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 false?

Options

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

C

Explanation

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