Physics Brainteaser

[Beaker]

Ok I nicked this off another site, but thought it could make an amusing topic of conversation on here :)

Here is the brainteaser:

A helium filled balloon is tethered inside a car such that it floats
at the top of a piece of string, without touching the car's ceiling.
All windows are shut and the car is driving along at a steady pace.
The driver suddenly brakes to a stop.
Relative to the car, does the balloon
a) move forwards,
b) move backwards,
c) stay in the same position
as the car decelerates?

....and why?

Lindsey

[MaestroMatt]

It stays in the same place because the air in the car is moving at the same speed as the car and decelerates at the same speed. The balloon has so little mass any inertia would have a negligible effect. That's what I think!

[Mat_C]

Thats a good one that. It should be (a) because it does have mass and the decceleration will provide a force via the tether.

But it also could go backwards (b) because it is acting as a sort of spirit level, the car tilts forwards under decelleration.

It all depends on the cars suspension and mass of the ballon for which effect is greatest.

[Dave]

You would assume.......Newtons first law states that an object in motion continues to travel with constant velocity unless acted on by an external force. Hense, the balloon will continue to travel at the same velocity until the horizontal component of force in the string stops it doing so....
But actually, the air in the car will also be doing this.... so the air will all 'slump' to the front of the car, so now we have high pressure air at the front of the car, low pressure at the back - thus, since the balloon is lighter than air, and the effects of newtons first law is so negligable, the ballon goes backwards!

Dave.

[D87 SMW]

I agree with Dave.

The air will be at higher pressure at the front of the cabin. This would then cause the balloon to move backwards. It would depend on exactly where in the car the balloon was to determine how far backwards it would go....

[D87 SMW]

Originally posted by Mat_C
But it also could go backwards (b) because it is acting as a sort of spirit level, the car tilts forwards under decelleration.


The balloon does go *backwards,* but only in relation to the car, now tilting forwards, rather than in relation to something which remains flat. This is what is asked in the question. It appears to be going backwards, but that is only because the car is tilting. The balloon remains vertical throughout.

:cool:

[Mat_C]

---
The balloon does go *backwards,* but only in relation to the car, now tilting forwards, rather than in relation to something which remains flat. This is what is asked in the question. It appears to be going backwards, but that is only because the car is tilting. The balloon remains vertical throughout
----

Thats what I was saying (acting as a spirit level) :)

Personally, I doubt variations in air pressure due to decelleration would be great enough.

[Quality]

Moves forwards, like every thing when the brakes are hit hard.

[SimonR]

Which way would it move if the headlining has sagged down far enough to touch it?!:)

[D87 SMW]

Originally posted by F153JUE
Which way would it move if the headlining has sagged down far enough to touch it?!:)

Down, if it was sagging enough. ;) :laugh:

[Dave]

>>Personally, I doubt variations in air pressure due to decelleration would be great enough<<

I think you'd be suprised about the air pressure - Im certain, for you to be able to notice the balloon move backwards under the effect of the car nosing down you'd have to have laser ranging vision! The angle must be maximum 5deg!!

Dave

[D87 SMW]

Originally posted by Dave
>>Personally, I doubt variations in air pressure due to decelleration would be great enough<<

I think you'd be suprised about the air pressure - Im certain, for you to be able to notice the balloon move backwards under the effect of the car nosing down you'd have to have laser ranging vision! The angle must be maximum 5deg!!

Dave

How can the air pressure possibly change, when it is all in a confined space, with no windows open etc? :confused:

[Dave]

The air pressure in the front of the car increases and the air pressure in the back decreases because every single molecule of air in the car is subject to netons first law so they all get forced into the front!

Dave

[Mat_C]

>>I think you'd be suprised about the air pressure - Im certain, for you to be able to notice the balloon move backwards under the effect of the car nosing down you'd have to have laser ranging vision! The angle must be maximum 5deg!! <<

Fair point!

You are right that there WILL be a pressure difference due to decelleration, but I doubt it would be a big enough difference. I may be wrong!

Also, considering that the ballon will have to decellerate with the car, the same pressure gradient will occur within the baloon itself.

In which case I put my money on a) it will go forwards.

[Dave]

The pressure in the balloon is much higher than the pressure outside it - its a balloon! The differential observed in the balloon, as a proportion of the pressure in the balloon is not comparable to the pressure gradient seen in the cab. But none of this matters...(its not relevant - it can be anything you fancy - its inside the balloon! and we are concerned with how the balloon as a whole moves)

Anyway - regardless of the pressure inside the balloon the balloon will be subject to the force imposed by the difference in pressure in the car -As will all items in the car. However, it is the only item light enough to be moved by this force.

Dave.

[tony]

i dont know,what was the question.
the balloon would move forward, - because i say so:)

[D87 SMW]

Originally posted by tony
i dont know,what was the question.
the balloon would move forward, - because i say so:)

Erm, the question is at the top of the page. The first post.... ;)

[Mat_C]

Sorry to stop the argument, but dave, you are totally RIGHT and I hang my head in shame :)

I looked thru my old physics textbooks (just now) and found an explaination of this effect. It does move backwards due to the density variations in air!

[Dave]

:) I like physics. (repeat whilst holding nose and wearing thick glasses)

Dave. (proud to be a geeky engineer type - the girls love it... no really they do)

[Mat_C]

I'm definately more sadder because i've got a degree in aforesaid subject and got it wrong!! Sometimes those blasted trees get in the way of the woods!

All that geeky work for nothing LOL

[Dave]

Similar - ive got mine in Mathematics... perhaps we should start a 'proud to be a geek' board on this forum.

Dave

[Mat_C]

Tell you what......let's not ;)

I've never used "hey, i've got a degree in physics" as a chat up line. And if I ever do, please castrate me!

[Ricky]

Originally posted by Mat_C
I've never used "hey, i've got a degree in physics" as a chat up line.

To be honest, i've never used "hey, i can't pass gcse physics" as a chat up line either, and I cant imagine that one having much effect either! I have got a good excuse though (for the physics!!), but i'm not going to bore you to death with that!!

All this talk of degrees..... the closest I get to a degree is a bleedin protractor!!:laugh: :laugh:

[Dave]

:D

[Jolly_MG_Maestro]

The balloon would move towards the back of the car, because if the car braked the front of the car would go downwards and because the ballon floats to the highest point of any space given no physical restrictions, the angle of the roof line at point of braking would be such that the back of the car would be slightly higher than the front therefore the ballon would shift to the highest point of the cabin, the back, I don't think braking G Force has any or little effect on an object filled with gas lighter than air.

Paul

[Dave]

'Braking G-Force' as you call it will affect EVERY object with any mass. Note, Mass not Weight - the helium balloon may appear to have a negative weight - but this is purely a consequence of it being lighter than air - in a vacuum, a helium balloon would sink just like anything else in a potential gravity field.

It is interesting that more than one person has noted the angle of the car under braking, have you all got shot suspension? The angle of the car would only vary by, maybe, 5o... You would not be able to percieve the balloon moving at all with respect to this angle change.

So, we have established the effect of momentum (Newtons First Law), meaning the balloon will lurch forward is negligable compared to the effect the air pressure/density variations have on it. The tendancy to move forward is vastly out-done by the air pressure pushing it backwards. You would also see the balloon 'lean in' on tight cornering!

Dave.

[Austin-Rover]

Will somebody please get a baloon and try this so we can know for sure...

;)

[e692wtt]

"The air pressure in the front of the car increases and the air pressure in the back decreases because every single molecule of air in the car is subject to netons first law so they all get forced into the front!"

Can't say I've noticed any effects of this, eg Rear Seat Passengers exploding when the car is braking heavily on the motorway, and then me getting The Bends after the car has stopped.

Won't the balloon move in an arc around the point at which the bottom of the string is tied, forward 'a bit' when the car is slowed then revert to upright soon after the car stops? Or mebbe it'll explode? Or half explode?

Only kidding...
:eek: :D

PS as I said as I flunked Maths and Physics A-levels, "what does it matter???"

[Maria]

I think this would be the perfect experiment for the car park at the Xmas do - and someone had better bring a digicam or camcorder!

[threelitre]

Originally posted by e692wtt
Won't the balloon move in an arc around the point at which the bottom of the string is tied, forward 'a bit' when the car is slowed then revert to upright soon after the car stops?

I was really waiting for the first one to mention the rope! And that rope has a mass, too. So when everybody here is discussion minor effects with unknown proportions... So the mass of the rope will lead to two effects: a) the balloon moving forwards and b) the balloon tilting around it's center of gravity in such a way, that the bottom end will be pointing forward a little. The same rotating movement will be happening, when the balloon will be forced backwards due to a pressure gradient... My personal estimate would be that the effect of the rope could be the biggest and the tilting well noticeable (braking really hard).

But the, one effect has gone by unnoticed so far. and that's the movement of the air in the car. If you go and consider the change of air density due to braking, you should also consider that there is a non-stationary action in the air to get the pressure evenly spread inside the car - and this will cause 'wind', not evenly spread in the interior, so that wind can move the ballon about, too... But then again, I would say that all these effects are hardly noticeable, as the re-equalizing of the pressure will be a _very_ quick action...

I hope to leave all confused now ;)

Regards,
Dr. rer. nat. Boucke

[Simon]

I dropped physics in 1979!! You could always inhale the helium and talk in a funny voice.
:confused:

[Dave]

The mass of the thread attached to the string MUST be insignificant, afterall the helium ballon is holding it up - so it cant be heavy, it is not 'wind' that carries the balloon backward or forward - it is the same principal (Bernouilli's First(?) law of hydrodynamics) that a balloon will rise in air if let go, because it tries to move to the region of lowest air density.
There is no 'wind' observed, since we are talking about a tiny mass of air moving, but the pressure differential can be significant.

Skatiechick?... whats the official answer?

Dave.

[Dave]

Right, lets get rid of this nosing down issue...

If we over do it a bit, i.e. completely remove the front wheel and plonk the car down, the angle of the car relative to the level road is only 3.6deg... yes 3.6deg. (assume car is 4m long, drop the front by 25cm quick bit of trigonometry gets us to 3.57deg)

Next, I'd assume we would only notice a deflection of min 3cm, relative to the size of a balloon we would only just see this... so, how long would the peice of string need to be for us to see this... 3cm/tan(3.57deg)=48cm!

So, with a half meter tether, the balloon would only tilt back 3cm with the front wheel completely removed! So i'd guess, same string, angle caused by braking would give 1.5cm deflection. ie. none!

Dave

[Beaker]

I have no idea, what the proper answer is. As i mentioned in my first post I stole the brain teaser off another site.

But seeing as it was car related, I thought it would make a great topic of discussion.

I will try and add my theory later.

Maybe it would be a good idea to try this experiment out in the car at the XMAS party :)


Skatty/Skatie/Lindz

[e692wtt]

The balloon, helium and string have a mass, it's just the helium that is less dense then the surrounding atmosphere so the balloon floats, which I think is a red herring. So they will continue at the car's original speed, at least initially, as the car slows. So they move forward? I'd guess so, but there are other things going on as per above...

I think Simon's idea is the best yet - mebbe an xmas party trick for Oxford?:laugh:

[Beaker]

My theory is B the balloon will move backwards.

Now when a car stops, we all want to shoot forward. Thats why we wear seatbelts to stop ourselves flying out of the windscreen. Now the air density/mass is greater than the balloon. Thats why the balloon floats in the air.

So seeing as the air, is relatively heavier than the balloon this will move forward when the car brakes. Thus forcing the lighter balloon backwards.

[tony]

what would having the heater or blower on do to all this, linds did they have a answer where u got it from?

[threelitre]

Originally posted by Dave

There is no 'wind' observed, since we are talking about a tiny mass of air moving, but the pressure differential can be significant.


How would you get a significant pressure differential (talking of front to rear) without air moving in the car (-> 'wind')?

And now think about how quick pressure in air will distribute... If you put a certain amount high pressure air in one place of the car's interior (like happening when letting an air-filled balloon explode), how long would it take to gain an evenly distributed pressure in the car again and no more air moving (the heater blower switched off etc...)? Definitly not a long time compared to your braking process!! You must brake really hard (like hitting a large rock at 100mph) to get even near that time scale. With usual braking the pressure in the car will be quasi-static! This leaves the pure mechanical effects due to the car decellerating and diving in with the nose - and it's not the angle being important here, but the angular velocity! This will lead to a movement in the air inside the car, giving the balloon backwards directed forces. (To see this effect: place a cup of tea before you and rotate the cup a small amount suddenly - and look what the fluid does...) After coming to halt, and assuming the change of accelleration is the same as when staring to brake, this effect will be turned around.

Going back to the mass of the balloon: When the car would brake, and assuming always constant pressure and neglicting the drag due to the balloon moving through the air: The balloon is held by a string... How could the _horizontal_ force needed to slow down the balloon together with the car be transfered? There is only and exactly one way: The balloon has to move forward, so that there is an angle smaller then 90 degree between the road before the car and the string!

So to me there are now 3 effects remaining: mass of balloon and string, moving air relative to the inside of the car and the quite large drag coefficient of the balloon.

If this thread keeps on going for longer time, I'll probably calculate a solution for that :)

Cheers,

Alexander

[Dave]

I hate to be a grouch... but I am right! Its a commonly observed phenomenon - the balloon moves backwards due to air pressure changes.

A wind is not felt because wind is created by a continual bombardment of a stream of air particles - there is no such stream here, just a few (relatively!) particles re-organising themselves. A few particles moving would not be able to transfer enough energy to the balloon via inelastic momentum to make it move.

Angular Velocity? I cant even begin to visualise how this is at all relevant. The car moves through, maybe 5o, in say 2secs... there is no angular velocity - the phenomena would be observed on a trolley with no suspension equally as well as in a car.

Dave.