is positive. (e) The direction of resistive force is opposite to the direction of motion of the bob, i.e., `theta=180^(@).`. Hence, work done, in this case, is negative. |
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