Question

A ball, filled with air has a volume of 500 cm^3. What should be the minimum force applied by a child to put it completely inside water? Take g=10 m/s^2.

Answer 1

volume of balloon (v)= 500cm³ 
If it is completely immersed in water, it will displace the same volume water.
So buoyant force would act on it in the upward direction.The child need to apply same amount of force as buoyant force to put it inside.

density of water = 1 g/cm³
$buoyant\ force=\rho gv$

$\rho v = 1\ g/cm^3 \times 500\ cm^3=500g=0.5kg$

$buoyant\ force=\rho vg=0.5 \times 10=5N$

So the child needs to apply 5N force to put it completely inside water.

(Note: The density of air is negligible compared t density of water. So you should ignore the mass of air.)

Answer 2

volume of balloon (v)= 500cm³ 
If it is completely immersed in water, it will displace the same volume water.
So buoyant force would act on it in the upward direction.The child need to apply same amount of force as buoyant force to put it inside.

density of water = 1 g/cm³






So the child needs to apply 5N force to put it completely inside water.

(Note: The density of air is negligible compared t density of water. So you should ignore the mass of air.)