I have used this example but need to ask a question.?

Question

We often refer to air pressure as the weight of air directly above our heads. But we typically do not "feel" this weight. Why? Since air surrounds our bodies, and all things, the pressure, as a result of the air, is applied equally on all sides. For example, if someone holds an 8½x11" sheet of paper by their hand at arms length, the weight of the air directly above the sheet is over 1,300 pounds.

Obviously the paper does not weight that much. Why? That same pressure (14.7 pounds per squar inch) is also pressing up on the bottom side of the paper. The equal pressure on all side cancel each other out so all that is left is the weight of the matrerial that comprises the paper.

How is the same pressure pressing up beneath the paper. Does this pressure act against gravity?

Answer 1

If you hang a piece of aluminum foil within a vacuum (perhaps a bell jar) there is no pressure on the foil. If you release a small amount of gas into the chamber the molecules will bounce off the walls of the chamber and both sides of the foil by the thousands per second. The "pressure" is balanced as both sides take as many hits but the pressure is no longer zero. The energy of the gas molecules (velocity) depends on the temperature of the chamber wall because cold molecules hitting a warmer wall will pick up energy and hot molecules hitting a cooler wall will give up energy. The pressure depends on the average energy of the gas molecules and the number of hits per second, etc. As more gas is let into the chamber, the number of hits per second will increase as will the pressure. Because the gas molecules are free to bounce in any direction; up, down or sideways, gravity has nil effect. on an individual gas molecule. However, collectively, the earth's gravity pulls down on every gas molecule in the atmosphere preventing them from escaping into outer space. The weight of a one square inch area column of air reaching from the ground all the way up to the upper edge of the atmosphere (and into the vacuum beyond?) will equal 14.7 pounds or more specifically 14.7 psi (pounds per square inch). On top of a mountain it may be only 14 psi because there is less gas above. Every gas molecule near the ground has to resist atmospheric pressure from every direction bouncing off all its immediate neighbors and any surface in the way and that is why the gas pushes with as much force up as down on your hand or a piece of paper. You can not feel the pressure because it is balanced inside and outside of your body except perhaps when you breath or before your ears pop coming down from a mountain.

Answer 2

Pressure is created by enclosing the space within which molecules can move. The molecules of air are bouncing around and yet enclosed by the atmosphere.

Within the enclosure, the molecules are bouncing off all sides of everything. The bouncing molecules are hitting the top and the bottom of the paper equally.

The example above regarding the straw is a good one. At the top, your finger is preventing the air molecules from bouncing off the water. The air molecules at the bottom of the straw are bouncing off the water so rapidly and with so much force they can push the water up into the straw. In fact, the force is so great that on a standard day a straw with an opening of 1 square inch could hold 14.7 pounds of water -- about 1.76 gallons!

(Of course, the viscosity of water will require that your "opening" actually be comprised of several small openings. Or you could do it with a liquid that has a higher viscosity like liquid mercury.)

Answer 3

hold your finger on the top of a straw filled w/ liquid....

ask "Why doesn't the liquid fall to the groung if gravity is pulling it? "

the answer is the air pressure is pressing the fluid up into the straw without pressure on top... when you let your finger off the straw, the air pressure forced the air to displace the liquid in the straw

Answer 4

pressure acts on all directions. Picture putting a float in water, it floats up, y?? because the pressure acting below is more than the pressure acting above.. Imagine yourself in a deep pool, swimming horizontally.. do u onli feel the pressure on the side facing up?? no, u feel the pressure from the bottom as well..

Answer 5

Air pressure acts in all directions, not simply up or down. The only directional force it exerts is air friction or resistance.

Air friction acts against gravity in opposing gravity's pull - you get terminal velocity as a function of gravity's pull versus wind resistance. However, these are dynamic forces, and not in play for static (Resting) objects.

Answer 6

It is neural so only gravilty effects the weight