First slide

Fluid statics

Question

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

Easy

A

work done against upthrust exerted by the water
B

work done against upthrust plus loss of gravitational potential energy of the block
C

work done against upthrust minus loss of gravitational potential energy of the block
D

none of the above

Solution

Initially the wooden block floats when partially immersed in water. Initially, upthrust exerted by water is exactly equal to the weight of the block. But when it is pressed down then more water is displaced. Hence up thrust exerted by water increases. The force required to press down the block is equal to F = upthrust -mg. Hence, work done by the force F will be equal to work against(upthrust -mg). It means, work done by the external force is equal to (work done against upthrust-loss of gravitational potential energy of the block.

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