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Old Feb 29th 2016, 06:36 AM   #1
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atmospheric pressure can be ignored?

in the second picture , the author said that the atmospheric pressure can be ignored , since the pressure act on 2 sides of the door ? my question is which 2 sides? up ? down ? or left ? right ?
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Old Feb 29th 2016, 07:02 AM   #2
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Pressure acts in a direction normal to the surface in question. In this case the surface is the car door, which is assumed to be vertical. So - the pressure acts in the "left - right" direction.

You've asked several times now about ignoring atmospheric pressure - is it clear to you why this is so?
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Old Feb 29th 2016, 07:10 AM   #3
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Originally Posted by ChipB View Post
Pressure acts in a direction normal to the surface in question. In this case the surface is the car door, which is assumed to be vertical. So - the pressure acts in the "left - right" direction.

You've asked several times now about ignoring atmospheric pressure - is it clear to you why this is so?
i do not really understand actually
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Old Feb 29th 2016, 08:01 AM   #4
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Include Atmospheric pressure...

Hi Ling233,

An easy way to go is to include atmospheric pressure. Just put the correct term(s) in the equation. Solve carefully.
Let the physics itself ignore what's small.

Here are three examples.

http://www.thermospokenhere.com/wp/0...lma_surge.html

http://www.thermospokenhere.com/wp/0...off_shore.html

http://www.thermospokenhere.com/wp/0..._guericke.html

Good Luck... (P.S. malware is gone at TSH).
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Old Feb 29th 2016, 08:13 AM   #5
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The reason you can ignore is in this problem is that atmospheric pressure is pressing on the surface of the water and also presses against the outside surface of the submerged door, and atmospheric pressure inside the car presses outward against the inside of the door. Hence they cancel out. Of course you can always include in your calculations - just be sure to include both so that they properly cancel out.

If the inside of the car was a vacuum, you would need to include the atmospheric pressure acting on the surface of the water.
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