Question

In air , a metallic sphere with an internal cavity weighs 40g and in water weighs 20g . the volume of the cavity if the density of material with cavity is 8g/cm^3

Answer 1

You can find volume of metal when ball is in air and volume of ball when ball is immered in water.
So. difference of two volume will give the answer.

now volume = weight / density.

when ball is in air; volume of metal = (40 gm) / (8 gm/cm³) = 5 cm³

when ball is in water; weight of water = 40gm - 20 gm = 20 gm

so, volume of ball = 20gm / (1gm/cm³) = 20 cm³.

So, volume of cavity = 20 - 5 = 15 cm³.

First calculate volume of solid part for weight in air and denisty of material. Vs=5 cm³.
When it is placed in water net pward force=net downward force
=> 40g-(V+Vs)g=20g
=>V+Vs=20
therefore
V=20-Vs
=15cm³