# How does atmospheric pressure work?

1. Aug 30, 2014

### yunocell

From what I know, the pressure of a gas is due to the collisions of its molecules against a surface. So I don't understand why atmospheric pressure is measured by taking the weight of all the air molecules above a surface including the ones that aren't contacting it. Gases have plenty of space between their molecules so I don't understand how I can feel the weight of a gas molecule one mile above me that isn't in contact with anything else. Can somebody please enlighten me? Thanks in advance.

2. Aug 30, 2014

### Simon Bridge

Welcome to PF;
The gas in the air all bumps into each other so how hard the molecules close to you bump into you depends also on how hard all the other molecules bump into each other. So it is reasonable to expect that the air way above you would have some effect.

The reason this effect comes to the weight of the air above a spot on the ground is that the air pressure is what stops all the air being a thin pancake under your feet. There has to be enough pressure to hold the whole atmosphere up.

3. Aug 30, 2014

### yunocell

Ok, I think I understand now. Thanks for the answer!

4. Aug 30, 2014

### Simon Bridge

If you has a cylinder of air with a piston on top that was being held up by the pressure in the container, then you'd have to figure the pressure in the container in terms of how heavy the piston was right?
It works like that with water pressure too.

The trouble with picturing air pressure it there is no obvious piston ;)

But you should have a decent mind-pic now to go on with.

5. Aug 30, 2014

### yunocell

So if I go outside and I get a container and a piece of paper, and I put the piece of paper on top of the container to seal it, does that mean that the inside of the container is 14.7psi (atmospheric pressure) just like the outside?

6. Aug 30, 2014

### jbriggs444

Yes.

7. Aug 30, 2014

### Simon Bridge

... if there were a difference, then the paper would bend.
This effect can be used to build a barometer.