Question

A 10 N metal block is suspended by a spring balance. A beaker containing some water is placed on a weighing machine which reads 20 N. The spring balance is now lowered so that the block gets immersed in water. The spring balance now reads 8 N. The reading of the weighing machine is

• A.
  18 N
• B.
  24 N
• C.
  22 N
• D.
  28 N

Answer 1

The correct option is C

22 N

Weight of the block, $W_1=10N$

Weight of the beaker, $W_2=20N$

Now, the spring balance is lowered and the block gets immersed in water.

For spring and block system:

T = Tension in the spring
$W_1$ = Weight of block

U = Upthrust applied by water

At equilibrium:

$T+U=W_1\Rightarrow U=W_1–T=10–8\Rightarrow U=2N\dots ..\left(i\right)$

The block applies same force downwards.

Now, for the system of water, beaker and weighing machine:

Reaction force applied by weighing machine, $R=U+W_2\Rightarrow R=2+20\Rightarrow R=22N$

The weighing machine always measures the value of normal reaction R.

Thus, the reading of the weighing machine is 22 N.

Hence, the correct answer is option (c).