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

1. (Original post by Puddles the Monkey)
If there was no air, the balloon would move downwards?
That's what I'd say I don't know if there's a difference between air being around and an air pressure differential being around
2. (Original post by purple-duck)
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?

If you've genuinely done first year physics at university I'm shocked you've not covered effective density. What are they teaching kids nowadays?

Think of the analogy with electric charge. If you have a positive charge, negative charges will feel an attractive force, positive charges will feel a repulsive force.

Well with gravity, objects with positive effective density feel an attractive force, objects with negative effective density feel a repulsive force.
3. (Original post by purple-duck)
That's what I'd say I don't know if there's a difference between air being around and an air pressure differential being around
Imagine a scenario in which a bubble of helium is surrounded by a medium of a homogeneous fluid with a density greater than helium, under the force of gravity.

homogeneous = no pressure differentials.

What would the force on the helium bubble be and why?
4. (Original post by cole-slaw)
If you've genuinely done first year physics at university I'm shocked you've not covered effective density. What are they teaching kids nowadays?

Think of the analogy with electric charge. If you have a positive charge, negative charges will feel an attractive force, positive charges will feel a repulsive force.

Well with gravity, objects with positive effective density feel an attractive force, objects with negative effective density feel a repulsive force.
I've done first year warwick maths, and chose the physics classical mechanics (and special relativity, but I don't think that'll help here ) module as an option. So far as I remember they weren't mentioned I got a good first in the module, but then it was more mathsy than physicsy, I suppose.

I've always been taught that gravity is a strictly attractive force - at least in the 3 physics modules we did it was taught as one.

(Original post by cole-slaw)
Imagine a scenario in which a bubble of helium is surrounded by a medium of a homogeneous fluid with a density greater than helium, under the force of gravity.

homogeneous = no pressure differentials.

What would the force on the helium bubble be and why?
Hmm. I'm afraid I haven't a clue what a pressure differential is (although I could make a guess)- I would guess you're meaning a substance which has the same pressure as the helium in the balloon, but has a greater mass/weight?

I'm not sure what would happen in that situation. I'm trying to decide whether its even possible to have such a surrounding material? I'm sketchy on the gas laws, but I would have thought that pressure is proportional to density - so you wouldn't be able to have a fluid with greater density without it having a higher pressure than the helium (?)
So, at least how I would explain it under a purely attractive gravitational force, the helium balloon would rise because of the pressure of this new fluid being greater than the pressure of the balloon. Maybe.

Though then I realise that that maybe doesn't give a reason for why the balloon should rise - the fluid could push it down? I'm going to assume that something to do with gases/stuff means that lower pressure always feels a force upwards (when surrounded by higher pressure)

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5. (Original post by purple-duck)
I've done first year warwick maths, and chose the physics classical mechanics (and special relativity, but I don't think that'll help here ) module as an option. So far as I remember they weren't mentioned I got a good first in the module, but then it was more mathsy than physicsy, I suppose.

I've always been taught that gravity is a strictly attractive force - at least in the 3 physics modules we did it was taught as one.

Hmm. I'm afraid I haven't a clue what a pressure differential is (although I could make a guess)- I would guess you're meaning a substance which has the same pressure as the helium in the balloon, but has a greater mass/weight?

I'm not sure what would happen in that situation. I'm trying to decide whether its even possible to have such a surrounding material? I'm sketchy on the gas laws, but I would have thought that pressure is proportional to density - so you wouldn't be able to have a fluid with greater density without it having a higher pressure than the helium (?)
So, at least how I would explain it under a purely attractive gravitational force, the helium balloon would rise because of the pressure of this new fluid being greater than the pressure of the balloon. Maybe.

Though then I realise that that maybe doesn't give a reason for why the balloon should rise - the fluid could push it down? I'm going to assume that something to do with gases/stuff means that lower pressure always feels a force upwards (when surrounded by higher pressure)

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No when we talk about pressure differentials, we mean that the pressure varies within the surrounding fluid.

If there was a pressure difference between the helium balloon/bubble and the medium, which is what you are describing, that would cause it to expand or contract, not rise or fall.

Think about it: pressure differences are isotropic, how could that be related to a vector force? It doesn't make sense.
6. If you dropped 2 spheres with negative charge of -1 into a negatively charged fluid of charge -2, and allowed them to move freely, would they move towards or away from each other?
7. (Original post by cole-slaw)
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.
This implies to me that gravity still attracts the helium balloon - right?

It's just that if the balloon is surrounded by something heavier than itself the other stuff will push the lighter thing upwards?
8. (Original post by cole-slaw)
No when we talk about pressure differentials, we mean that the pressure varies within the surrounding fluid.

If there was a pressure difference between the helium balloon/bubble and the medium, which is what you are describing, that would cause it to expand or contract, not rise or fall.

Think about it: pressure differences are isotropic, how could that be related to a vector force? It doesn't make sense.
Ah I see now - understand now what this differential business means!

In which case I'm not really sure what would happen - I'd like to say that the balloon doesn't really do much/anything - I can't see why the net force would be up, though I suppose the fluid has a greater force downwards - so possibly the fluid having a greater force down means that the helium is somehow pushed up? I'm not entirely sure.

(Original post by cole-slaw)
If you dropped 2 spheres with negative charge of -1 into a negatively charged fluid of charge -2, and allowed them to move freely, would they move towards or away from each other?
I'm going to say they move away from each other. The fluid is everywhere around them so can't really do anything? Like if you're being pushed by 2 people either side of you. But then the other sphere is an extra force pushing it, so they'll repel still.
At least that's how I'm trying to think about it...
9. (Original post by Puddles the Monkey)
This implies to me that gravity still attracts the helium balloon - right?

It's just that if the balloon is surrounded by something heavier than itself the other stuff will push the lighter thing upwards?
I don't think its a helpful or particularly accurate way of looking at it because it credits the medium with a far more active role than is strictly correct, as opposed to simply considering the relative density, but basically, yes.

I also don't think its strictly accurate to say that gravity does attract the helium balloon. On the contrary, it repels it.
10. (Original post by purple-duck)
Ah I see now - understand now what this differential business means!

In which case I'm not really sure what would happen - I'd like to say that the balloon doesn't really do much/anything - I can't see why the net force would be up, though I suppose the fluid has a greater force downwards - so possibly the fluid having a greater force down means that the helium is somehow pushed up? I'm not entirely sure.

I'm going to say they move away from each other. The fluid is everywhere around them so can't really do anything? Like if you're being pushed by 2 people either side of you. But then the other sphere is an extra force pushing it, so they'll repel still.
At least that's how I'm trying to think about it...
But they would both repel the fluid more than they repel each other... meaning... they would move together...

11. (Original post by Puddles the Monkey)
This implies to me that gravity still attracts the helium balloon - right?

It's just that if the balloon is surrounded by something heavier than itself the other stuff will push the lighter thing upwards?
That's what I'd say

(Original post by cole-slaw)
But they would both repel the fluid more than they repel each other... meaning... they would move together...

Ah okay - so the spheres aren't point charges/they take up space where the -2 fluid would have been.
I was thinking of them more like point charges, I think.

I still don't agree/can't get my head around this gravity repels thing - I'm not sure comparing it to charges helps.
So far as I can tell from a quick trawl through the internet - gravity only repels in very odd circumstances/with dark energy and stuff () - I.e. it won't ever repel in a hypothetic experiment involving a helium balloon and some fluid
12. (Original post by purple-duck)
That's what I'd say

Ah okay - so the spheres aren't point charges/they take up space where the -2 fluid would have been.
I was thinking of them more like point charges, I think.

I still don't agree/can't get my head around this gravity repels thing - I'm not sure comparing it to charges helps.
So far as I can tell from a quick trawl through the internet - gravity only repels in very odd circumstances/with dark energy and stuff () - I.e. it won't ever repel in a hypothetic experiment involving a helium balloon and some fluid
We just agreed that two like charges can be attracted towards each other because of a charged medium, so in exactly the same way, gravity can be repulsive in a dense medium. Can you see that?
13. (Original post by cole-slaw)
We just agreed that two like charges can be attracted towards each other because of a charged medium, so in exactly the same way, gravity can be repulsive in a dense medium. Can you see that?
Not really, sorry - Gravity still acts by F = GMm/r^2 - in the direction of mass M (earth) - in no way does the gravitational force on the helium balloon repel the balloon away/make it rise.

Disagree with that and you disagree with newton
14. (Original post by purple-duck)
Not really, sorry - Gravity still acts by F = GMm/r^2 - in the direction of mass M (earth) - in no way does the gravitational force on the helium balloon repel the balloon away/make it rise.

Disagree with that and you disagree with newton
I completely agree with you. It seems like the poster is getting confused (or trolling the interpretation) with the negative terms used in atmospheric pressure models:

where P = pressure, z = height, = density, g = acceleration due to gravity.

solving yields where

i.e. the -ve sign in the first equation signifies a negative gradient so that pressure decreases with height and in no way is it meant to imply that gravity is a repulsive force which is simply not true.
15. (Original post by uberteknik)
I completely agree with you. It seems like the poster is getting confused (or trolling the interpretation) with the negative terms used in atmospheric pressure models:

where P = pressure, z = height, = density, g = acceleration due to gravity.

solving yields where

i.e. the -ve sign in the first equation signifies a negative gradient so that pressure decreases with height and in no way is it meant to imply that gravity is a repulsive force which is simply not true.
I assure you I am not getting confused. I am simply attempting to explain to an undergraduate mathematician a few pieces of elementary physics.

If you don't understand it either, that's fine, I am happy to explain. But don't claim that you know everything when you clearly don't.
16. (Original post by purple-duck)
Not really, sorry - Gravity still acts by F = GMm/r^2 - in the direction of mass M (earth) - in no way does the gravitational force on the helium balloon repel the balloon away/make it rise.

Disagree with that and you disagree with newton

But if m is effectively negative, as it is in the case of a helium balloon surrounded by air, then it acts in the opposite direction, you see?
17. (Original post by cole-slaw)
But if m is effectively negative, as it is in the case of a helium balloon surrounded by air, then it acts in the opposite direction, you see?
You can't have negative mass

If what you're doing involves negative mass then it is not elementary physics though - maybe super advanced weird physics with blackholes and stuff, but not elementary (or at least I'd say so)

From everything I've learnt this just seems wrong and not true - university lecturers have never given any hint that gravity would ever repel - it is an attracting force. Nor has anyone mentioned negative mass - if you get a negative mass from an equation or something, then it's always an indicator I've mucked up the calculation

I'm sorry I'm not understanding Where did you learn this/could we read anything written about it?

(Original post by uberteknik)
I completely agree with you. It seems like the poster is getting confused (or trolling the interpretation) with the negative terms used in atmospheric pressure models:

where P = pressure, z = height, = density, g = acceleration due to gravity.

solving yields where

i.e. the -ve sign in the first equation signifies a negative gradient so that pressure decreases with height and in no way is it meant to imply that gravity is a repulsive force which is simply not true.
I'm glad I'm not going crazy with gravity-ness

So that is the pressure differential everyone is talking about
18. (Original post by purple-duck)
You can't have negative mass

If what you're doing involves negative mass then it is not elementary physics though - maybe super advanced weird physics with blackholes and stuff, but not elementary (or at least I'd say so)

From everything I've learnt this just seems wrong and not true - university lecturers have never given any hint that gravity would ever repel - it is an attracting force. Nor has anyone mentioned negative mass - if you get a negative mass from an equation or something, then it's always an indicator I've mucked up the calculation

I'm sorry I'm not understanding Where did you learn this/could we read anything written about it?

I'm glad I'm not going crazy with gravity-ness

So that is the pressure differential everyone is talking about

Not negative mass, negative effective mass. We're not talking about dark matter, we're talking about helium balloons in air or air bubbles in water, both of which rise under gravity. Really everyday stuff.

Just like how we agreed that two like charges can attract, two masses can repel by the same logic, depending on their properties relevant to the medium.
19. (Original post by cole-slaw)
Not negative mass, negative effective mass. We're not talking about dark matter, we're talking about helium balloons in air or air bubbles in water, both of which rise under gravity. Really everyday stuff.

Just like how we agreed that two like charges can attract, two masses can repel by the same logic, depending on their properties relevant to the medium.
But gravity isn't calculated/influenced by effective mass, it's influenced by the mass.
Both rise under the force of gravity, yes - but neither rise directly by gravity, as in, there isn't a force directly between the balloon/bubble and the earth that pushes the balloon/bubble upwards.

I don't think it's anything to do with logic though The fact is that gravity isn't the same as magnetic force, in that the influencing "charge" for gravitational force is mass - which can (so far as we know) only be positive, hence why it is a purely attractive force.

I'm confused how you can have a repelling force of gravity, and still agree with the gravitational force equation. The fact of the matter is, that if you did put a repelling force into the equation, you would get out a negative mass of the object. Which is wrong - which means that the repelling force can't be gravity.
20. (Original post by purple-duck)
But gravity isn't calculated/influenced by effective mass, it's influenced by the mass.
Both rise under the force of gravity, yes - but neither rise directly by gravity, as in, there isn't a force directly between the balloon/bubble and the earth that pushes the balloon/bubble upwards.

I don't think it's anything to do with logic though The fact is that gravity isn't the same as magnetic force, in that the influencing "charge" for gravitational force is mass - which can (so far as we know) only be positive, hence why it is a purely attractive force.

I'm confused how you can have a repelling force of gravity, and still agree with the gravitational force equation. The fact of the matter is, that if you did put a repelling force into the equation, you would get out a negative mass of the object. Which is wrong - which means that the repelling force can't be gravity.
Of course you use effective mass.

In simplified cases that you may have studied in your introductory courses, mass and effective mass are the same thing. But in this case they are not, so you have to think a little bit more carefully than simply blindly lumping numbers into an equation.

The gravitational force is given by the 3-integral of the effective density distribution within the gravitational vector field. (You can use the 4-interval if you want to get into GM and tensor analysis but its not necessary). Your equation is simply a simplification of this.

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