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# Please can someone explain the classic helium balloon in a car? Watch

1. What sorcery is this?!

Thank you!

(Original post by Motorbiker)
x
2. O_O
3. (Original post by Puddles the Monkey)

What sorcery is this?!

Thank you!
Normally when accelerating, you are thrown backwards. I assume the helium balloon goes forwards because it is lighter than the surrounding air, and so the air going back wrt to it makes it go forwards.
4. (Original post by Puddles the Monkey)

What sorcery is this?!

Thank you!
Isn't it down to the balloon moving at a slower velocity to the car so as such it's now experiencing the movement in that particular direction?
5. Is this not what normally happens to people in cars, but the helium balloon has less mass and so the effects are more obvious??

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6. Saw this a while back. Hope it helps.
7. (Original post by rayquaza17)
Is this not what normally happens to people in cars, but the helium balloon has less mass and so the effects are more obvious??

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I normally get pushed backwards into the seat when a car accelerates. Heavy breaking throws you forwards.

The balloon goes in the opposite direction!
8. Air is a fluid so sloshes around with the movement of the vehicle and is still moving when the vehicle stops, more so than the occupants because it is lighter.
As the car accelerates the air is being forced (more dense) into the rear of the vehicle
The balloon moves towards the less dense air in the car (front) because the higher density air forces it that way.
9. (Original post by Puddles the Monkey)

What sorcery is this?!

Thank you!
When you brake all the air rushes to the front, pushing the balloon backwards.
10. (Original post by Puddles the Monkey)
I normally get pushed backwards into the seat when a car accelerates. Heavy breaking throws you forwards.

The balloon goes in the opposite direction!
You're heavier than air.

The balloon is lighter than air.

Heavier things go forward, therefore logically, lighter things must go backwards.
11. (Original post by mphysical)
Air is a fluid so sloshes around with the movement of the vehicle and is still moving when the vehicle stops, more so than the occupants because it is lighter.
As the car accelerates the air is being forced (more dense) into the rear of the vehicle
The balloon moves towards the less dense air in the car (front) because the higher density air forces it that way.

This is the only explanation i've found that makes sense. It's what i was told yesterday as well.
12. (Original post by Motorbiker)
This is the only explanation i've found that makes sense. It's what i was told yesterday as well.

Unfortunately its *******s.

If the air was "sloshing around" as the car moved, you would feel it, wouldn't you. It would be like being outside on a windy day.

Gravity and acceleration are equivalent. Hence anything that rises under the force of gravity is correspondingly going to go in the opposite direction under acceleration.
13. (Original post by cole-slaw)
Unfortunately its *******s.

If the air was "sloshing around" as the car moved, you would feel it, wouldn't you. It would be like being outside on a windy day.

Gravity and acceleration are equivalent. Hence anything that rises under the force of gravity is correspondingly going to go in the opposite direction under acceleration.

Well, air obviously is moving around somewhat as that's how gases work.

That final part makes good sense though.
14. (Original post by Motorbiker)
Well, air obviously is moving around somewhat as that's how gases work.

That final part makes good sense though.

If there was no balloon in the air, would the air move? If so in which direction?
15. (Original post by cole-slaw)
If there was no balloon in the air, would the air move? If so in which direction?

Would there not still be a higher density of air briefly in the back of the car during acceleration?
16. (Original post by cole-slaw)
Gravity and acceleration are equivalent. Hence anything that rises under the force of gravity is correspondingly going to go in the opposite direction under acceleration.
I swear this doesn't make any sense
The balloon is still pulled down by its weight - it isn't some weird opposite force object!?
By that argument if you blow the balloon (apply a force/acceleration to it) then it should go towards your mouth/where you're blowing from. I don't think that would happen
17. (Original post by Motorbiker)
Would there not still be a higher density of air briefly in the back of the car during acceleration?
A very, very slight one. The accelerative force backwards would not override the gas pressure enough to maintain a significant pressure gradient from back to front.

Assuming that some physical objects in the car move backwards during acceleration, the net movement of the air molecules is probably actually forward.
18. (Original post by purple-duck)
I swear this doesn't make any sense
The balloon is still pulled down by its weight - it isn't some weird opposite force object!?
By that argument if you blow the balloon (apply a force/acceleration to it) then it should go towards your mouth/where you're blowing from. I don't think that would happen

Gravity and the effects of accelerating references frames interact with the mass of the constituent particles of the balloon, whereas applying a convective force onto a localised area of the balloon's periphery is a surface interaction. So they don't correspond.

It doesn't matter what is inside the balloon when you blow on it (ie create a pressure gradient across it), the force created will be identical (although the balloons acceleration will be given by F/m of course).
19. (Original post by cole-slaw)
Unfortunately its *******s.

If the air was "sloshing around" as the car moved, you would feel it, wouldn't you. It would be like being outside on a windy day.

Gravity and acceleration are equivalent. Hence anything that rises under the force of gravity is correspondingly going to go in the opposite direction under acceleration.
You're sort of half right. The balloon rises in air and moves forward in an accelerating car for the same reason - the air around the balloon is heavier than it, and therefore is more affected by the acceleration of the car and acceleration due to gravity. In both scenarios the acceleration causes a larger force to be put on the air than on the balloon. A force is being put on the balloon to move backwards or down, but more​ force is being put on the air, so the air pushes the balloon out of the way and forwards or upwards.
20. (Original post by Puddles the Monkey)
I normally get pushed backwards into the seat when a car accelerates. Heavy breaking throws you forwards.

The balloon goes in the opposite direction!
Have you ever stirred a glass of water with ice in it? The ice goes the wrong way, because the surrounding fluid is denser and is pushed more.

What is surrounding the helium balloon? Why woudn't they have used a different type of balloon? What differs between you and helium?

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