Question

Will an empty balloon have precisely the same apparent weight on a scale as a balloon filled with air? Explain.

Answer 1

The air inside the balloon is under higher pressure than that outside

What effect does that have on its density?

There is a 'buoyancy effect' when the balloon displaces air around it. What is the weight of the air displaced compared with the weight of the air in the balloon?

Answer 2

It depends if the balloon floats. But since it's no denser than the air around it, I don't think the balloon floats in air, in fact it should just sink. The weight of air displaced compared to the weight of air in the balloon is the same since they're the same gas in the balloon. So the buoyancy effect is almost irrelevant. If knowing that the mass of air has weight, as it should, the full balloon should just "apparently weigh" more than the empty balloon.

Answer 3

It is full of air so it won't float. (It needs to be full of a gas less dense than air to float - Hydrogen or more normally Helium)

If it was just for instance a plastic bag full of air, at atmospheric pressure then you would naturally assume that it weighs the same whether it is 'full' or 'empty'

However, because the air inside the balloon is under pressure it is MORE dense than the surrounding air. The buoyancy effect IS significant - and is equal to the weight of the air displaced by the balloon BUT the weight of air displaced is LESS than the weight of the air in the balloon and hence the balloon weighs more when full of air under pressure than when empty.

Answer 4

Actually if you apply Bernoulli's principle and law of conservation of energy, the "apparent weight" of the full balloon comes out less than the apparent weight of the empty balloon. Use Newton's 2nd and solve for the "apparent weight", which you can treat as the tension force Ft if the balloons were hanging from a string under a scale. Then the apparent weight (w' or Ft) of the full balloon is less than the empty balloon if you consider the buoyancy force Fb, which is why this problem is so confusing in the first place. Instead just logically reason that air in the full balloon has weight, and the full balloon, being full, has a greater "apparent weight"

Answer 5

@Onikage781

I'm afraid I cannot make a clear picture of your description.

Could you provide some diagram?

As far as I see - the balloon is full of a material more dense than it's surrounds.

The material and weight of the skin of the balloon remains the same.

The weight of the air inside is greater than the buoyancy, and hence a resultant 'downwards' weight is created by the air (= weight of air in balloon - weight of air displaced).

SO it seems to me that the inflated balloon has
(Weight of skin + weight of pressurised air - weight of displaced air)

The uninflated balloon has only (weight of skin).

I am willing to believe there is another explanation, but your post does not explain it to me. Please post free body diagrams or similar to explain your point....

Answer 6

Bernoulli has to do with moving fluids, and conservation of energy has no relevance here because there is no transfer of energy in the question. SO I cannot see that those points are relevant to this discussion. Newton's 2nd law is related to acceleration - again not pertinent.

Answer 7

You also stated above that
"The weight of air displaced compared to the weight of air in the balloon is the same since they're the same gas in the balloon"
However you have ignored the fact that the air in the balloon is under pressure ,and hence is more dense than the air surrounding it. Thus the net force of weight - buoyancy is a downward force adding to the total weight.