Question

Weight of a body in air is $40g$. If its volume is $10cc$, then its weight in water is
(A) $30g$
(B) $40g$
(C) $50g$
(D) None of these

Answer 1

Hint We know that weight of a body is net force applied on the body. When we put an object in water, its weight decreased by the weight of the volume of the object inside the water. Weight of the body is decreased due to buoyant force applied by water.

Complete step by step solution
Given, the weight of a body in air is $40g$ and its volume is $10cc$, where cc is $c{m^3}$.
Then the density of the body is $\dfrac{{40g}}{{10cc}} = 4g/cc$.

When we put the body in water, the body will replace the water of volume $10cc$ due to which water applies some buoyant force on the body in an upward direction. We know the density of water is $1g/cc$.
Here we can see that the density of the body is greater than water then the body completely sinks into water.
Then buoyant force on the body or decrease in weight is $1 \times 10 = 10g$.
Now, there are two forces applied on the body: one is $40g$ due to its weight in downward direction and another is $10g$ due to buoyant force applied by water. Then the net weight of the body is $40 - 10 = 30g$.

Hence the correct answer is option A.

Note In above question the body is completely submerged into water because its density is more than that of water but if its density is less than water then its some is inside the water remaining is outside the water. Or we can say that the body floats on the surface of water in that case.