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Low pressure air means a lack of air molecules. Normal pressure air means a normal amount of air molecules.

Say you have a stationary (can't move no matter what) cube of say steel, with low pressure on all 6 sides. We can just focus on one side for now. To take it to the extreme, say the low pressure is a complete vacuum. This cube is floating in an atmosphere of normal pressure air.

When the air molecules travel into this vacuum zone, they will continue in a straight line. Usually they would bounce off of their neighbors.

My question is, why will this make the cube have a force exerted on it? The air molecules traveling in a straight line towards the cube shouldn't really do anything. Similar to how pulling a heavy weight towards you quickly will exert a force on you in the direction the weight. The difference here is the air molecules aren't being pulled towards anything.

What is happening in this situation is the low pressure (vacuum) is pulling the surrounding air towards it. But as I explained, why would this make thrust?

(For some reason I found this really hard to explain, so if this needs improvement let me know!)

Also, on each side of the cube, say the vacuum low pressure area extends 6 inches out.

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    $\begingroup$ You're contradicting yourself: "low pressure on all 6 sides" versus "floating in an atmosphere of normal pressure air". $\endgroup$
    – John Doty
    Commented Jul 14 at 2:08
  • $\begingroup$ @JohnDoty The 6 sides of the cube are low pressure. The atmosphere surrounding those sides is not. If it’s not clear, say 6 inches off of each side is a vacuum. $\endgroup$
    – Wyatt
    Commented Jul 14 at 2:32
  • $\begingroup$ It is not clear what you mean by "low pressure on all 6 sides". Are you referring to the pressure inside an empty cubical container? Is your cube solid or hollow? $\endgroup$
    – Vincent Thacker
    Commented Jul 14 at 10:11

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Low pressure air means a lack of air molecules. Normal pressure air means a normal amount of air molecules.

Low pressure means that the combination of the number of molecules and the speed of the molecules over an area is less than that corresponding to some other area. It doesn't have to mean a lack of molecules.

When the air molecules travel into this vacuum zone, they will continue in a straight line. Usually they would bounce off of their neighbors.

For an ideal gas, we simply ignore the fact that they might bounce off neighbors. We can just imagine the gas particles move in a straight line until it hits a wall or other surface. So for now, don't worry about bouncing off a neighbor.

My question is, why will this make the cube have a force exerted on it?

Gas particles have mass and therefore momentum. When they bounce off a surface, they exert a force on the surface they bounce from, just like a tennis ball or basketball would. The combination of the number of particles in a given time and the speed of those particles will give you the total force on the surface.

What is happening in this situation is the low pressure (vacuum) is pulling the surrounding air towards it.

No. Low pressure doesn't "pull", it just pushes less than a higher pressure region would. If you have high pressure on one side and low pressure on the other side, then you will have a net force exerted because the higher pressure is pushing harder.

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  • $\begingroup$ I see, thanks. So in the case where you have something like a wing at 45 degrees. The top of the airfoil is low pressure, and because of that the air is bent towards it. Why does this bending of air create a force? I understand why the pressure difference between the surfaces would make a force, but not why the bending of air over the top of something like an airfoil would. $\endgroup$
    – Wyatt
    Commented Jul 14 at 3:09
  • $\begingroup$ (if that doesn't make sense I can clarify) $\endgroup$
    – Wyatt
    Commented Jul 14 at 4:09
  • $\begingroup$ The pressure difference creates the force (not the bending). The reasoning of why the pressure difference is created is more complex. $\endgroup$
    – BowlOfRed
    Commented Jul 14 at 5:31

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