Explaining Pressure With Interactive Animations

It’s All Just Bouncy Balls

Imagine if I threw a tennis ball at you, what would you feel? Well firstly you’d probably feel quite annoyed because I just threw a tennis ball at you, but at the point of impact when it hits you’d feel a small knock backwards.

Now if I kept repeatedly throwing several tennis balls at you a second, you’d experience almost a constant backwards force, like a strong wind blowing against you.

Animation Time

Exactly the same would happen if you threw a tennis ball into a closed container and watched it bounce around inside. Every time it hits a wall, it causes an outwards force. This can be seen in the below animation – add a ball and see how it bounces about. Every time it hits a wall, the wall brightens up to show the outward force.

As you add more balls, you will see that the walls are hit more and more frequently until there is essentially a constant outwards force.


Swapping Tennis Balls for Molecules

Now imagine if instead of having a small number of tennis balls bouncing around inside the container, we instead have billions and trillions of molecules of gases like nitrogen and oxygen which are in the air all around us.

In fact, in one cubic metre of air, there are 2500, 000, 000, 000, 000, 000, 000, 000, 000 gas molecules. Unfortunately, my bouncy ball animation only goes up to 50, but you get the idea!

(Like a good writer, I’ll define my terms: A molecule is a group of atoms bonded together, like two oxygen atoms bonding to make O2, or two hydrogen atoms and an oxygen atom bonding to create H2O. Also, gas molecules really do bounce around and behave like tiny tennis balls – it’s a realistic analogy, promise!)

The Big Reveal

I think you’re ready for the big reveal. After all this talk of molecules bouncing around, we get to the point of this article:

The outwards force exerted by molecules as they bounce is called… pressure!

The key to understanding pressure is realising that there are so many molecules that they act as one constant force, like the wind blowing on you – it’s caused by squillions of molecules bouncing off you every second, but you only feel the overall combination of their individual efforts.

This can be experienced in a pressurised pop bottle – the molecules inside are bouncing around faster and with more energy than the molecules outside. Therefore, when you unscrew the lid, they force their way out of the top. It’s like two teams of rugby/american football players running into each other – the team which goes in with the biggest force will push the other team out of the way.

In this case, the molecules inside the bottle are the stronger team and so they force their way out of the bottle.

Pump It Up!

So now we have an idea of what pressure is, how do we cause a higher pressure? Well, there are three ways:

  1. Increase the temperature – this makes the molecules bounce around faster and so they hit into the walls more frequently and with a greater force.
  2. Add more molecules – as we saw in the above animation, the more molecules we add, the more frequently we get bouncing and therefore there is a greater outwards force.
  3. Make the container smaller – by keeping the same number of molecules but compressing them into a smaller container, we also get more frequent bouncing which leads to a higher pressure.

Try it out! See how temperature and compression affect pressure in this animation – how do you get the highest and lowest pressure?

You should find that the highest pressure (most bouncing) occurs when the container is compressed and at a high temperature, whilst the lowest pressure occurs when it is expanded and at low temperature.

Wrapping It Up

I hope this post helped clarify what pressure is. It seems like a simple thing on the surface but actually getting your head around it is quite tricky – it took me a long time. The key is in understanding that each molecule bounces around in a crazy, erratic way, but there are so many of them that overall, they create one smooth and highly predictable motion, just like the wind.

If you have any questions then feel free to leave a comment.

As a side note, I hope you enjoyed the animations, they were a fun challenge to create and get working on my website. If you’re interested in how I got them working with WordPress, you can check out this post here

Bye for now!


Leave a Reply