Question

A $20\text{N}$ metal block is suspended by a spring balance. A beaker containing some water is placed on a weighing machine which reads $40\text{N}$. The spring balance is now lowered slowly into the beaker such that the block gets immersed in the water. The spring balance now reads $16\text{N}$. The reading of the weighing machine will be

• A. $36\text{N}$
• B. $60\text{N}$
• C. $44\text{N}$
• D. $56\text{N}$

Answer 1

The correct option is C $44\text{N}$

Loss in weight of block after being immersed in water is,
$\Delta W=20-16=4\text{N}$
This decrease in weight is due to buoyant force by water acting on the block in vertically upward direction.
$\Rightarrow F_b=\Delta W=4\text{N}$
According to Newton's $3^{\text{rd}}$ law, block will apply equal amount of force (i.e $F_b$) on water in downward direction.
$\Rightarrow$ Reading of weighing machine is given by,
$WM=\text{Initial reading}+F_b$
$\therefore WM=40+4=44\text{N}$