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  What is the amount of work done in the following cases? Justify your answer by giving the appropriate reason.      (a) By an electron revolving in a circular orbit of radius r around a nucleus. (b) By the force of gravity, when a stone of mass m is dropped from the top of a multi-storied building of height h.

Answer:

            (a) Work done is zero, as shown in the figure below, When electron revolves around the nucleus, a centripetal force Facts along the radius towards the centre O. The displacement (s) acts tangentially therefore, the angle between the force and the displacement is 90°. Therefore, W = Fs cos 90° = 0  [] (b)W =mgh As the stone is dropped, its PE starts to convert into KE. Let its speed is v, then from v2 = 2gh (when stone reaches the ground)