Even though they’re too tiny to see, all the molecules of air in the atmosphere above your head weigh something. And the combined weight of these molecules causes a pressure pressing down on your body of 10,000 kg per square metre. This means that the mass of the air above the 0.1 square metre cross section of your body is 1,000 kg, or a tonne.

I would agree with the argument that the atmospheric pressure is a result of the weight of the air above me were I standing in an open area. I do not understand how, by this model of atmospheric pressure, the reason of atmospheric pressure can be explained in a closed room say.


  • $\begingroup$ Are you sure that pressure of the atmosphere is caused because of air weigh? Doesn't the motion of the air molecules have any effect on the atmosphere pressure? $\endgroup$ – lucas Apr 16 '16 at 14:30
  • $\begingroup$ physics.org/facts/air-really.asp $\endgroup$ – The Cryptic Cat Apr 16 '16 at 14:35
  • $\begingroup$ I know air has weigh! I said that the pressure of atmosphere is caused by air weigh and molecules motions, both. $\endgroup$ – lucas Apr 16 '16 at 15:41
  • $\begingroup$ Are you asking how the pressure can act horizontally, or are you asking how, if the room is separate from the atmosphere, the pressure in the room can be equal to atmospheric pressure? $\endgroup$ – Chet Miller Apr 16 '16 at 16:25
  • $\begingroup$ @ChrisWhite Fixed! $\endgroup$ – The Cryptic Cat Apr 17 '16 at 3:19

From Pascal's law, we know that pressure is isotropic, which means that at a given location in a fluid, it acts equally in all directions. So, at a given location, the horizontal force per unit area acting on a small vertical surface is the same as the vertical force per unit area acting on a small horizontal surface.

Usually, a room is not hermetically sealed, so it is not totally separated from the atmosphere. Any connection between the room and the atmosphere allows the pressure to equalize (by air flowing in or out). As we said above, pressure acts horizontally also, so air can come through a vertical crack just as easily as through a horizontal crack. In a house, there are typically vents in the attic which allow communication with the atmosphere.

If the room were totally hermetically sealed from the atmosphere, then you could impose any air pressure you wanted inside the room. It would not have to match the outside atmospheric pressure. But, the forces on the walls could get pretty large between inside and outside as a result of the pressure difference, and you would have to be pretty careful so that the room didn't implode or explode. When tornadoes occur, the atmospheric pressure outside drops substantially, and people are recommended to open the windows (to allow the pressures to equalize) in order to avoid the windows blowing out (or even worse).


This is a duplicate as far as atmospheric pressure goes.

As long as the container is not air tight there will come equalization of pressure. To understand why pressure equalizes one has to see the derivations of the ideal gas law, PV=RT using statistical mechanics, for example here. The attribute "law" is indicative of a thermodynamic law, which was observed to hold, not derived. Only after the understanding using statistical mechanics it could be derived.

Gas in an air tight container will keep the pressure it had when in equilibrium with the atmosphere unless temperatures change. The motions of the gas molecules exert an effective kinetic pressure on any surface they impinge on ( remember pressure is force over area) according to the ideal gas law.


"Closed room" word is ambiguos. If first the room was open , then was closed, without pumping all of air out(creating a vacuum), the pressure conditions would remain same, because there will be still air inside the room.

However, had we pumped all air out, our blood capillaries would collapse, since our internal pressure is 1 pascal, and it exceed the external pressure by 100000 Newtons.

To understand what i mean is, think about how a lizard can stick to a wall,or how do suction pumps work(in bow and arrow toys).The basic principle is that, there is vacuum created at the surface of contact.enter image description hereFrom your mechanics course you could easily draw FREE BODY DIAGRAM and notice forces due to pressure, explaining the suction.

Note that forces along horizontal direction dont mater because air pressure would be equal on both sidesenter image description here


I do not understand how, by this model of atmospheric pressure, the reason of atmospheric pressure can be explained in a closed room say.

Let's take your closed room to it's most extreme, a sealed tank containing a gas in deep space. There's no need to worry about gravity, or the fact that the Earth's atmosphere is exposed to vacuum. The pressure of the gas in the tank is a function of density, makeup, and temperature of the gas.

Ideally, the relation between pressure, density, makeup, and temperature is a very simple function, $P=\rho R^\ast T$, where $P$ is the pressure of the gas, $\rho$ is the density of the gas, $R^\ast$ is a constant that is specific to the makeup of the gas, and $T$ is the absolute temperature of the gas. Alternatively, $PV=nRT$, where $n$ is the number of particles and $R$ is the universal gas constant. Both $P=\rho R^\ast T$ and $PV=nRT$ are expressions of the ideal gas law. You need to understand the ideal gas law and its ramifications to have any understanding of the behavior of real gases.

The behavior of the gas in that sealed tank is not quite so simple in a domain where gravitation is non-negligible. The pressure in that idealized sealed tank of gas is no longer uniform when that gas tank is at rest on the surface of the Earth and the gas is in an equilibrium state. A careful experimenter will find that the pressure varies with height. This is because the gas is in a state of hydrostatic equilibrium.


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