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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.

Answer» <html><body><p></p>Solution :Work done `W=vecF. vecs=Fs cos theta` where `theta` is smaller angle between force `<a href="https://interviewquestions.tuteehub.com/tag/vec-723433" style="font-weight:bold;" target="_blank" title="Click to know more about VEC">VEC</a> F` and displacement `vec s`. <br/>(a) To lift the bucket, force equal to weight of bucket has to be <a href="https://interviewquestions.tuteehub.com/tag/applied-380664" style="font-weight:bold;" target="_blank" title="Click to know more about APPLIED">APPLIED</a> <a href="https://interviewquestions.tuteehub.com/tag/vertically-3260386" style="font-weight:bold;" target="_blank" title="Click to know more about VERTICALLY">VERTICALLY</a> upwards and the bucket moves along the same direction i.e., `theta=0^(@)` therefore `W=F s cos 0^(@)=F s.` It si positive.<br/> (<a href="https://interviewquestions.tuteehub.com/tag/b-387190" style="font-weight:bold;" target="_blank" title="Click to know more about B">B</a>) The bucket moves in a direction opposite to the gravitational force which <a href="https://interviewquestions.tuteehub.com/tag/acts-848461" style="font-weight:bold;" target="_blank" title="Click to know more about ACTS">ACTS</a> vertically downwards. <br/> `:. theta=180^(@), W=F s cos 180^(@)=-Fs.` It is negative. <br/> (c) Friction always opposes the relative motion `:. theta =180^(@), W=F s cos 180^(@)=-Fs`. It is negative. <br/> (d) As the body moves along the direction of applied force `theta=0^(@), :. W=F s cos 0^(@)=Fs.`. It is positive. <br/> (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.</body></html>


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