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Calculate the work done in raising a stone of mass 5 kg and specific gravity 3 lying at the bed of a lake through a height of 5 metre.

force * displacement = work

force =mass *accelerleration

     =5 *3 =15

work = 15 *5 =75

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 Loss of weight in water = weight in air / specific gravity = 5/3

Weight of stone in water = 5-5/3 = 10/3

Force = 10/3 x 9.8 = 98/3

Work done = 5 x 98/3 = 163.3

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calculate the work done in raising a stone of mass 5 kg and specific gravity 3, lying at the bed of a lake through a height of 4 m?
 
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net force = W - boyancy                  

ds , dw are dencity  of stone & water respectively & V  is volume of stone then ,

boyancy = dwVg

W = mg = dsVg

Fnet = Vg(ds-dw)

Fnet = ma = dsVg

a = g(1-dw/ds) 

relative dencity = ds/dw = 2 (given)

so  ,a = g/2

now work is done against this force

work = maX                 (X displacement)

       =5*(g/2)3 = 75j

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