# Simple Physics Experiment

I was required to blow up two identical balloons, one more than the other. Then I joined the two balloons via a short length of tubing and watched as the smaller balloon began to shrink at the expense of the larger balloon expanding. I was thinking that this had something to do with newtons third law, for every action there is an equal and opposite reaction, but I am sure it is much more in depth, thanks

Drakkith
Staff Emeritus
Interesting. I would have expected the smaller balloon to expand until both balloons equalized at the same size.

That is an unexpected result. I would expect the larger balloon to be under higher pressure and force air into the smaller balloon until they were equal.

Since you stated that the opposite happened I will put forward a theory. Perhaps the larger balloon was inflated to the point that the rubber was permanently stretched and therefore lost some tension as compared to the less inflated balloon with undamaged rubber. You could test this theory by inflating 2 new balloons to the larger size, then partially deflating one before connecting them. This should ensure that the rubber of both balloons has been stretched beyond its elastic limit.

K^2
Surprised as well, but it makes sense in retrospect. If we say E~A, V~R³, A~R², and P=dE/dV, then P~1/R.

Nothing to do with Newton's 3rd, though.

Edit: And come to think of it, it also agrees with experience of inflating balloons. It's always the hardest to get it going, but once it expands a little, inflating it further becomes easier. So the above dimensional analysis appears to be correct.

sophiecentaur
Gold Member
2020 Award
The pressure in a balloon is not just simply proportional to the amount of air in it so the experiment is not a simple one as with joining two columns of water with a pipe (which would just balance out). Our memory tells us that, when a balloon is only partially inflated, it is actually quite hard to blow up (stiff) and it gets easier, once it is really stretching. Finally, it gets more difficult to inflate when it's almost at its maximum size. The Stress / Strain (Pressure / Volume, actually) curve is S shaped and the stiffness changes as the shape changes.

If you were to join two balloons which were both 'well inflated', I think you would get a stable situation where the smaller one would inflate and the larger one would deflate until equilibrium was reached. That is because they would both be operating in the same region of their pressure / volume curve.

I'm myself astonished about what you said. Never thought something like that could actually happen.

But what Sophiecentaur mentioned could make sense.

What tubing did you use ? I want to try it myself.

Last edited by a moderator:
sophiecentaur