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Q & A: Air Pressure

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Most recent answer: 10/22/2007
Q:
In another answer you said that air pressure was due to the force excerted by the air molecules as they bounce off a surface. To me this means that the speed of the molecules is causing the pressure. But I’ve also read that airpressure was caused by the gravitational force on the molecules. But the molecules don’t just sit there, stacked together excerting a downward force, they are in motion. So what should I believe?
- Frank (age 18)
denmark
A:
First, you're very close, but saying that pressure is caused by the speed of molecules bouncing off a surface is not quite right. The force on the surface due to a single molecule is actually the result of the mass of the molecule times the acceleration (or change in velocity) of the molecule. When a molecule hits the surface, it bounces off at the same speed, but in the opposite direction. So the change in velocity (which depends on both speed and direction) is twice the speed of the molecule. The force of one molecule on a surface isn't very much, but molecules are constantly hitting it. So pressure on a surface is the average force acting on the surface over the area of the surface.

Now when we talk about the pressure of the entire atmosphere, we're talking about a lot more than one molecule. And all of the different molecules are moving in different directions so that the average velocity of the air is zero, assuming there's no wind causing many particles to move in the same direction. So even though the individual molecules are moving, the atmosphere as a whole actually is just sitting there exerting a downward force. And since gravity tends to hold the molecules down, there are more molecules near to the surface of the Earth than higher up.

So the force on a surface is caused by the average force exerted on it by molecules bouncing off, and the average force exerted by the air molecules depends on the pressure of the surrounding air. For example, if you go high up into the mountains, the atmospheric pressure is less, so fewer air molecules hit you and you feel less air pressure.

Hope this answers you question,
Tim

(published on 10/22/2007)

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