Question

can you help me understand a concept here quick involving buoyancy force.

Ive got a question here that states as a balloon filled with air with (contant density) is pushed deeper and deeper into water. the buoyancy force decreases.

If F(B) = p.V.g

how is this so.

Answer 1

The buoyancy force will increase at first (till the particle is completely submerged) and then remain constant. (This is applicable for solid objects)

Tell me if the question is from thermodynamics because I think it is using the concept of pressure change inside the balloon.

Answer 2

(thats what i thought) LDW, but ive seen a couple answers online aswell as my memo which state that F(B) decreases.

This question is from first year university (Hydrostatics)

Answer 3

Its possible if the water is very cold.. cause then volume would decrease as temperature decreases (isobaric process)

Answer 4

But by going down, pressure will increase (and hence we are doomed to go to mathematics :P)

Answer 5

no no.. when u are pushing ur balloon down.. u can completely ignore the increase in the pressure :P.. unless u have a asteroid size balloon :D

Answer 6

i dont think the question would be so ambiguous at this level.

I do imagine the Volume displaced will decrease as the pressure compresses the balloon with depth.

i made a mispelling with the equation

F(B) = density x Volume x gravitational acceleration.

so im goign to assume volume is decreasing with depth due to pressure from the water above the balloon, like lungs would be

Answer 7

@Mashy
Consider the case of an air bubble inside water

Answer 8

yes i understand.. hydrostatic pressure comes into picture if the depth is considerable..

fine then.. consider increase in pressure.. then take it as isothermal process.. volume decreases as well.. :P.. i think that would be easier !

Answer 9

Doesn't the relation between pressure inside and outside the bubble depends on radius. I cant remember the formula but I am positive about this.

Answer 10

^^ that is the excess pressure due to surface tension of a BUBBLE.. u can completely neglect that when u are dealing with balloons :P

Answer 11

I cannot read all that preceded. As you go down, the pressure of the water will make the balloon smaller due to compressing the gas. Theoretically, it could get to be so small as not to have enough buoyancy to float back up.

Answer 12

"...I cannot read all that preceded..."
Nice punch :D