# The Naked Scientists Forum

### Author Topic: A question about laws of motion and balloons?  (Read 4026 times)

• Neilep Level Member
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##### A question about laws of motion and balloons?
« on: 22/01/2010 17:08:48 »
When you let the air out of a balloon and it whizzes around the room, why does it suddenly go really fast on its last bit of released air?

#### LeeE

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##### A question about laws of motion and balloons?
« Reply #1 on: 22/01/2010 17:43:30 »
Hmm...  I think we really need a rubber scientist to answer this one, but I think most people have noticed that it seems to take a lot more effort to start inflating a balloon than it takes to fully inflate it once you've got it started, which would tie in with the air being forced out more vigorously just before the balloon is entirely deflated.  Another factor may be that the cross section of the balloon will decrease as it deflates, so there'll be less resistance and drag as it moves through the air.

#### syhprum

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##### A question about laws of motion and balloons?
« Reply #2 on: 22/01/2010 18:59:25 »
I think its an area versus volume thing, one varies as the second power while the other varies as the third power.
the energy stored in the stretched surface powers a rapidly decreasing volume.

#### Geezer

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##### A question about laws of motion and balloons?
« Reply #3 on: 23/01/2010 06:29:14 »
I think these are great answers. However, a minor detail. A rubber scientist, in US parlance, might be someone with much knowledge of rubbers.

On my first day at work in the US, I embarrassed the heck out of our department admin by asking her if she had any rubbers. No kidding!

#### Soul Surfer

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##### A question about laws of motion and balloons?
« Reply #4 on: 23/01/2010 10:27:19 »

The problem does not require a great deal of knowledge of rubber to explain just the basic physics of surface tension in elastic films, which is probably not taught generally to degree students these days.

The tension in the balloon skin is approximately proportional to the amount it is stretched up to the maximum amount of stretch when it suddenly goes up a lot.   The pressure in a bubble or a balloon is related to the tension in the skin multiplied by the sum of the reciprocals of the two radii of curvature of that skin in each of the two orthogonal dimensions of the skin. So as the balloon gets smaller at some point the pressure starts to rise.  This is in effect the inverse of the process that anyone who has blown up a balloon knows.  The first bit is the most difficult because although the tension is less the curvature is greater.  Then it gets easy for a while and then it gets more difficult again just before the balloon bursts as the rubber reaches it's elastic limit.  You can also see this effect if you partially blow up a long balloon. It starts to blow up at one end and stays thin in other parts.  The pressure inside the balloon is all the same but only part of the length is inflated and this can be moved along the balloon you just can't make the balloon have a small even inflation all along its length because that condition is not stable because it would require higher pressure than having part of the length of the balloon fully inflated.  This is also the reason why there is a little lump at the end of a long balloon because there, the curvature is small in two directions and therefore the pressure needed to stretch the rubber is greatest.
« Last Edit: 23/01/2010 11:41:14 by Soul Surfer »

• Neilep Level Member
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##### A question about laws of motion and balloons?
« Reply #5 on: 25/01/2010 21:51:40 »
Wow, I didn't realise how much there was to this. I thought it was just a question of drag. Now I've got to try and explain this to 10 year olds. I might need to scratch my head for a while in order to get it down to their level. 10 year olds ask the best questions.

#### Don_1

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##### A question about laws of motion and balloons?
« Reply #6 on: 26/01/2010 08:12:01 »
.... but then the P-λ relationship encounters the hump......... grows disproportionately large ............ then experiences a wild burst of speed.

Are we still talking about balloons?

#### LeeE

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##### A question about laws of motion and balloons?
« Reply #7 on: 29/01/2010 00:42:02 »
Lol - the testing machine I used at college was hand powered i.e. you increased the tension by a manually operated geared screw mechanism, and the recording 'device' was a strip of paper tape that you manually advanced beneath a (manually operated) pointer to poke a series of holes in the paper strip

#### Geezer

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##### A question about laws of motion and balloons?
« Reply #8 on: 29/01/2010 05:13:24 »
Lol - the testing machine I used at college was hand powered i.e. you increased the tension by a manually operated geared screw mechanism, and the recording 'device' was a strip of paper tape that you manually advanced beneath a (manually operated) pointer to poke a series of holes in the paper strip

which probably gave you a very good appreciation for the principles involved, even though it was a pain in the neck!

#### syhprum

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##### A question about laws of motion and balloons?
« Reply #9 on: 29/01/2010 12:43:57 »
When I was at school we had even cruder machines where the tension was applied by stacking weights on a balance.
We kids where not allowed to break any specimens as too much work was involved in preparing them that privilege was reserved for the night students.

#### The Naked Scientists Forum

##### A question about laws of motion and balloons?
« Reply #9 on: 29/01/2010 12:43:57 »