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Dr. Christopher S. Baird

Why don't I feel the miles of air above me that are crushing me down?

Category: Physics      Published: September 14, 2015

air pressure on a hand
Because air is a fluid, the weight of the air is transmitted to the palm of your hand and to the back of your hand at the same time. These forces cancel so that your hand overall feels no net force. Public Domain Image, source: Christopher S. Baird.

Air does not crush you down. As a fluid, air flows around you and tries to crush you in. Fortunately, there is typically just as much pressure inside your body pressing outward as there is air pressure outside your body pushing inward. They typically cancel out, meaning that there is no overall force on you and you don't get crushed. Even when the internal and external pressure don't exactly cancel each other out, your skin, muscles, and other tissues are usually strong enough and flexible enough to not be damaged by the force.

Stand outside in a field and look straight up. You are looking at a one-hundred kilometer column of air that is being pulled down toward you by gravity. Although air is very tenuous compared to other materials, it is indeed composed of atoms and indeed has mass. As such, air is pulled down by gravity just like everything else that has mass. A handful of air may not have much mass, but one hundred kilometers of air is a different story. If you draw a one-meter by one-meter square on the ground (which is about the footprint of fridge), then all of the air directly over that square has a total mass of about ten thousand kilograms. Because of earth's gravity, this mass pushes down on the square meter with a force of about twenty thousand pounds, which is the same as the weight of an empty school bus. In other words, if you placed an empty school bus on a square meter pedestal (to focus the force to the right area), the patch of ground under the pedestal would be experiencing just as much weight from the bus as it normally does from air.

Now draw a one-inch by one-inch square on your shoulder. The one hundred kilometers of air directly above that square has a total mass of 7 kilograms. Under the influence of earth's gravity, this mass pushes down on the square inch of your shoulder with a force of about 15 pounds. This is the same as the weight of a large bowling ball. In other words, the air pushes down on the square inch on your shoulder just as hard as if there was a large bowling ball sitting on that square inch and no atmosphere. Furthermore, there is nothing special about that particular square inch on your shoulder. Every square inch on the surface of your body feels 15 pounds pushing on it because of the air in the atmosphere. For instance, if the palm of your hand has a surface area of 10 square inches (which is a typical value), then the air pushes on your palm with a total force that is equal to the weight of 10 large bowling balls. But why don't you feel it? It certainly doesn't feel like you are constantly holding 10 large bowling balls in the palm of your hand. The answer is that air is a fluid.

As a fluid, air is able to flow in every direction and take any shape. As it does so, air transmits the crushing force of its weight in all directions, even up. Hold out your hand with the palm facing up. While it's true that the air is pushing down on your palm with the force of 10 bowling balls, at the same time the air is pushing up on the back of your hand with the force of 10 bowling balls. As a result, the total force on your entire hand is zero. The individual surfaces (the top and bottom of your hand) feel the air pressure, but the hand as a whole experiences no net force from the atmosphere. That is why it does not feel like you are constantly lugging around ten bowling balls in the palm of your hand, and that is why one hundred kilometers of air above you does not pin your hand to the ground. Similarly, although air is pressing down on the top of your head with hundreds of pounds of force, it is also pressing up on your chin and neck with hundreds of pounds of force. As a result, your head does not get crushed down against the sidewalk. In this way, the fluid nature of air makes it so that the weight of the atmosphere pushes everywhere in on you, but does not push you down.

This canceling of forces on your hand only happens if atmospheric air is able to reach both the palm and the back of your hand. If you remove the air that is pushing up on the back of your hand, there will no longer be a canceling of forces and you will experience 10 bowling balls worth of weight on your hand overall. You can partially do this by turning on an extremely strong vacuum machine while pressing the opening at the end of the hose directly against the back of your hand. The vacuum machine sucks and removes the air that was pressing on the back of your hand, allowing your hand overall to feel the downward force of the air pressure on the palm of your hand. You would probably describe this experience as the hose sucking your hand down towards it, and therefore would say that the force that your hand overall feels is a pulling force exerted by the hose. In reality, the weight of the air above your hand is pushing your hand down towards the vacuum hose and is the source of the force.

Any time we talk about suction we really mean removing the air from one side of an object so that the immense weight of the air in the atmosphere can be experienced by the object overall without being canceled. Any time you have had difficulty removing a suction cup from a window, that resistive force you are experiencing is literally the weight of the atmosphere's air crushing it against the glass. If the suction cup is large and has a good seal, you will not be strong enough to directly pull the cup off. Simply put, you are not strong enough to oppose the force of the air crushing the suction cup against the glass. However, you can get the suction cup off of the window easily by letting atmospheric air get to the other side of the suction cup. Poke a pencil or pin in between the cup and the glass to break the seal, thereby letting air flow behind the suction cup. Once this happens, the forces on the cup from the air on both sides cancel each other and the cup is no longer crushed against the glass. With these concepts in mind, it should now be obvious that suction cups don't work in the vacuum of space.

Even when the force of the air on the back of your hand cancels the force of the air on the palm of your hand, your hand is still getting crushed in the middle between these two opposing forces. Fortunately, there is also pressure inside your hand directed outwards which cancels the inward force of air pressure. As a result, there is no net force on the surface of your hand. The internal pressure is not caused by air but is caused by trapped water. Internal body pressure is created and maintained by having semi-rigid cells that are pumped up with water using the chemical attractive forces between water and ions such as sodium. Each cell is a bit like a water balloon. If you pump water into a balloon, the internal water pressure can cancel out the external air pressure and the balloon holds its shape without being crushed.

If you make a container air tight and then reduce its internal pressure so that the canceling effect goes away, the weight of atmospheric air is indeed strong enough to inwardly crush the container. For instance, take an empty metal barrel and heat it up with the cap off so that the air inside the barrel heats up, expands, and partly escapes. Now, place the cap on tightly and place the barrel in a pool of ice. As the remaining air in the barrel cools, it loses its pressure and is no longer able to cancel the air pressure outside the barrel. As a result, the barrel implodes.

Note that even if the internal body pressure and the external air pressure are not exactly equal and therefore don't cancel each other out, most tissues in your body are strong enough to withstand the resulting net force. For instance, if you place a human in the vacuum of space without a space suit, there will be the regular internal body pressure but no external air pressure to cancel it out. Despite this difference, such a person will not explode since the skin is strong enough to withstand the pressure. Vacuum exposure does indeed harm humans and even causes death after a few minutes, but it does not cause humans to explode from the pressure difference.

Topics: air, air pressure, atmosphere, pressure