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Offline DoctorBeaver

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Air pressure
« on: 20/01/2006 04:13:04 »
I'm confused... again. [|)]

I've always been told that air pressure acts on us from all directions. But why? Gravity is pulling the atmosphere down; not pushing it up again. Surely the weight of all that air pressing down is greater than any pressure from underneath. If you pour water onto the ground, it spreads out; it doesn't go back up.
Why doesn't a house of cards collapse under the weight from above?


 

Offline ukmicky

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Re: Air pressure
« Reply #1 on: 20/01/2006 04:52:36 »
Air is a fluid so isnt it the same  buoyancy force that lets us float in the sea of water.

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« Last Edit: 20/01/2006 04:53:16 by ukmicky »
 

Offline Dr B

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Re: Air pressure
« Reply #2 on: 20/01/2006 05:45:44 »
Air molecules hitting an object cause air pressure on it.  They hit you from all directions, not just downwards, as they bounce around.





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Offline Soul Surfer

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Re: Air pressure
« Reply #3 on: 20/01/2006 09:32:26 »
If you pour water into a bucket it fills the bucket and the water pressure in the bucket or swimming pool is higher at the bottom than the top.  Try swimming to the bottom of a 3-5 metere deep pool and you'll soon notice!

The gravity just causes the air to fill the bucket that is the earth (just like the oceans) and not drift off into the space too fast.

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« Last Edit: 20/01/2006 09:33:04 by Soul Surfer »
 

Offline DoctorBeaver

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Re: Air pressure
« Reply #4 on: 20/01/2006 14:31:32 »
Simon - I'm aware of that. But as I said, the air is being pulled down by gravity. Assume there's no wind anywhere. The weight of the air around you would be pressing down on the ground. But surely the column of air directly above you must weigh quite a lot. Doesn't that add a force in addition to that exerted by the action of molecules hitting you?

Ian - I know that. What I'm asking is why doesn't gravity make the air/water above you press down on you with greater force than that exerted by the air/water around you?
 

Offline rosy

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Re: Air pressure
« Reply #5 on: 20/01/2006 17:18:04 »
OK, I'll give it a go.
It is the case that the air pressure pushing down on you is huge.
I believe the analogy quoted is about one elephant per square metre. It doesn't much matter what sort of elephant it's still enormous! The reason that you're not crushed by it is the equal and opposite push of your internal pressure.

However, since the air doesn't just sink through the planet's surface, the ground must exert an equal and opposite force *upwards* on the air (the same reason your dinner, and the plate it's on, won't fall through the table).

Now, air is a fluid. If it's being squashed from above, and pushed up from below, then in order to stay put (for there to be no wind blowing) it must be being pushed in from all sides at the same time. Otherwise it'd just squash out the side. Infact, this is how winds form: air rushes toward an area of low pressure.

I'm not sure if that helps...
 

Offline Soul Surfer

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Re: Air pressure
« Reply #6 on: 21/01/2006 08:58:36 »
Rosy's got it Beaver. the reason that the air presses on you equally on all sides is the same reason that the water sperads out when you pour it on the ground air and water move around if there is a force on them so they sperad out until the forces on all sides balance.

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Offline DoctorBeaver

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Re: Air pressure
« Reply #7 on: 21/01/2006 11:42:57 »
I think I get that. Thanks.
Next question - what causes the internal pressure?
 

Offline chris

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Re: Air pressure
« Reply #8 on: 21/01/2006 11:47:55 »
Of What ?

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Offline DoctorBeaver

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Re: Air pressure
« Reply #9 on: 21/01/2006 12:03:33 »
Of the human body
 

Offline daveshorts

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Re: Air pressure
« Reply #10 on: 21/01/2006 13:33:41 »
Do you mean that the human body is at about 1 atmosphere internally?

If you get something a bit flexible and push it from all directions with one atmosphere of pressure, all the molecules will get slightly closer together, repell each other and push outwards with the same 1 atmosphere of pressure.
 

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Re: Air pressure
« Reply #10 on: 21/01/2006 13:33:41 »

 

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