Question

A 20 N metal block is suspended by a spring balance. A beaker containing some water is placed on a weighing machine which reads 40 N. The spring balance is now lowered so that the block gets immersed in the water. The spring balance now reads 16 N. The reading of the weighing machine will be
(a) 36 N
(b) 60 N
(c) 44 N
(d) 56 N.

Answer 1

Answer ⇒ Option (c). 44 N.

Explanation ⇒ Weight of the metal block as measured by the spring balance in air is 20 N.

When the metal block is suspended in water, its weight becomes, 16 N.

This means upthrust is involved which has decreased its weight.

We know, that Upthrust = loss in weight.

  Upthrust = 20 - 16

= 4 N.

We know, In equilibrium of fluid,

Weight of fluid  = Upthrust + Normal reaction.

Weight of fluid = 4 + weight of fluid measured separately.

Weight of fluid  =4 + 40

Weight of fluid = 44 N.

Hence, the weight of the fluid measured when the spring balance containing metal block is suspended is equal to 44 N.

Hence, Option (c). is correct.