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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 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 inclined plane.(d) Work done by an applied force on a body moving on a rough horizontal 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» As W = Fs cosθ (a) θ = 0; work is positive (b) θ = 180º; work is negative (c) θ = 180º; work is negative (d) θ = 0º; work is positive (e) The resistive force of air opposes the motion, hence work done is negative. |
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