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Q & A: what causes atmospheric pressure?

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Most recent answer: 01/11/2013
Q:
Consider the three statements 1)Air pressure acts from all directions 2)Air pressure is due to the motion of air molecules 3)Air pressure is caused by the weight of air above Statements 1 and 2 seem logical to me. Statement 3 seems illogical to me -obviously pressure in gases is dependent upon temperature and pressure and will decrease as one rises in the atmosphere but not because of a decrease in the weight of air molecules pushing down from above. My question. is there a fault to my reasoning? Many thanks for your help Stephen
- Stephen (age 57)
Risch, Zug, Switzerland
A:
Many questions about causation raise philosophical issues, since the meaning of causation isn't very clear.Here I think we all have no problem with the first statement, which is just an empirical fact. Your second statement is a pretty non-controversial one as well, since if you take away the air molecules the pressure goes away and if you keep the same density but have them move slower the pressure is reduced. In fact, you might say that the moving air molecules are the proximate cause of the pressure.

So the third statement is the one that bugs you. The atmospheric weight isn't quite as proximate a cause of the pressure as is the local atmosphere. If you quickly removed all the atmosphere above say 1 km (see Spaceballs) it would take a little bit of time before the pressure down here dropped. Essentially no effect would be noticed until time of 1km/(speed of sound) or about 3 seconds. Still, if you removed that weight pretty soon the pressure here would indeed drop, so there's a decent sense in which you can say the weight causes the pressure. The basic point is this: The net force on a column of air above us is zero, in equilibrium. The downward force on it is just gravity, its weight. The upward force is just the pressure of the air down here times the cross-sectional area of the bottom of the column. So the weight does determine the pressure, in equilibrium although not quite instantly.

Mike W.

(published on 01/11/2013)

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