# Force and Particle Density

So take a look at the two boxes below. The box on the left has 10 air particles in it. The box on the right has 20 air particles in it. Which box will have more force pushing on its walls?

To find out, we are going to replace one of the walls in both boxes with a movable wall and put a spring behind it. Since the particles in both boxes have the same mass and the same speed, any difference in the forces pushing on the walls of the two boxes will be caused by differences in the particle collision rates.

The box with 20 air particles in it will have more force pushing on its walls because the particle collision rate is higher. When the particle density increases, the number of collisions per second also increases.

In the simulation below, adjust the particle density to see how it affects the particle collision rate and the force pushing on the wall. A particle density of 1.0 represents our standard air particle density.

You should notice that the particle density goes down a little bit as the movable wall gets pushed to the right and the volume inside the box increases.

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