Question

A body weighs 21 newton in air and when immersed in water which weighs 3 newton find its relative density

Answer 1

W1 = 21 N
W2 = 3 N
R.D = 21/(21-3) = 21/18 =7/6

Answer 2

Given that, a body weighs 21 newton in air and when immersed in water, weighs 3 newton.
We have to find it's relative density.
We know that, relative density is the ratio of density of body in air to density of body in water.
Let F₁ be the weight of the body in air.
Then,
 F₁ = 21 N
and F₂ be the weight of the body in water.
Then,
F₂ = 3 N
So,
F₁ / F₂ = 21 / 3
F₁ / F₂ = 7 N
or,
m₁ / m₂ = 7 N   .......... (1)
Since,
Density = Mass / Volume
As volume remains the same.
So,
Relative density = Relative mass
d₁ / d₂ = 7 N  (Using equation (1) )
Which is the required answer.
Hope it helps.
Thanks.