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Consider a 10 inch by 10 inch sheet of paper lying flat on the ground. If atmospheric pressure is 15 pounds per square inch (sea level), then there is 1500 pounds of force acting downwards on the papers surface. How am I able to lift the sheet of paper? If air gets underneath the sheet of paper, then atmospheric pressure on either side will cancel and I'd only have to lift against gravity. But for the most part, I'm assuming there is more pressure on top than below.

My guess is that the seal between the sheet of paper and the ground is not great, allowing air to flow underneath. However, consider the following demonstration: a ruler lies on a table with 5 inches hanging off the side. If I spread a sheet of newspaper over the part of the ruler on the table, I increase the surface area for air pressure to press down on the ruler (without the sheet of newspaper, the air pressure is just acting on the ruler which doesn't have a lot of surface area). Now I can karate chop the ruler and break it in half due to atmospheric pressure giving a counter torque. However, why is it that I can easily lift the ruler with the newspaper on it (meaning, instead of karate chopping, I grab the ruler and just lift)? My guess is that, even if there is 3000 lbs of force acting downwards on the newspaper, I can still lift the ruler newspaper ensemble with say 1 lb of force because air instantly rushes into the cracks. But why don't I feel 2999 lbs of resistive force pushing down on my hand? Air flows in too fast and neutralizes the air pressure instantaneously?

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  • $\begingroup$ Same idea but much higher pressures involved, think about how oil rig divers can pick up objects from the sea floor or how deep sea submersibles can do the same at the depth of say, the Titanic or even deeper? $\endgroup$
    – user81619
    Jul 28, 2015 at 2:23

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A few things happen. One, the paper would bend, but lets pretend it's rigid, what happens when you lift. Your assumption is correct, air moves very fast, reducing but not completely eliminating the difference in air pressure between above and below. Air (molecules) moves at about 1,000 miles per hour.

http://www.phy.mtu.edu/~suits/SpeedofSound.html

For typical air at room conditions, the average molecule is moving at about 500 m/s (close to 1000 miles per hour). Note that the speed of sound is largely determined by how fast the molecules move between collisions, and not on how often they make collisions. This is because no energy is lost during the collisions. The collisions do not "slow things down"

So, in theory, if you could lift rigid paper at 1,000 MPH, you might approach the air pressure on that paper of 1,500 lbs, perhaps even exceed it due to compressive forces above (and there's probably people here who could do that math - I'd prefer not to try), but you'd only get that kind of pressure if you are able to lift faster, or about as fast as the air can move in below the paper.

Another thing to keep in mind about air is that it's compressible, so, even in a sealed tube where air couldn't get past what you were lifting, the air could compress so movement would be possible even if Air couldn't get past what you were lifting.

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Atmospheric pressure has nothing to do with it as you have not created a seal. You just have more surface area and more resistance. Karate chop versus lift is just speed. If you suspended the paper (assuming it would hold its shape) on fulcrum (no surface below) you would have the same effect. Ruler alone and enough seed and you could snap it. In a pure vacuum with zero suction or air resistance you could snap the ruler based on conservation of momentum alone.

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  • $\begingroup$ What about a tile on the ground? It's laying there and there's more atmospheric pressure pushing down than the pressure in the cracks pushing up. So initially and instantaneously, I have to fight atmospheric pressure? Or is atmospheric pressure neutralized on the tile because its not a perfect perfect seal? But even if its not a perfect seal, should there be some substantial net pressure downwards? $\endgroup$
    – DWade64
    Jul 28, 2015 at 2:45
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    $\begingroup$ For this situation, the speed of air flow is important. When the ruler is hit abruptly, the air does not have enough time to flow under the paper to counter the pressure from above. So the paper appears to be very heavy and the ruler breaks. If you slowly push down on the ruler, it will lift the paper and not break. $\endgroup$
    – LDC3
    Jul 28, 2015 at 2:46
  • $\begingroup$ @LDC3 I see, so does my final conclusion for lifting the paper correct? even if the paper (tile, any flat object) is very heavy, I don't feel that weight of the atmosphere because its neutralized by air rushing in. That neutralization is almost instantaneous? $\endgroup$
    – DWade64
    Jul 28, 2015 at 2:54
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    $\begingroup$ @DWade64 The air flow can be measured, but it is very fast. Push an inverted toy pail into the sand (about 2 cm). You should be able to pull the pail out quickly. Doing the same with water on the sand will make it nearly impossible to lift out quickly because the flow of water is slower than air. $\endgroup$
    – LDC3
    Jul 28, 2015 at 3:02
  • $\begingroup$ @LDC3 I removed prior comments as not helping with the stated question. You are the same guy that came back with double pane glass when I tried to answer a question from you about temperature profile. I am not going to criticize your post or down vote but I chose not to try and help you. $\endgroup$
    – paparazzo
    Jul 28, 2015 at 14:32

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