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# The paradox of the Helium Balloon watch

1. I wonder if anyone's heard this one before-

A Helium balloon is tied to the seat of a car, not touching the roof or sides. When you accelerate, the balloon moves forwards, and similarly when you turn, say, left the balloon leans left.

Can anyone explain?.
2. yes, it's lighter than air, and so it will always rise up away from the "gravitational force", and when you turn left, the apparent "gravitational force" is to the right, and when you accelerate forwards, the apparent "gravitational force" is to the back, so the balloon will "rise" to the left and forward respectively.
3. what do you mean gravitational force?

if this was done in deep space the effect would be the same becuase the air is more dense than the helium in the balloon
4. i put gravitational force in brackets. (i meant using acceleration-gravitational field equivalence)
5. lol okay

general relativity is a bit over my head at the moment
6. its not general relativity
7. I thought it said being accelerated or being in a gravitational field are equivalent
8. i was only using the gravitational fiel analogy because it is something we take for granted (the balloon will rise, away from the source of gravity). in the acceleration cases i could have called it an arbitrary inertial force, but decided to call it (note the quotes) "gravittional force" just so it would fit in with the analogu of the balloon rising.
9. imagine applying general relativity to this problem........
10. Isn't this just a simple pressure difference problem? ie as accelerate, air moves back, thus higher pressure back than forward, thus ballon moves forward.
11. This is just inertia. The balloon resists the motion. Relatively speaking it seems to the driver that the balloon experiences a force ma in the opposite direction to the acceleration.
12. (Original post by AntiMagicMan)
This is just inertia. The balloon resists the motion. Relatively speaking it seems to the driver that the balloon experiences a force ma in the opposite direction to the acceleration.

"When you accelerate, the balloon moves forwards, and similarly when you turn, say, left the balloon leans left."

not opposite, in the same direction

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