# The Naked Scientists Forum

### Author Topic: Why do bubbles seem to fall slowly on their own but quickly when there are many?  (Read 4260 times)

#### gazza711

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##### Why do bubbles seem to fall slowly on their own but quickly when there are many?
« on: 23/05/2015 19:33:45 »
I understand this all about density.
« Last Edit: 25/05/2015 11:14:42 by chris »

#### Colin2B

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##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #1 on: 24/05/2015 07:22:23 »
I understand this all about density.
If you blow 2 bubbles of similar size and watch the rate of fall, when you then push them together they have the same weight and volume so should sink at the same rate. The only way they can sink faster is if they have gained weight or lost volume. They clearly can't gain weight.

How have you performed this experiment? Have you eliminated air currents from the area?. How do you get the bubble to combine?

#### alancalverd

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##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #2 on: 24/05/2015 08:09:22 »
An excellent question!

Surface to volume ratio is one of the most important numbers in physics, but is not emphasised in elementary syllabuses.

If all the bubbles have the same skin thickness and radius r, they will have the same mass m and a surface/volume ratio of  S = 4πr2/(4/3)πr3 = 3/r

Now pack 1000 bubbles into an approximation of a sphere. The effective radius will be about 10r so S = 3/10r.

Now the downward force on a single bubble = gm where g is the acceleration due to gravity, and the drag force due to air resistance is proportional to its surface area a. So the terminal speed will be a function of m/a.

In the case of 1000 bubbles, the mass M = 1000m but the surface area A is only 100a, so M/A = 10 m/a and we can expect the cluster to fall at 10 times the terminal speed of a single bubble.

For small numbers of joined bubbles, the equation is complicated by the fact that a group of say, 2 or 3 bubbles does not look like a smooth sphere at all, but the group will still have a smaller value of S than a single bubble - two identical bubbles, for instance, will be joined by a plane disc which contributes mass but not surface area.

#### Colin2B

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##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #3 on: 24/05/2015 08:29:15 »

Now the downward force on a single bubble = gm where g is the acceleration due to gravity, and the drag force due to air resistance is proportional to its surface area a. So the terminal speed will be a function of m/a.
I see what you are saying, the fall rate is not only due to the degree of negative buoyancy, but more affected by the air resistance, a feather effect.
Neat explanation, thanks

Edit: A thought. This poster has the theory that gravity does not exist and it is due to air pressure. If that is so then the ratio of air resistance to downward presure - both fns of effective area - would mean that both types of bubble would decend at the same rate. Whereas under gravity they dont.
« Last Edit: 24/05/2015 09:53:26 by Colin2B »

#### gazza711

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##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #4 on: 25/05/2015 08:33:11 »
I understand this all about density.
If you blow 2 bubbles of similar size and watch the rate of fall, when you then push them together they have the same weight and volume so should sink at the same rate. The only way they can sink faster is if they have gained weight or lost volume. They clearly can't gain weight.

How have you performed this experiment? Have you eliminated air currents from the area?. How do you get the bubble to combine?

Hi Colin,

I was blowing bubbles with bubble mix in the bathroom while my 3yr old daughter was having a bath.I simply blew a bout 10 bubbles.there were three larger ones left.they fell slowly from 6ft and combined at 5ft and accelerated very fast to the ground.There were no drafts or any other forces involved.

#### Thebox

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##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #5 on: 25/05/2015 08:43:25 »
Hello, an interesting question.  If objects all fall at a constant , both combined bubbles should still fall at the same rate. From 6ft to 5ft the bubbles are accelerating, does it make any difference because the bubbles are filled with exhale?

#### gazza711

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##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #6 on: 25/05/2015 09:00:33 »
the statement that helium is heavier compressed in a cylinder comes to mind.The more atoms/molecules together, the heavier or more dense.there is less air resistance around the one combined bubble than three bubbles.therefore the combined bubble wouldn't weigh more,but experienced less air resistance maybe?

I apologise to all the regulars on here for sounding or coming across as a troll.I realise that I need to ask more suitable questions first to make others understand my logic.I am not a scientist and love this forum totally.The proof of gravity's direction is yet to be proved(if that's not to offensive?)

#### chris

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##### Re: Why do bubbles seem to fall slowly on their own but quickly when there are many?
« Reply #7 on: 25/05/2015 13:18:17 »
Alan - I love your take on this; I had assumed, when he said "there are many" that he meant many individual bubbles, rather than a linked raft, which is the interpretation you have used, isn't it? Many individual bubbles should all behave individually and independently like a single bubble, assuming that they don't interfere with their neighbours.

#### jccc

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##### Re: Why do bubbles seem to fall slowly on their own but quickly when there are many?
« Reply #8 on: 25/05/2015 16:10:58 »
gravity is an attraction force. like the force between 2 opposite magnet poles.

the difference is gravity inversely proportional to distance square, magnetic force inversely proportional to distance cube. do you know why?

gravity direction is 2 way. you attract your wife, she attracts you.

you attract other woman, your wife attracts other man. so what's the attraction direction?

#### David Cooper

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##### Re: Why do bubbles seem to fall slowly on their own but quickly when there are many?
« Reply #9 on: 25/05/2015 17:08:16 »
If two bubbles join, the heaver one should end up underneath and the collective drag should then reduce. Repeat the experiment with a video camera and then you can make accurate timings by going through it frame by frame to see the actual speed difference between before and after the bubbles combine. I would not expect the speed to increase to more than double. If it does though, you've found something that merits further study and which could well lead to an ig-nobel prize.

There may also be a collective downdraught effect where lots of individual bubbles falling close together may create a downward air current.
« Last Edit: 25/05/2015 17:10:56 by David Cooper »

#### gazza711

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##### Re: Why do bubbles seem to fall slowly on their own but quickly when there are many?
« Reply #10 on: 25/05/2015 18:03:25 »
Hi David,

Great answer.Just going to recreate the experiment now as its bathtime.I might video it.As for the ig nobel prize-its a little far from London no.

Gav

#### yamo

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##### Re: Why do bubbles seem to fall slowly on their own but quickly when there are many?
« Reply #11 on: 25/05/2015 19:57:45 »
Air pressure compression between bubles?

#### gazza711

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##### Re: Why do bubbles seem to fall slowly on their own but quickly when there are many?
« Reply #12 on: 25/05/2015 19:59:41 »
Air pressure compression between bubles?

#### alancalverd

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##### Re: Why do bubbles seem to fall slowly on their own but quickly when there are many?
« Reply #13 on: 25/05/2015 22:56:11 »
Alan - I love your take on this; I had assumed, when he said "there are many" that he meant many individual bubbles, rather than a linked raft, which is the interpretation you have used, isn't it? Many individual bubbles should all behave individually and independently like a single bubble, assuming that they don't interfere with their neighbours.

Up to a point, but if you want to be really pernickety you can regard a formation of individual bubbles as analogous to a skein of geese or a formation of aeroplanes. Since each is flying in the wake of its predecessor, those at the back of the formation gradually close up on the leader, and just before they make contact with the leader they actually improve the leader's aerodynamics by partially streamlining its wake, so I guess if you observed a stream of bubbles in still air, you would indeed find a higher group velocity than for a single bubble in free fall.

But only a really geeky nitpicker would bother to look, let alone write my paper on the subject.

#### Colin2B

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##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #14 on: 29/05/2015 23:18:33 »
I simply blew a bout 10 bubbles.there were three larger ones left.they fell slowly from 6ft and combined at 5ft and accelerated very fast to the ground.There were no drafts or any other forces involved.

When the bubbles combine they don't gain weight ie weight of bubble 1 + bubble 2 = weight of combined bubble. Also they don't lose air volume, none has seeped out. So it's not a question of density.
Alan is suggesting it is air resistance. Try these 2 experiments with your daughter.
Take 2 equal sized pieces of card, drop them one flat side down the other on edge. Clearly the one on edge falls faster, but their weight and volume are the same.
Next take 2 equal sized downy feathers like you get in pillows. Drop both together they will fall slowly. Now crush one so no fluffy bits and then drop them. Crushed one falls faster, but they both still have same weight.
In both cases air resistance causes the different fall rate.
It's the same with the bubbles, the air resistance has changed.
Will be interested to see the video.

Edit: forgot to say, look out for what David mentioned, smaller bubble hanging below larger. These should fall faster because of lower resisting area, but increased weight of combined bubble.

Also, air currents in bathroom. Over the bath warm moist air will rise, when it reaches the ceiling it will move outwards and fall over the cooler area outside the bath edges. So as long as the bubbles are within the bath perimeter you could view them as being in the same current, but don't let them go outside otherwise they will fall even faster.

This is great, real science, I'm almost tempted to get in the bath with my bubble kit!
« Last Edit: 30/05/2015 04:05:57 by Colin2B »

#### gazza711

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• Posts: 144
##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #15 on: 31/05/2015 19:10:12 »
I simply blew a bout 10 bubbles.there were three larger ones left.they fell slowly from 6ft and combined at 5ft and accelerated very fast to the ground.There were no drafts or any other forces involved.

When the bubbles combine they don't gain weight ie weight of bubble 1 + bubble 2 = weight of combined bubble. Also they don't lose air volume, none has seeped out. So it's not a question of density.
Alan is suggesting it is air resistance. Try these 2 experiments with your daughter.
Take 2 equal sized pieces of card, drop them one flat side down the other on edge. Clearly the one on edge falls faster, but their weight and volume are the same.
Next take 2 equal sized downy feathers like you get in pillows. Drop both together they will fall slowly. Now crush one so no fluffy bits and then drop them. Crushed one falls faster, but they both still have same weight.
In both cases air resistance causes the different fall rate.
It's the same with the bubbles, the air resistance has changed.
Will be interested to see the video.

Edit: forgot to say, look out for what David mentioned, smaller bubble hanging below larger. These should fall faster because of lower resisting area, but increased weight of combined bubble.

Also, air currents in bathroom. Over the bath warm moist air will rise, when it reaches the ceiling it will move outwards and fall over the cooler area outside the bath edges. So as long as the bubbles are within the bath perimeter you could view them as being in the same current, but don't let them go outside otherwise they will fall even faster.

This is great, real science, I'm almost tempted to get in the bath with my bubble kit!

Good answer.The smaller bubble was accelerating faster than the larger ones.Hard to see in a blink of an eye.Dont forget that if you inflated the bubbles with a fan instead,they wouldn't really drop that fast.###

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##### Re: why do bubbles drop slowly on theyre own,but fall quicker when combined?
« Reply #15 on: 31/05/2015 19:10:12 »