Is this an easy question?

[Kedas]

The air filled balloon will move back relative to the car. Newton's first law.

 

so you do realize now that a helium filled balloon will not certainly move to the front of the car? (it will depend on the weight of the ballon in relation to its size)

[jordan]

Now you've got me stuck on a question, Kedas.

 

I understand that the helium balloon would move forward in the van with the air pressure and stuff, but why does it do so outside? If the air pressure is equal all around it, how does it always go straight up? I know there's got to be a simple answer, but I'm drawing a blank right now.

[Kedas]

Now you've got me stuck on a question' date=' Kedas.

 

I understand that the helium balloon would move forward in the van with the air pressure and stuff, but why does it do so outside? If the air pressure is equal all around it, how does it always go straight up? I know there's got to be a simple answer, but I'm drawing a blank right now.[/quote']

 

Because the pressure at the bottom of the balloon is bigger than at the top, same like water.

about the same effect but horizontal if the car would accelarate with 9.8m/s² (g)

[swansont]

so you do realize now that a helium filled balloon will not certainly move to the front of the car? (it will depend on the weight of the ballon in relation to its size)

 

Well, I've assumed the balloon was not made out of lead, or anything similar. But the original question strongly implied that thw whole system was less dense than air. Under those circumstances it will move toward the front. Or toward the inside, in bloodhound's case of circular motion.

[Kedas]

anyone feeling lucky to write a formula down to know what the weight of the ballon can be in a certain situation?

[Severian]

I think you forgot a force/mass:

 

No I didn't. I was answering the question of why does the balloon rise. Any motion tangentional to the direction the frame is being accelerated is unaffected by that accelaration.

 

so what you say is a balloon filled with air will not move in the car?

then what is accelerating the balloon itself to math the speed of the car??

 

Your question is rather unfair since we have previously been applying the assumption that the weight of the baloon itself (ie the rubber, or whatever it is) is negligable compared to the weight difference between air and the gas inside. Clearly this is not true for a baloon filled with air, but if it were (ie in the limit of the baloon mass approaching zero) the ballon would not move.

 

The way to think of this problem is in my opinion to use relativity. Do not think of the car as accelerating as x ms^-2 (I forget my original figure) but realize that it has the same affect as if there were an additional gravitational force in the (backwards) horizontal direction. So if the accelaration due to gravity is y ms^-2, then we have to add the two 'gravitational forces' together and find that rreluting force (seen from inside the car) is [math]\sqrt{x^2+y^2} \: {\rm ms}^{-2}[/math] at an angle [math]\sin^{-1} \frac{y}{x}[/math] to the horizontal. Therefore it is natural that the He filled ballon moves forward, since in a 'gravitational sense', this is 'up'.

[Kedas]

Nice math that isn't helping us.

 

"Your question is rather unfair" I didn't stop you to say that your answers was only valid for certain conditions.

 

but forget about all that lets try to get the formula:

only about the horizontal force:

-What is the force on the balloon due to the diff. air pressure due to the acceleration?

That is the mass of the air volume of the balloon multiplied with the acceleration of car.

F1 = RHOair . V . a

 

-Now what should be the force on the balloon to keep it the same speed as the car?

That is simple: the total mass of the balloon multiplied with the acceleration of the car. (Mt= total mass balloon)

F2 = Mt . a

 

so F2 must be < F1 for the balloon to move to the front.

This is the condition for the force on the balloon.

like you can see the acceleration doesn't matter or: (Mb=mass empty balloon)

Mb + RHOhe . V < RHOair . V

 

But this is the same condition to know if a balloon will go up or not!!

 

So a balloon that goes up in the car will also move forward when the car accelerate. (and that condition was more or less given in the question.)

so the weight of the balloon doesn't have to be mentiond only that it goes up.

 

anyone saw any mistakes?

[Severian]

I think you have beautifully demonstrated the difference between a physicist and an engineer.

 

I still like my relativity explanation better.

[drz]

Describe the behaviour of the ballons as seen from inside the van.

 

are the windows down?

 

just a guess in the dark, I know helium balloons bouce all around the car, regardless of which way the car is moving. Especially with a draft, window or vent. I've never seen a heavier then air ballon, but my guess with it being attached to the sealing is it would tend to move backwards while the van accels, right when it turns left, etc.

[jordan]

Because the pressure at the bottom of the balloon is bigger than at the top' date=' same like water.

about the same effect but horizontal if the car would accelarate with 9.8m/s² (g)[/quote']

So if I turned it upside-down would it move towards the Earth?

[Kedas]

So if I turned it upside-down would it move towards the Earth?

 

The balloon? please tell that you are joking the top would be the bottom if you turned it upside down.

[Kedas]

I think you have beautifully demonstrated the difference between a physicist and an engineer.

 

I still like my relativity explanation better.

 

if you are the physicist then how come that I have to come up with the little bit of math that is needed to calculate/explain it?

 

also you started with 'negligable weight' of the balloon while I took it into account.

 

yes, an acceleration has the same effect as gravity (thanks A. Einstein)

it's a good base to come up with the answer but I doubt that you would get your points if that was all you have to say about that question.

[Severian]

if you are the physicist then how come that I have to come up with the little bit of math that is needed to calculate/explain it?

 

Excuse me? As I already stated' date=' I think [b']my[/b] 'little bit of math' before your post was a lot more elegant explanation than yours.

 

also you started with 'negligable weight' of the balloon while I took it into account.

 

No I didn't. It was Swansont who (implicitly) used that (reasonable) assumption. I pointed out to you that your objection to his argument (of the air filled baloon) was not a reasonable objection. My explanation requires no assumption.

 

yes, an acceleration has the same effect as gravity (thanks A. Einstein)

it's a good base to come up with the answer but I doubt that you would get your points if that was all you have to say about that question.

 

Well, since I set the question, I don't really need to give myself points now do I?

[Kedas]

Excuse me? As I already stated' date=' I think [b']my[/b] 'little bit of math' before your post was a lot more elegant explanation than yours.

saying how you can add two vectors doesn't tell anything about those vectors.

(we only needed the horizontal components)

did I miss other math?

 

No I didn't. It was Swansont who (implicitly) used that (reasonable) assumption. I pointed out to you that your objection to his argument (of the air filled baloon) was not a reasonable objection. My explanation requires no assumption.

whatever, you can turn it like you want

maybe your thoughts are good but you'r sure bad in explaining them.

[Severian]

saying how you can add two vectors doesn't tell anything about those vectors.

(we only needed the horizontal components)

did I miss other math?

 

That is not quite true. The two accelarations work on the baloons in the same way. Once you have the vector you don't in principle yet know in which direction the balloons move (with the combined acceleration or against). However' date=' the fact that the He balloon moves upwards when there is no horizontal acceleration tells you that it will have the same response to the horizontal acceleration and will move forwards.

 

maybe your thoughts are good but you'r sure bad in explaining them.

 

0.5/10 (I give you the half mark since I bothered to respond to the troll )

[Kedas]

To answer the question:

It's an easy question "but this year all answers are different."

[jordan]

The balloon? please tell that you are joking the top would be the bottom if you turned it upside down.

Yep. I was joking .

 

No, I guess what I'm getting at is this: If you put a perfect sphere of helium in a container of air, which way will it go and why? I don't really know what the up and down side of the balloon have to do with it because the density inside is still the same at the top and bottom.

[Kedas]

The direction will depend on the directions of the accelerations of the container and if the balloon meet the condition that I defined a previous post.

You can include gravity effect that is also expressed with an acceleration value.

 

if you want to know the direction of the balloon then you have to calculate if the condition is met to go in the opside direction or same direction of the acceleration.

 

so the shape of container or balloon have notting to do with it it's only that condition and the direction of the acceleration that will tell where the balloon is going.