# Discuss rabidrobot's answer to: What is air pressure inside rubber birthday balloon?

Consider a normal rubber balloon inflated to a reasonable size; where the initial resistance of the rubber to stretching is overcome, and the balloon is not so full that the rubber can no longer ...

I went a bit deeper into the search results looking for information about this, and I believe I have found confirmation of my original position.

Here is an interesting PDF on balloon physics;
http://www.eurekaeducation.net/files/physics/playing_with_balloons.pdf
Which is from a collection;
http://www.eurekaeducation.net/sub_files/physics.html

As described at the top of page three of the "playing_with_balloons" pdf, air pressure in a flexible container approaches that of ambient.  If a discrepancy exists between these pressures, this results in a change in the shape/volume of the balloon to maintain the equilibrium demanded from Boyle's law.

Think of the elasticity of the rubber as capable of storing potential energy.  When air is added to the balloon, work is done to stretch the rubber.  But once it is stretched, it does not require a constant greater pressure or influx of energy to maintain the shape--this would essentially mean free energy or perpetual motion.  As a rock lifted and placed on a table, the force of the rock down on the table & the table back on the rock is independent of the fact that the rock was once on the ground.

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