Question

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 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 ravitational potential energy of the block
D. all the above

Answer 1

Correct Answer - C
Initially the wooden block floats with partially immersed in water. Initiall upthrust exerted by water is equal to weight of the block. But when it is pressed down more water is displaced. Hence upthrust exerted by water increases. The force requried to press down the block is equal to `F="upthrust"-mg` Hence, work done by the force F will be equal to work done against (upthrust -mg). It means, work done by external force is equal to work done against upthrust loss of gravitational potential energy of the block.