Question

A bucket of water contains a wooden block floating in water with ($4/5)$th of its volume sub merged in the water. The bucket is placed on the floor of a lift and the lift now starts moving down with uniform acceleration. The block of wood now:

• A. moves upward.
• B. moves downward.
• C. remains at same place.
• D. moves horizontally.

Answer 1

Answer: (C)

remains at same place.

${\rho }_{s}Vg={\rho }_w\left(\frac{4}{5}V\right)g\Rightarrow {\rho }_s=\frac{4}{5}{\rho }_w$

If effective gravity is changed to g - a;

${\rho }_{s}V(g-a)={\rho }_w\left(xV\right)(g-a)\Rightarrow x=\frac{4}{5}$

So, 4/5 th volume of the block is still submerged, thus the block does not move.