I mean your average party balloon, when it’s completely deflated, it seems you have to put extra effort into getting it going. As soon as it starts inflating, you need less effort.
In: Physics
Boyle’s law, also called Mariotte’s law, a relation concerning the compression and expansion of a gas at constant temperature. This empirical relation, formulated by the physicist Robert Boyle in 1662, states that the pressure (p) of a given quantity of gas varies inversely with its volume (v) at constant temperature; i.e., in equation form, pv = k, a constant. The relationship was also discovered by the French physicist Edme Mariotte (1676).
Source: https://www.britannica.com/science/Boyles-law
The volume increase of the balloon decreases for each inflation as it gets closer to its maximum capacity. The resistance for each inflation also grows as your get closer to the maximum capacity. The first blow has the least resistance and the last will have the most since the tension in the rubber is increasing for each inflation.
It starts stiff and needs to be stretched for smoother inflation. Stretching the balloon a couple of times makes it easier to blow up too.
https://www.youtube.com/watch?v=rSnnScAyIjI video by Steve
Mould which ends up answering that question.
There is less surface area of a small balloon for the pressure of your breath to work against. So let’s say you can exhale 2 lb of pressure per square inch. When the balloon is small the inside of it might only measure three square inches. So that 6 lb of pressure trying to stretch the balloon.
Double the size of the balloon and now you are exerting 12 lb of pressure on 6 square inches.
Double it again to 12 square inches and you’re exerting 24 lb of pressure on the balloon.
You’re still only providing the 2 lb of pressure per square inch but the number of inches has grown a lot.
Then as the balloon becomes nearly full, the total number of pounds of force it takes for the balloon to expand another inch starts steadily rising. This is sort of like a spring, but it’s at the molecular rubber level of spring. Literally the springiness of the rubber.
So then as you get to the maximum expansion of the balloon it may not be able to expand any further. Now you have to provide enough air to pop the balloon or you have to stop blowing.
So the easiest point in the inflation is when there’s plenty of surface area inside the balloon for the breath of your longest depressed against, but there’s still plenty of balloon stretchiness left so it’s still easy for the balloon to get bigger.
There is no pressure inside the balloon when start blowing into it. The material is also more dense in the tiny amount of area available for air.
Once the material has expanded and there is more pressure inside the balloon, blowing it up requires left effort because the material is more pliable and there is already air inside the balloon to help.
Not to completely disregard a lot of people’s answers, but it’s all about pressure.
When it’s small, you increase the pressure in the balloon significantly more when blowing because the surface area of the balloon is smaller. As it expands, there is increasingly more surface area, which makes it easier to apply more pressure, but you have to blow significantly more air to increase pressure since there is more surface area.
If you were to increase pressure linearly in the balloon, you would find the effort/force behind your blow would not actually change from small to large, but since our lungs have a small capacity this isn’t particularly feasible.
Also, when it gets more full it doesn’t push back with significant force because of how small the hole is, the pressure is still technically the same, but you only feel the force of the pressure multiplied by the size of the hole. If you’ve ever blown up a balloon with a larger hole, you’ll notice it’s much harder to hold the air in when it gets full because of this.
Understanding pressure/stress is just a force over an area (P = F/A), i.e. psi (pounds per square foot) is key to understanding BALLOON MECHANICS!!
It’s a matter of surface area to volume ratio. When you start, adding a breath’s worth of air has to stretch the balloon a lot, but near the end, a single breath only stretches it a little.
[There is no ELI5](https://www.sciencedirect.com/science/article/abs/pii/S0020746214001292)
Rubber has complex mechanical properties when it’s stretched, which cause the effect described by OP.
PDF of the paper whose abstract I linked to above is available on arxiv.org.
There is a certain amount of rubber in the balloon. The bigger the balloon becomes, the more the rubber is stretched, which means more force needed to inflate.
However, the bigger the balloon gets, the thinner the rubber gets, which makes the “stretching force” less!. This not only cancels out the original stretch force, but since the surface area is dependent on the square of the radius, this actually beats the linear relationship of the rubber stretching, thus making it easier to blow an inflated balloon than an uninflated one.
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