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 blocks get 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 the metal block is suspended from one end of the spring balance. We are given a 20 N metal block, so we are given with the weight of the block since weight is nothing but the gravitational pull of the earth. Here when the block goes into the water, Archimedes principle can be used.

Complete step by step answer:
Weight of Block in air= true weight= 40 N
Weight of block in water= apparent weight= 16 N
Loss in weight= true weight- apparent weight
=(20-16)
=4 N
Now, what will the machine read?
= weight of water + loss in weight
=40+4
=44 N
Hence,option (C) is the correct answer.

Note:Apparent weight comes into existence because the block when it goes into the water it displaces the fluid and it experiences a buoyant force which acts in the upward direction. The weight acts downward and a buoyant force acts upwards and thus the net force is less than the true force acting on the block when it was in the air. Apparent weight is also responsible for our sensation of the weight of our bodies.