# An old one I posted years ago: helium balloon

sonhouse
Posers and Puzzles 07 Apr '10 15:13
1. sonhouse
Fast and Curious
07 Apr '10 15:13
You have a rocket, picture the gas tank from the shuttle, big sucker, but mostly empty.
It is way in space a light year from home, in other words, away from any large gravitational field. Inside there is a bulkhead midway between the front and back. You have a helium filled balloon and the upper chamber is filled with standard air, pressure and composition the same as on Earth.

You very gently place the helium balloon in the middle of that chamber and very gently move back to the bulkhead where your rocket controls are and the balloon is still centered in the chamber.

You fire off the rocket, it thrusts from the back just like any rocket. What happens to the helium filled balloon?

If you remember the solution, don't shout it out, let someone who hasn't heard it before come up with it. You can PM me the solution and I will tell the first one who gets it.
2. 07 Apr '10 15:591 edit
Originally posted by sonhouse
You have a rocket, picture the gas tank from the shuttle, big sucker, but mostly empty.
It is way in space a light year from home, in other words, away from any large gravitational field. Inside there is a bulkhead midway between the front and back. You have a helium filled balloon and the upper chamber is filled with standard air, pressure and composition before come up with it. You can PM me the solution and I will tell the first one who gets it.
I've seen the same thing whan I was a boy and had a helium baloon in the car. It acted strangely.
So, yes, I know the answer.
3. 13 Apr '10 15:361 edit
EDIT: PM
4. sonhouse
Fast and Curious
13 Apr '10 17:50
Originally posted by Thomaster
EDIT: PM
One or two correct answers so far.
5. 13 Apr '10 19:56
Originally posted by FabianFnas
I've seen the same thing whan I was a boy and had a helium baloon in the car. It acted strangely.
So, yes, I know the answer.

me too
6. wolfgang59
Mr. Wolf
16 Apr '10 08:39
Originally posted by sonhouse
You have a rocket, picture the gas tank from the shuttle, big sucker, but mostly empty.
It is way in space a light year from home, in other words, away from any large gravitational field. Inside there is a bulkhead midway between the front and back. You have a helium filled balloon and the upper chamber is filled with standard air, pressure and composition ...[text shortened]... before come up with it. You can PM me the solution and I will tell the first one who gets it.
F=ma
7. 16 Apr '10 08:56
Einstein asserted that gravitational force is the same as the "force" caused by constant acceleration; so, in a rocket 1G accelerating at 1G the balloon should do exactly the same thing that it would if released when the rocket was stationary in a 1G gravitational field (e.g. on earth).
8. wolfgang59
Mr. Wolf
16 Apr '10 09:02
Originally posted by iamatiger
Einstein asserted that gravitational force is the same as the "force" caused by constant acceleration; so, in a rocket 1G accelerating at 1G the balloon should do exactly the same thing that it would if released when the rocket was stationary in a 1G gravitational field (e.g. on earth).
Which way does the balloon go?
Forward or backward or stay in position (relative to ship)?
9. 16 Apr '10 10:18
Originally posted by wolfgang59
Which way does the balloon go?
Forward or backward or stay in position (relative to ship)?
It moves in the most surprising direction.
But when you have the proper explanation it isn't surprising at all.
10. wolfgang59
Mr. Wolf
16 Apr '10 11:51
Originally posted by FabianFnas
It moves in the most surprising direction.
But when you have the proper explanation it isn't surprising at all.
I know!

I was asking iamatiger to elaborate.

F=ma
11. 16 Apr '10 17:31
If the rocket is on the earth, and we are standing inside it and release the balloon the balloon will rise relative to us and the rocket.

So, by equivalence, if the rocket is accelerating at one G and we are standing inside it (with our feet pointing in the opposite direction to the acceleration), and we release the balloon, then the balloon will "rise" in the direction of the acceleration.
12. ua41
Sharp Edge
16 Apr '10 17:482 edits
Originally posted by iamatiger
If the rocket is on the earth, and we are standing inside it and release the balloon the balloon will rise relative to us and the rocket.

So, by equivalence, if the rocket is accelerating at one G and we are standing inside it (with our feet pointing in the opposite direction to the acceleration), and we release the balloon, then the balloon will "rise" in the direction of the acceleration.
But, it rises on earth not due to gravity but due to buoyancy.
If you were on the moon, it would fall to the surface
13. AThousandYoung
16 Apr '10 18:32
Originally posted by ua41
But, it rises on earth not due to gravity but due to buoyancy.
If you were on the moon, it would fall to the surface
In one case, there is air; in the other, there is no air.

Is there air in the spaceship?
14. ua41
Sharp Edge
16 Apr '10 18:59
Originally posted by AThousandYoung
In one case, there is air; in the other, there is no air.

Is there air in the spaceship?
I realize. It was more of a nitpick comment
15. wolfgang59
Mr. Wolf
16 Apr '10 18:59
Originally posted by iamatiger
If the rocket is on the earth, and we are standing inside it and release the balloon the balloon will rise relative to us and the rocket.

So, by equivalence, if the rocket is accelerating at one G and we are standing inside it (with our feet pointing in the opposite direction to the acceleration), and we release the balloon, then the balloon will "rise" in the direction of the acceleration.
Providing that the balloon is in an air-filled room
Yes I agree. The helium balloon is subject to the same force as everything else in the rocket. Therefore with a smaller mass (per unit volume) than the surrounding air-molecules the helium balloon will have a greater acceleration.

In a vacuum there would be nothing to impart a force to the balloon until it hit a bulkhead.