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I know that the particles that constitute air move freely about. There must be a significant amount of empty space between the bouncing particles. So why is it so hard to compress air without any machines/devices?

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  • $\begingroup$ What do you mean by "hard"? Are you basically just asking why decreasing the volume of a gas increases its pressure (at constant temperature)? $\endgroup$ Commented Jun 2, 2023 at 22:13
  • $\begingroup$ Compress by how much? I can exhale through a tube a foot or more below the water surface, so my lungs can compress air by at least a few percent. Beyond that I need mechanical advantage... but I also need that to lift more than a hundred lbs. or so. Why are my biological limits important to physics? That 99.9% of air at normal conditions is "free space" follows directly from the fact that the density increases by a factor of 1000 if we cool it down below its boiling point. $\endgroup$ Commented Jun 2, 2023 at 22:14
  • $\begingroup$ @BioPhysicist Since air has a significant amount of empty space, it should in theory be easy to compress, so I'm asking why that isn't the case. $\endgroup$
    – VV_721
    Commented Jun 2, 2023 at 22:17

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By decreasing the volume of the gas, you're increasing the number of collisions the particles make with the walls of the container (over some time interval), hence the walls feel a larger force. It becomes harder and harder to compress the gas because the gas is pushing back with more and more force.

Yes, there is space between the particles still, but the force from the collisions between the particles and the container wall doesn't arise from lack of space between the gas particles. The force the gas exerts on the container doesn't come from interactions between the gas particles (at least if we are still in the regime of an ideal gas).

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    $\begingroup$ Thanks, I didn't think that the gas particles would exert a significant amount of force on the container until most of the empty space became filled in. I assumed that all the empty space had to be filled first before the gas would exert more force on the container. $\endgroup$
    – VV_721
    Commented Jun 2, 2023 at 22:22
  • $\begingroup$ @VV_721 oh ok. If the gas particles weren't moving then you would be correct $\endgroup$ Commented Jun 3, 2023 at 12:01

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