# Balloon in car

1. Dec 2, 2004

### daveed

how come when a car accelerates w/ a helium balloon, the balloon goes forward?

2. Dec 2, 2004

### Averagesupernova

Because a helium filled ballon will always go AWAY from the most dense gas. When you accelerate the air in the back of the car is squeezed together a little more than the air in the front. This happens the same way a bunch of rubber balls would bunch up in the back instead of laying out evenly on the floor when accelerating. I can't say I've ever seen it happen so I wouldn't know if you are lying to me. But if it actually does, that is why.

3. Dec 3, 2004

### ceptimus

It's exactly the same reason that explains why when you accelerate in a car with a half full drink bottle, the air in the bottle moves forward. The denser fluid (the drink) moves to the back, and that means the less dense fluid (the air) has nowhere to go but forward. With the helium balloon, the helium is the least dense, and the surrounding air is denser.

A helium balloon in a vacuum however, would move towards the back.

4. Dec 4, 2004

### Cyrus

A helium balloon in a vacuum would fall to the ground because it has no air to displace in order to float. Its the minor details that always get you. lol

5. Dec 4, 2004

6. Dec 5, 2004

### Physics is Phun

I have a different expanation. If you have a helium balloon and let it go it goes up, because a) it is less dense than the air, and b) because gravity is pulling the balloon and air down down. If you simply had a container of air with an H/balloon in it and it was not in a gravitational field the balloon would not move. So now in a car we are flipping gravity/acceleration on its side. when the car is sitting there or going a constant velocity the balloon does not move horizontally. when you accelerate the balloon moves away from the acceleration just as the balloon rises when let go. it isn't caused by the air in the back being more dense but by the air in general being denser than the balloon and the balloon moving away from the acceleration direction.
I hope I have made this coherent enough to be understandable.

7. Dec 5, 2004

### Cyrus

This is not true, the balloon does not go up because it moves away from gravity/acceleration, which points down. It goes up becasue it displaces the heavier air. Take away the air and the balloon will fall down.

8. Dec 5, 2004

### PhilipF

Kaku says that it is to be explained by the equivalence principal 'the laws of nature in an accelerating frame are equivalent to the laws in a gravitational field'.he says ' imagine a gravitational field pulling on the car from the right . gravity will make us lurch to the right so the balloon which is lighter than air and always floats 'up' i.e. opposite the pull of gravity [or its equivalent] must float to the left into the direction of the swerve.'
so it seems 'Phun' is right

9. Dec 5, 2004

### Cyrus

Maybe I am not interpreting what he stated correctly. I dont follow the authors reasoning. If the car is pulled from the right, we should lurch in reistance to change, to the left, same way your head tilts back and not forward when a car acclerates. I also dont see why he used the word gravity making us lurch to the right. Gravity acts down. Its the accleration that makes a force that acts to the right.

Ah never mind I see what you are saying now. I appologize. Its really quite the same thing though. You are saying this sideways gravity, due to the accelration, pushes the air to the back of the car. and so the lighter balloon moves to an area of lower density, the front of the car. But i see what you two mean now.

Last edited: Dec 5, 2004
10. Dec 5, 2004

### PhilipF

I dont think variation in air density has anything to do with it . What would happen to a helium balloon in a falling lift or in the orbiting space shuttle?

11. Dec 5, 2004

### Cyrus

For a falling lift, provided that it is tied down, I would think that the balloon floats up, and it gets jerked downwards at the lift descents. In a space shuttle, I think that the air in there is pressurized to atmosphereic pressure. I wonder if the pressurized air tends to fall to the floor of the shuttle, becuase there is less gravity at that high an altitude, or does it remain fairly uniform. IF it stays uniform, I would expect the balloon to displace the air just the same as on the ground and go to the roof. If not, I would imagine that the balloon floats up to a certain heigh and stays there.

12. Dec 5, 2004

### Averagesupernova

The balloon does nothing unless there is gravity or the air in the enclosure is being accelerated in a certain direction. So, in the weightless environment of the shuttle, the balloon would just float along with everything else. Bottom line is that the densest matter displaces the least dense matter out away from the direction that all matter is being pulled.

13. Dec 5, 2004

### Cyrus

that sounds reasonable. thanks average. But I wonder, why the dense matter moves to the back of the car more favorably. The heavier the air particles, the more resistant to change they will be. Also, as the heavier air is being sent to the back, the frontal area of the balloon will have a force due to the air trying to go back also. I wonder why this does not push the balloon back.

Last edited: Dec 5, 2004
14. Dec 5, 2004

### Physics is Phun

I still don't think you get it cyrusa. It doesn't have to do with the air at the back of the car being denser. I only has to do with the fact that the air is denser than the balloon, and since everything accelerates backwards the balloon floats up ( in this case forward.) The subtle differences if any of air density does not affect the balloon.

15. Dec 5, 2004

### Physics is Phun

Here is a good experiment for you. Take a pop,(something carbonated) and pour it into a transparent glass. Now, as the bubbles rise, accelerate the glass. You will notice that the bubbles do not go backward but indeed forward in the direction you are accelerating. So there is an force backwards therefore the bubbles go in the forward direction because they are less dense than the pop.

16. Dec 5, 2004

### Cyrus

Ah I see what you mean. Thanks that clears things up. Even in the case when the balloon rises in normal atmosphere, there is a force acting down. This is equivalent to the case with the sideways accleration.

17. Dec 6, 2004

### DaveC426913

"...why the dense matter moves to the back of the car more favorably. The heavier the air particles, the more resistant to change they will be..."

The air particles are not "heavy". The mass of air near the back of the car gets dense, as air piles on top of it. The helium ballon would "like" to fall back, but loses the fight with the air, so it gets pushed out of the way.

"...Also, as the heavier air is being sent to the back, the frontal area of the balloon will have a force due to the air trying to go back also. I wonder why this does not push the balloon back..."

There is indeed, pressure on the balloon, pushing it backwards. Of course, there is also pressure on the air all around the balloon, pushing the air backwards too. The air wins the race.

Imagine the car filled with one helium balloon amongst one hundred balloons filled with regular air. The air balloons, being heavier, will fall towards the back of the car moreso than the heliuum balloon will.