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

Original post by cole-slaw

Q) What is a repulsive force
A) Any force which induces objects to be pushed apart

Consider a system in equilibrium, a helium bubble encased in a homogeneous fluid. No gravitational force is felt, the system is completely in eqbm.

Now turn on gravity, what happens to the bubble? It moves AWAY from the source of the gravity. the movement can't have been caused by of any other force because the system was in equilibrium. So gravity has pushed the helium bubble away.

BY DEFINTION, the new force that has disturbed the equilibrium- gravity - must have exerted a repulsive force on the helium bubble.

When you switch the gravity on it acts on the 'heavier' fluid cause it to become more dense to the bottom and less dense at the top.The bubble seeks equilibrium and therefore rises to the less dense fluid, that is up.

General Relativity, which you have introduced into this scenario shows negative gravity is possible and that pressure is a factor in the calculations.
But just as SR is only of consideration at speeds approaching c, so GR and in particular negative gravity is only significant with massive systems. An Earth/balloon frame of reference is hardly a massive system

Reply 103

Original post by mphysical

When you switch the gravity on it acts on the 'heavier' fluid cause it to become more dense to the bottom and less dense at the top.The bubble seeks equilibrium and therefore rises to the less dense fluid, that is up.

fluid is homogeneous and remains so. Try again.

Reply 104

Original post by cole-slaw

fluid is homogeneous and remains so. Try again.

All your experiment proves is that Archimedes principle does not work in zero gravity because there is no difference in weight.between objects.

Homogenous or not there there is still a pressure differential due to gravity.
My error in quoting density, but come on, do you have to be so obnoxious when debating?

(edited 9 years ago)

Reply 105

Original post by mphysical

All your experiment proves is that Archimedes principle does not work in zero gravity because there is no difference in weight.between objects.

There is no need for GR.

I have no idea what you're on about. There is no need for GR, I don't know why you're bringing it up.

A helium bubble enclosed in a homogeneous medium of greater density in a gravitational field will feel an upwards force, true or false?

Reply 106

Original post by cole-slaw

I have no idea what you're on about. There is no need for GR, I don't know why you're bringing it up.

A helium bubble enclosed in a homogeneous medium of greater density in a gravitational field will feel an upwards force, true or false?

Will you stop asking people questions, it sounds so silly.

It will feel an upward force due to buoyancy caused by gravity.

You brought up GR with negative gravity and pressure.

Reply 107

Original post by mphysical

Will you stop asking people questions, it sounds so silly.

It will feel an upward force due to buoyancy caused by gravity.

You brought up GR with negative gravity and pressure.

So you finally agree it will feel an upward force due to gravity, thank **** for that.

Reply 108

Original post by cole-slaw

So you finally agree it will feel an upward force due to gravity, thank **** for that.

No, the upward force is due to buoyancy which is created by gravity.
There is no negative gravity in this situation

Reply 109

DrQuantum

Think of the car braking causing perceived gravity towards the front of the car with a tiny component downwards. In normal gravity balloons will move in an opposite direction to gravity due to there being less air pressure away from the direction of gravity. Because of this the balloon moves away. - Buoyancy acts to oppose gravity and according the Einstein being in an elevator accelerating up at 9.8ms^-1 in space is the same as standing on a still elevator on earth.

Note this experiment only works in a pretty confined space as it relies on the gas in the car creating a pressure gradient. If you ran outside with a balloon this stuff wouldn't occur so you would have the effect you would normally expect:

https://www.youtube.com/watch?v=jyS3argEKmU

(edited 9 years ago)

Reply 110

Original post by mphysical

No, the upward force is due to buoyancy which is created by gravity.
There is no negative gravity in this situation

The fact that you can't see that those statements are effectively identical suggests that physics is possibly not the subject for you...

Reply 111

0le

21

Original post by cole-slaw

The fact that you can't see that those statements are effectively identical suggests that physics is possibly not the subject for you...

I personally think you believe you have a "better" understanding of a situation because you perceive the same phenomenon in a different way. The truth is, the person you quoted is correct and you just have an unusual way of looking at the problem. It doesn't make you or the other person wrong. Quite frankly, no one cares how you, or anyone else views any problem. So long as you understand it, others understand it and the OP understands it then what is the problem? Its actually a terrible thing to imply (which is my opinion of what you are saying) that there is only one way to view a physical problem.

You shouldn't tell people what is and what isn't suited to them. Others could easily turn the tables and mention you doing economics and suggest physics wasn't for you either...

Reply 112

Original post by djpailo

I personally think you believe you have a "better" understanding of a situation because you perceive the same phenomenon in a different way. The truth is, the person you quoted is correct and you just have an unusual way of looking at the problem. It doesn't make you or the other person wrong. Quite frankly, no one cares how you, or anyone else views any problem. So long as you understand it, others understand it and the OP understands it then what is the problem? Its actually a terrible thing to imply (which is my opinion of what you are saying) that there is only one way to view a physical problem.

You shouldn't tell people what is and what isn't suited to them. Others could easily turn the tables and mention you doing economics and suggest physics wasn't for you either...

Several people have claimed that it is pressure differentials that cause helium balloons to rise. That is simply incorrect. I cannot leave that uncorrected.

Reply 113

Stonebridge

13

http://www.thestudentroom.co.uk/showthread.php?t=1375220

The answer to this question now is the same as it was in the thread above from a few years back.
The principle is the same for the balloon in the accelerating car as it is for the balloon rising in still air. Buoyancy. Archimedes Principle.
Check out any basic physics text book for an analysis of buoyancy in terms of pressure differential.

Reply 114

uberteknik

21

Original post by cole-slaw

Several people have claimed that it is pressure differentials that cause helium balloons to rise. That is simply incorrect. I cannot leave that uncorrected.

What part of this do you not understand?

Pressure exerted on the walls of a container by a gas is defined as force exerted per unit area.

Gravitational acceleration acts on the atmopshere to create a pressure gradient starting at the earths surface (the surface provides a mass for the accelerting air molecules to react against.) i.e. the air has weight.

The difference in pressure between the top and bottom of the balloon creates a net force imbalance enough to overcome the weight of the helium filled balloon and it rises.

The three components needed for the effect are:

Any contained matter whose mass (including container) is less than the volume of air it displaces;
An acceleration force acting on either: the air and the balloon, or, a surface in contact with the static air;
Somethng for the air to react against (rear of car cabin or surface of the earth).

Reply 115