# Why is it to fill a balloon hard initially ?

1. Jul 5, 2014

### Murtuza Tipu

Why is it hard to get the air through the balloon when you first fill the balloon ? But after inflating it z bit it becomes easier to further inflate ?

2. Jul 5, 2014

### A.T.

The rubber is not linearly elastic. Initially there is some plastic deformation of the thick rubber, which requires more pressure. When you deflate it, and inflate for a second time, it is easy from the start because the rubber is thinner already.

3. Jul 5, 2014

### jbriggs444

Even if the rubber were linearly elastic, the reduction in curvature as the balloon inflates more than compensates for the increase in tension.

Consider, for instance the total tensile force of the rubber along an imaginary line between two hemispheres of the balloon. This force is what holds the hemispheres together. Inflate the balloon to double its dimensions and you've doubled this inward force.

If the air pressure inside the balloon remained constant during this inflation, the result would be to quadruple the total outward force. In order to keep the two forces in balance, the pressure inside the balloon must decrease as it inflates.

Ideally, pressure would vary in inverse proportion to the radius. This means that, ideally, pressure becomes arbitrarily high as the radius becomes arbitrarily small. Fortunately for your lungs, tension in rubber is not linear. It has a non-zero relaxed length.

Last edited: Jul 5, 2014