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

Hint
Here, we use the Archimedes’ principle, which states that when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Complete step by step answer
It is given that a 20 N metal block is suspended by a spring balance. The reading of the beaker filled with water on a weighing machine is 40 N. And the reading, on the spring balance, when the metal block is immersed in the water is 16 N.
We should find the reading of the weighing machine when the metal block is immersed in water.
On applying the Archimedes’ principle, we can say that the reading of the weighing machine (the metal block is immersed in water) = the weight of the water displaced by the metal block.
The weight of the water displaced by the metal block = 40 N + (20-16) N = 44 N.
Thus, the reading of the weighing machine when the metal block is immersed in water is 44 N, which is the option (C).

Note
Archimedes’ principle is useful for calculating the volume of an object that does not have a regular shape. The principle is also used in designing ships and submarines.