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?

Nomme

None of the above - it would be deflated on the floor, as, being the big kid that I am, I would have inhaled the helium for a moment of amusement caused by the temporary high pitch of my voice, long before the journey even started.

I'm even sat here laughing at the thought of it, getting strange looks from others in the office.

OK, I do actually know this one, but don't want to post the answer and ruin it for anyone else. It's quite interesting though.

I guess A, but don't know why.

And Hodges (?) - you made me laught too - luckily no one else here at the moment.
:P

It'd have to be C, surely!... maybe not.

Surely it has to be C. If it's tethered to the car then it must be moving the same speed as the car and the air in the car. The more I think about this though, the more I think it's wrong.

I would say A.

Why?

The balloon has very little mass thus very little inertia so the car coming to a sudden halt wouldn't affect the balloons motion (relative to the car) much at all.

However the balloon is tethered by a piece of string which has mass, therefore inertia so when the driver suddenly brakes to a stop the string moves forward pulling the balloon with it.

:confused:

I'd say backwards ... because the balloon had less density than the surrounding air ...

Does the car have a sunroof?
If so is that open?

And the driver that suddenly brakes to a stop- is he in the same car as the balloon?

C: Stay in the same position. The air inside the car is not moving. The balloon is not attached or touching anything so therefore cannot be affected by the movement of the vehicle or any outside forces like the wind. It cannot experience g-force as it is not in contact with any part of the car.
Thats my theory and I'm sticking to it. Probably wrong but I did think it through. I feel like I'm back at school.

Nomme, i wonder how many people have bought balloons and testing this theory.
Interesting one though helium makes the balloon float which must mean that helium is lighter than air.
I think the heavier air around it must cause a smll vacumm around the balloon and spin it.

Oooh look at me trying to look all clever and educated.
My answer is probably a load of rubbish.

Having bought my balloon this morning and tethered it to the inside of my van I now know what happens.

Just kidding :thumbsup:

Ignore what I said above but it does have something to do with inertia, I think.

My answer is now B.

B - when the car stops EVERYTHING wants to keep moving forwards, but the air has greater weight (strictly: "mass") than the balloon (that's why the balloon floats in air) so the relatively "heavy" air moves forwards forcing the lighter balloon backwards.

Originally posted by robh
B - when the car stops EVERYTHING wants to keep moving forwards, but the air has greater weight (strictly: "mass") than the balloon (that's why the balloon floats in air) so the relatively "heavy" air moves forwards forcing the lighter balloon backwards.

Spot on.

This puzzle was apparently on a show called Mindgames on BBC4. They actually did the experiment in order to prove it too.

Nomme

Question. The physics sounds fine, but the balloon has ‘physical’ mass. So, initially, wouldn’t the mass of air ‘behind’ the balloon, impart a force on the ‘back’ surface-area of the balloon, and tend to move it forward?

Originally posted by Carlwarker
Question. The physics sounds fine, but the balloon has ‘physical’ mass. So, initially, wouldn’t the mass of air ‘behind’ the balloon, impart a force on the ‘back’ surface-area of the balloon, and tend to move it forward?
If they did the experiment to prove it, then obviously not.

Originally posted by Sidla
If they did the experiment to prove it, then obviously not.

I suggest you read your own signature.

And my question deserves an answer - not an ass-brained retort.

Originally posted by Carlwarker
Question. The physics sounds fine, but the balloon has ‘physical’ mass. So, initially, wouldn’t the mass of air ‘behind’ the balloon, impart a force on the ‘back’ surface-area of the balloon, and tend to move it forward?

I don't know. I thought it would go forward due to inertia.

The actual answer I have is this :

"the decelleration makes the air inside the car move forward, relative to the car, but the helium, being lighter and subject to less force, is displaced backwards by the air rushing forward"

which sounds reasonable to me.

With Xmas partys coming up with helium ballons about I suggest people try it out themselves. (..but I suspect people like Hodge will get to them first)

Nomme

Originally posted by nomme
I don't know. I thought it would go forward due to inertia.

The actual answer I have is this :

"the decelleration makes the air inside the car move forward, relative to the car, but the helium, being lighter and subject to less force, is displaced backwards by the air rushing forward"

which sounds reasonable to me.
Nomme

And it did to me N. But, when I considered the physical mass and surface-area of the balloon, and using Newton's Laws of Motion, then... hence, my question.:)

Quite often, Physics 'theory' doesn't work out in practice.

And, I said, initially.

Tend to agree with Carlwarker on this one.

Surely if the air moves forward when the car slows down it would take the balloon with it and then move backwards when the air hits the windscreen.

So the balloon moves forwards and then backwards.

I did a bit of googling after my first post (yes, I need to get out more) and came across This (http://www.quantumscientific.com/ballpen.html) site which offers a different explanation, vector forces, inertia etc.

Also This (http://www.amherst.edu/~physicsqanda/Helans.htm) site which gives the air movement answer and a far more technical one :confused:

:thumbsup:

I suppose it could be demonstrated using a lit cigarette.
The smoke rising being less dense than the air, would (should) behave similarly.
I agree with other forum folk in that the air in the car would move forward on braking and drag any object lighter than it forwards with it. The balloon containing the helium has the same mass inflated as deflated so should behave accordingly (shouldn't it?) by moving forwards initially.
I'm not a scientist, but I'll have a go at justifying this. HERE GOES.

A helium-filled balloon in air resembles an object floating in water.
If the container is suddenly brought to a halt the water carries on in the direction it was moving momentarily and takes the object with it.

Well that's my twopennuth of dodgy science. I'm off to complete my studies on time travel now.:loopy: :banana: