Say a completely deflated balloon weighs 5 grams. When you add air, the balloon becomes lighter. But the air just adds more weight and does not decrease any weight. Let's say the air weighed 0.5 grams (random numbers). The inflated balloon will then weigh 5.5 grams, which is heavier than before. How did it become lighter?
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1How have you measured the balloon becoming lighter after air is added? – BowlOfRed Mar 15 '22 at 17:50
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look up buoyancy https://en.wikipedia.org/wiki/Buoyancy – anna v Mar 15 '22 at 18:33
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1Are you confusing density with weight? – DKNguyen Mar 15 '22 at 19:10
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The balloon does not become lighter (with respect to mass). It becomes heavier, as you'd expect. Your ability to weigh it using a scale becomes harder because of buoyancy, but its mass still increases.
Dr. Momo
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The actual weight of the balloon does increase, as you would expect:
$$\text{actual weight} = \text{weight of balloon} + \text{weight of gas inside balloon}$$
However, the apparent weight of the balloon may decrease if the gas that you fill the balloon with is less dense than the surrounding air:
$\text{apparent weight} \\ = \text{weight of balloon} + \text{weight of gas inside balloon} - \text{weight of air displaced by balloon} \\ = \text{weight of balloon} - \text{buoyancy}$
You will get a small decrease in apparent weight if you fill the balloon with warm air (which is less dense than cold air), and a larger decrease if you fill the balloon with, say, helium.
If the balloon is large enough then its buoyancy when inflated (weight of air displaced by balloon - weight of gas inside the balloon) can be greater than the weight of the balloon itself, and its apparent weight becomes negative - this is the principle behind the hot air balloon.
gandalf61
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When you add air to the balloon the downward force on the balloon due to gravity has to be greater than without the added air because its mass increases and the downward force due to gravity is $(M+m)g$ where $M$ is the mass of the balloon and $m$ is the mass of the added air.
But it won't show up on a scale because when you add air the total volume of the air displaced outside the balloon is greater than that displaced by the balloon material itself, resulting in an increase in the upward buoyant force on the balloon opposing the force of gravity. The result is a lower net downward force sensed by a scale.
Hope this helps.
Bob D
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