Explanation

(a) Upthrust is independent of all factors of the body such as its mass, size, density etc, except the volume of the body inside the fluid. Fraction of volume immersed in the liquid$V_{in} = \left( \frac{\rho}{\sigma} \right)\ V\ ie,$ it depends upon the densities of the block and liquid. So, there will be no change in it if system moves upward or downward with constant velocity or some acceleration. Therefore, the upthrrust on the body due to liquid is equal tto the weight of the body in air.