# Weight of air-filled balloon vs empty balloon

1. Jan 21, 2007

### ritwik06

1. The problem statement, all variables and given/known data
Take 2 rubber balloons A and B of equal mass. Balloon A is filled with air and balloon B is left empty and the two are weighed in a pan balance. What will you observe and why?

2. The attempt at a solution

The balloon with air will appear lighter if experiment is conducted in air and vice versa if in vacuum. Am I right?

2. Jan 21, 2007

### Kurdt

Staff Emeritus
The question is asking which balloon is heavier its has nothing to do with the conditions of the experiment.

3. Jan 22, 2007

### ritwik06

Then I think the one with air is heavier. What do you think? This was a 10 marks question. They wont take just a one line answer. What would you say?

4. Jan 22, 2007

### Gokul43201

Staff Emeritus
You are right about the second situation, but not about the first. What is the net force on each balloon?

5. Jan 23, 2007

### ritwik06

In the first case, that is in air, the baloon with air has less density, thus, an upthrust will act on the balloon due to air present. While in the second case, there will be no upthrust on the balloon filled with air.

6. Jan 23, 2007

### Gokul43201

Staff Emeritus
How big is the upthrust? What about the weight? Isn't the weight also greater, due to the air in it? Does one of these two dominate? If yes, which one; if no, why not?

What does Archimedes say?

7. Jan 24, 2007

### ritwik06

I think the upthrust will be slight greater. Accordind to archimedes. Wt. of fluid displaced= upthrust.

8. Jan 24, 2007

### Gokul43201

Staff Emeritus
And why do you say the weight of the air displaced by the balloon is greater than the weight of the air in the balloon?

9. Jan 24, 2007

### ritwik06

Oh!! I am sorry they will be equal !!!

10. Jan 24, 2007

### Gokul43201

Staff Emeritus
Yes, to a resonable approximation, they will be equal.

But if you look at it carefully, the air inside the balloon actually weighs a tiny bit more than the air displaced by the balloon, because the air pressure inside the balloon is a little bit higher than atmospheric pressure.

11. Jan 25, 2007

### ritwik06

Thanks a lot :D