Why are balloons harder to inflate when you start, and feel easier once they start expanding?


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

Think of the forces on a small piece of the balloon rubber. There is tension around the outside from the elasticity of the balloon material. Because of the curvature of the balloon, some of that elastic tension is pulling that piece of rubber inward. This force is countered by the pressure difference providing an outward force. If the balloon wall is in equilibrium, those forces should all cancel out. If you blow air into the balloon the pressure increases, and if the balloon expands the pressure decreases, so one way to think of it is that the balloon will change its size until the equilibrium state is reached.

Now, consider that the bigger a balloon is, the less curved any small segment of it will be. Even if the balloon material gets tighter as the balloon stretches, less of that tension is directed inward, meaning that less pressure is needed in the balloon to maintain equilibrium. When you blow up a balloon, your lungs are working against the pressure inside the balloon.

Overall, this means bigger balloon leads to less pressure needed to balance the tension of the balloon, which leads to less work your lungs have to do to blow it up.

When you start blowing up the baloon, the rubber its made of is stiff and thick (compared to what happens next). When you blow air into it, you can feel that it quickly stretches a lot, and then it gets easier to blow it up completely. That is also why its much easier to blow up a baloon which was once filled with air and deflated.

Kind of like with tight pants. You struggle to get them on because fabric is new. Once you wear them for even few minutes, they become easier to put on again.

It’s all folded up and cold at first. Stretching heats it up and straightens it out. When it’s warmer and smoother it will stretch easier.


Action Lab actually did [a great video](https://www.youtube.com/watch?v=GiG0e1s6nV4) on the elastics of a balloon!

It is from two intuitive reasons ..

The big part is how tension works .. have you noticed when two droplets of water in say a glass surface meet? .. the bigger droplet always seems to ‘suck’ the smaller droplet when they merge .. thats because the bigger droplet has lower pressure inside it! .. same with balloons .. think of it this way, if you were to allow balloons/droplets to merge, if the bigger sucks in the smaller droplet, it will only slightly get bigger, so no much change in pressure/energy required, while the smaller one get to be completely flat so all the pressure there can be gone! .. this is because the energy required to hold pressure depends on surface area which grows slower than the volume inside .. hence spheres held by surface-tension (like a balloon or droplets) have lower pressure in them the bigger they get!

The second part is simply that rubber is a type of material that can be ‘preconditioned’ .. basically if you stretch a balloon several times before you try to blow into it, you’ll find its a lot easier to get it started! .. all the stretching you do before hand ‘loosens up’ the rubber (and makes it warmer etc) and if you dont do that it is stiffer and therefore harder to blow up!

Can anyone explain why my cheeks get crazy painful when I do it? I’m serious. I can’t blow up balloons my cheeks hurt something fucking serious after…

Some interesting answers. I’m sure the tension of the materials against the air has SOME effect. But largely, in my opinion, you’re overcoming the atmospheric pressure / gravity. Once you’ve inflated the balloon slightly, you’ve pressurized the system into something closer to atmospheric pressure. and the atmosphere will have a delicate balancing act with your balloon to try to meet equilibrium of pressure.

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.

[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.

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.

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!!

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.

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.

https://www.youtube.com/watch?v=rSnnScAyIjI video by Steve
Mould which ends up answering that question.

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.

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.

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

I assume that after the first couple of breaths the energy in the rubber atoms is higher from the friction caused by expansion, allowing easier further expansion

If the deflated balloon has one square inch of surface area and you put one pound per square inch (psi) into it, there is one pound of force trying to stretch the balloon. If the balloon has 10 square inches of surface area and you put one psi into it, there are 10 pounds of force stretching the balloon.