Question

A wooden block is floating in a water tank. The block is pressed to the bottom. During the process, a work is done. Which of the following statements are incorrect?

• 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.

Answer 1

Answer: (A), (B), (D)

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

The force required to press down the block is equal to
$F=\text{Upthrust}-mg$
Hence, work done by the force $F$ is equal to the work done against $(\text{Upthrust}-mg)$. It means, work done by the external force is equal to the work done against upthrust - loss of gravitational potential energy of the block. Hence the correct choice is 'C'.