# Why dont we explode when we go indoors?

1. Mar 18, 2013

### jaydnul

I mean we evolved under miles and miles of atmosphere pushing down on us then we walk inside a building, like 10 ft of air pressure, and we are fine? Wouldnt that big of a pressure difference have an impact on our bodies?

2. Mar 18, 2013

### Staff: Mentor

If the air pressure inside a building were equivalent to only 10 feet of air, the building itself would collapse from the weight of the air above it, long before we have a chance to enter it.

3. Mar 18, 2013

### Crazymechanic

Well above ground level the air pressure is pretty much the same everywhere it decreases with going higher up and increases when going lower.Although the official mark from which they count the pressure is sea level.
That's why mountain climbers have to be trained and physically capable because they do physical stuff where air pressure and oxygen is less , also that means you can inhale less air per breath than at lower altitudes.

P.S. When I first read the questions I was laughing :D:D:D I mean exploding people at their houses... :D

4. Mar 18, 2013

### Staff: Mentor

The pressure with which the ceiling is pushing down is not zero. It is the same as the outside pressure that exists 10 feet above the ground.

5. Mar 18, 2013

### Drakkith

Staff Emeritus
The building is not airtight nor is it perfectly rigid. The air inside is pressed upon by all the air around it so the air pressure is the same as it is outside. If you hook up a powerful fan or a vacuum you could reduce the pressure inside, but this would put stress on the building, especially where air is leaking in.

6. Mar 18, 2013

### Staff: Mentor

Really???

7. Mar 18, 2013

### Crazymechanic

the pressure I believe in the case of the ceiling would be the pressure of air at that height + the gravitational potential from the exact mass of the ceiling structure.
Now that is more than just air outside at the same height.

Just like a cylinder put upside down and the piston now facing it's rod upwards.Put some weight on the rod and the piston will get lower compressing the air beneath it because of the weight it now has.

8. Mar 18, 2013

### AlephZero

Yes, in the sense that there is a compressive stress through the thickness of the ceiling that is in equilibrium with the air pressure below and above it.

Of course this stress is so small compared (about 0.1MPa) compared with the failure stress of most materials (hundreds of MPa) that it is usually ignored.

9. Mar 18, 2013

### cjl

Nope - this would be true if the air pressure inside the building is what was supporting the ceiling, but the ceiling is actually supported by the walls of the building. If the air pressure was supporting the ceiling, the pressure inside the building would be higher than outside the building, and every time someone opened a door, air would rush out of the building.

10. Mar 18, 2013

### Crazymechanic

yes the air would also come out and the piston would drop if someone mechanically opened a valve.
Well I didn't say that the air supports the ceiling which it couldn't I just said that there is a pressure which is that of air + that of gravity in the case of ceiling.yes the walls hold the ceiling but that doesn't switch or cancel out the pressure the structure has or exerts on the walls and to the ground together with the air upon it.

11. Mar 18, 2013

### cjl

The pressure of the air at the level of the ceiling is unaffected by the presence of the ceiling though - it'll be the same inside as it is outside at the same height. Internally within the ceiling, you can't really view it as just a pressure either (it'll be some combination of shear and normal stress, depending on the building design, and it won't simply be the weight of the ceiling per unit area - it'll probably be quite a bit higher than that).

12. Mar 18, 2013

### Crazymechanic

@cjl with all respect that is just semantics.
The point is what i wanted to say that you have air pressure and you have gravity which works just as much on the air by making it's pressure just as much on everything else.ceilings, iron H beams cars etc.
Now we may argue where does the ceiling pressure end , at the walls or connections to the walls or some other parts depending on different building construction methods but that doesn't change the fact that there is a pressure.

13. Mar 18, 2013

### cjl

With all respect, it isn't just semantics - the mechanical stresses within the ceiling (which are not simply pressures) don't affect the air underneath in any way unless the building is using the air to help support the ceiling.

14. Mar 18, 2013

### Staff: Mentor

What you said is, of course, correct. The stress vector exerted by the air (acting on the ceiling) at the air/ceiling interface is just equal to the atmospheric pressure surrounding the building at an elevation of 10 ft, and it points upward. The stress vector exerted by the ceiling on the air at the air/ceiling interface has the same magnitude, and, by Newton's third law, points downward.

15. Mar 19, 2013

### SteamKing

Staff Emeritus
Obviously, the OP must live outdoors, otherwise he would have felt that large pressure difference when walking indoors and promptly exploded.

16. Mar 19, 2013

### jaydnul

Well of course I knew i was wrong when I posted this. Just wanted to know why.

17. Mar 19, 2013

### Staff: Mentor

The why is straightforward. Unless the structure is airtight, if there is any pressure difference then air will move in or out until the pressure difference is equalized.

18. Mar 20, 2013

### SteamKing

Staff Emeritus
I wondered why you didn't have an airlock on your front door.