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A wooden block is floating in a water ta...

A wooden block is floating in a water tank. The block is pressed to its bottom. During this process, work done is equal to :

A

Work done is equal to work done against upthrust exerted by the water

B

Work done is equal to work done against upthrust plus loss of gravitational potential energy of the block

C

Work done is equal to work done against upthrust minus loss of gravitational potential energy of the block

D

Work done is equal to loss of gravitational potential energy of the block

Text Solution

Verified by Experts

The correct Answer is:
C
Initially, the wooden block floats partially immersed in water. Initially, the upthrust exerted by water is exactly equal to weight of the block. But when it is pressed down, then more water is displaced. Hence, upthrust exerted by water increases.
The force required to press down the block is F = upthurst – mg
Hence ` W_("by "F_(ext)) = W_("against (upthrust - mg)") or W _ ("by "F _ (ext)) = W _ ("against upthrust" )` - Loss in GPE of block
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