Q39 of 146 Page 145

When is the work done by a force on a body?

(a) Positive


(b) Negative, and


(c) Zero?

(a) Work done is positive, when a force acts in the direction of motion of the body.

For e.g. kicking a football


(b) Work done is negative, when a force acts opposite to the direction of motion of the body


For e.g. when a body is being lifted in the upward direction


(c) Work done is zero when a force acts at right angles to the direction of motion of the body


For e.g. No work is done by the porter in carrying the load.


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37

A body of mass 100 kg is lifted up by 10 m, Find:

(i) The amount of work done.


(ii) Potential energy of the body at their height (value of g = 10 m/s2)

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A boy weighing 50 kg climbs up a vertical height of 100 m. Calculate the amount of work done by him. How much potential energy does he gain? (g = 9.8 m/s)

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To what height should a box of mass 150 kg be lifted, so that its potential energy may become 7350 joules? (g = 9.8 m/s2)

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A body of mass 2 kg is thrown vertically upwards with an initial velocity of 20 m/s. What will be its potential energy at the end of 2 s? (Assume g = 10 m/s2).