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

1. (Original post by uberteknik)
Every molecule of air has mass and therefore exhibits both properties of inertia and momentum. (Newton's laws - an object continues in its line of motion or rest unless acted upon by an external force).

As the car accelerates forward, the air inside the car becomes compressed towards the rear of the car because of inertia. This creates a higher pressure region of air towards the rear of the cabin and a lower pressure region towards the front. That pressure differential causes the balloon to move towards the front of the cabin under acceleration.

Once the acceleration stops and the car moves at a constant velocity, the air pressure will equalise and the balloon will revert to the upright after some oscillation.

Reverse the process for deceleration and the inertia of the air inside the car, causes a higher pressure region to develop to the front of the cabin with a consequential force pushing the balloon to the rear.

Turning the car causes the air inside to compress against the outside radius of the turning circle and hence the pressure now pushes the balloon towards the centre of rotation and will continue to dos so for as long as the car keeps turning.
How does this relate to gravity?
2. (Original post by cole-slaw)
Oh dear oh dear.
You seem confused.
The post following yours [uberteknik] seems to agree with my version.
I am fascinated to hear your explanation of how gravity forces a balloon up

By the way. In my last post where I stated hydraulic, I think I meant pneumatic.
3. (Original post by mphysical)
You seem confused.
The post following yours [uberteknik] seems to agree with my version.
I am fascinated to hear your explanation of how gravity forces a balloon up
The same reason gravity forces a brick down. It pulls on it with tiny little bosonic particles called gravitons.

With a brick the interactions are stronger than that with the surrounding air, so the brick wins the race to the floor. Hooray!

With a helium balloon, the interactions are weaker than with the surrounding air, so the poor old balloon gets elbowed aside by the naughty air particles and pushed away from the earth! Oh no
4. (Original post by cole-slaw)
The same reason gravity forces a brick down. It pulls on it with tiny little bosonic particles called gravitons.

With a brick the interactions are stronger than that with the surrounding air, so the brick wins the race to the floor. Hooray!

With a helium balloon, the interactions are weaker than with the surrounding air, so the poor old balloon gets elbowed aside by the naughty air particles and pushed away from the earth! Oh no
Is that the best you can do? Why introduce a theoretical particle when the force of gravity is explanation enough? Do you actually understand the force of gravity? Please explain how gravity forces the balloon up? Your explanation is very poor.
5. (Original post by mphysical)
A helium balloon rises because it is less dense than the surrounding air.
It will continue to rise until it meets an atmospheric pressure equal to the balloon/helium combination.

I am not sure why you believe gravity forces the balloon up.
Newton asked a similar question 400 years ago.

Granted, the air is more dense at lower altitudes because of the force of gravity.
But the only gravitational force on a free balloon is pulling it down.

Yes, in a car the force produced when turning, braking accelerating all mimic gravity and will act on the balloon until the much greater effect of air pressure takes over.
I think what the poster is saying (not very well) is that gravity causes the air to pile up at the surface of the earth. This creates an air pressure differential with pressure decreasing as height above the surface increases.

Since the pressure force acting on the balloon is always towards lower pressure, the balloon must experience an upwards force and hence rises.
6. (Original post by uberteknik)
I think what the poster is saying (not very well) is that gravity causes the air to pile up at the surface of the earth. This creates an air pressure differential with pressure decreasing as height above the surface increases.

Since the pressure force acting on the balloon is always towards lower pressure, the balloon must experience an upwards force and hence rises.
This is correct.

So why was cole-slaw wittering on about the battle of gravitons and air molecules.
Unnecessary prattle. The balloon rises because the force of the pressure differential is greater than gravitational attraction.
7. (Original post by mphysical)
Is that the best you can do? Why introduce a theoretical particle when the force of gravity is explanation enough? Do you actually understand the force of gravity? Please explain how gravity forces the balloon up? Your explanation is very poor.
At least its correct, unlike yours.
8. (Original post by cole-slaw)
At least its correct, unlike yours.
You have still not explained how gravity forces a balloon up.
Use Newtonian mechanics and leave QM out if it. You are impressing no-one.
9. (Original post by mphysical)
You have still not explained how gravity forces a balloon up.
Use Newtonian mechanics and leave QM out if it. You are impressing no-one.
Mate, get over it and leave out the petty childish insults. You're entirely wrong, and when you do your A-levels you will understand why.

Are you this rude to all your teachers?
10. (Original post by mphysical)
You have still not explained how gravity forces a balloon up.
Use Newtonian mechanics and leave QM out if it. You are impressing no-one.
(Original post by cole-slaw)
Mate, get over it and leave out the petty childish insults. You're entirely wrong, and when you do your A-levels you will understand why.

Are you this rude to all your teachers?
11. (Original post by Puddles the Monkey)
The guy denies the effect of the force of gravity on objects in the atmosphere. He's clearly either trolling you or at a very low level of physics education and probably shouldn't be commenting. Its just bizarre.
12. (Original post by cole-slaw)
The guy denies the effect of the force of gravity on objects in the atmosphere. He's clearly either trolling you or at a very low level of physics education and probably shouldn't be commenting. Its just bizarre.
Its that your explanation seems to imply that the force of gravity acts upward, but only against balloons, and on everything else gravity acts downward. It seems bizarre too, though I'm sure that its not what you mean to imply (?)
From what I remember, gravity always acts downwards on a mass - unless I missed something in my classical mechanics module this year (which is possible, I guess )
13. (Original post by purple-duck)
Its that your explanation seems to imply that the force of gravity acts upward, but only against balloons, and on everything else gravity acts downward. It seems bizarre too, though I'm sure that its not what you mean to imply (?)
From what I remember, gravity always acts downwards on a mass - unless I missed something in my classical mechanics module this year (which is possible, I guess )
This is my point. I am not looking for a fight.
I am waiting for an explanation as to how gravity can exert a negative force and make the balloon go up, when obviously the air pressure differential forces the balloon up.
14. (Original post by purple-duck)
Its that your explanation seems to imply that the force of gravity acts upward, but only against balloons, and on everything else gravity acts downward. It seems bizarre too, though I'm sure that its not what you mean to imply (?)
From what I remember, gravity always acts downwards on a mass - unless I missed something in my classical mechanics module this year (which is possible, I guess )

The force of gravity DOES act upwards. Gravity is negative, the effective density of a helium balloon is negative, negative times negative is positive, hence gravity acts upwards.
15. (Original post by mphysical)
This is my point. I am not looking for a fight.
I am waiting for an explanation as to how gravity can exert a negative force and make the balloon go up, when obviously the air pressure differential forces the balloon up.
You realise that if for some reason there was no air pressure differential across different altitudes, a helium balloon would still rise under the force of gravity, right?

Or are you continuing to deny basic physics?
16. (Original post by cole-slaw)
The force of gravity DOES act upwards. Gravity is negative, the effective density of a helium balloon is negative, negative times negative is positive, hence gravity acts upwards.
This is where I just get confused.

The weight/gravitational force on the helium balloon is calculated by its mass (positive) and the gravitational constant, g. And this force of gravity/weight points, and acts, downwards.
So if I were to draw a force diagram, I'd have an arrow down for the balloons weight, and then a bigger arrow for the air pressure/upwards force/something, so that overall the balloon just has a force upwards.

I guess maybe the "effective density" is something that takes into account surrounding air and air pressure/other stuff?
17. (Original post by purple-duck)
This is where I just get confused.

The weight/gravitational force on the helium balloon is calculated by its mass (positive) and the gravitational constant, g. And this force of gravity/weight points, and acts, downwards.
So if I were to draw a force diagram, I'd have an arrow down for the balloons weight, and then a bigger arrow for the air pressure/upwards force/something, so that overall the balloon just has a force upwards.

I guess maybe the "effective density" is something that takes into account surrounding air and air pressure/other stuff?

At GCSE or maybe A level, you might just forget about the air, and model the effects of gravity as if the balloon is surrounded by a vacuum.

But if you want to do the calculation thoroughly, you have to take the air into effect. A helium balloon has a lower density than the air around it, so it has a negative effective density, meaning it feels an upwards force from gravity, because - times - equals +. So you would just draw a big arrow upwards.

Its the same gravitational force that acts on an air bubble in water. They also go upwards because they have negative effective density in that medium.
18. (Original post by cole-slaw)
At GCSE or maybe A level, you might just forget about the air, and model the effects of gravity as if the balloon is surrounded by a vacuum.

But if you want to do the calculation thoroughly, you have to take the air into effect. A helium balloon has a lower density than the air around it, so it has a negative effective density, meaning it feels an upwards force from gravity, because - times - equals +. So you would just draw a big arrow upwards.

Its the same gravitational force that acts on an air bubble in water. They also go upwards because they have negative effective density in that medium.
If there was no air, the balloon would move downwards?
19. (Original post by cole-slaw)
At GCSE or maybe A level, you might just forget about the air, and model the effects of gravity as if the balloon is surrounded by a vacuum.

But if you want to do the calculation thoroughly, you have to take the air into effect. A helium balloon has a lower density than the air around it, so it has a negative effective density, meaning it feels an upwards force from gravity, because - times - equals +. So you would just draw a big arrow upwards.

Its the same gravitational force that acts on an air bubble in water. They also go upwards because they have negative effective density in that medium.
For A level and first year Uni Physics modules (what I'm basing my thoughts on) you would not ignore the air, you just wouldn't take it into account when calculating the weight, or force of gravity, on an object - which is calculated by -GMm/r^2 (I think?) Or in this case just mg

You then add the effects of air after calculating the weight.

This is why I believe you are incorrect in your statement that gravity makes the balloon rise - because by all I've learnt, it most certainly doesn't.
I'll agree that the air being there, and the fact that helium is lighter than air, contributes/causes the balloon to rise - but I can't see how gravity would act upwards in any way on the balloon.

Personally I haven't ever used a "negative effective density" - from what you say it sounds like some sort of combination/short cut to calculate the overall force on an object?
20. (Original post by Puddles the Monkey)
If there was no air, the balloon would move downwards?
In a vacuum, yes. It would have positive effective density so the effect of gravity would be to pull it down.

But its worth noting that in a hypothetic scenario in which it was encased in a homogeneous fluid medium with a greater spatial density than helium, it would STILL move upwards. Air pressure differentials are irrelevant.

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