Figure 4 This series of photos (1–4) illustrates that the atmosphere can exert enough force to crush a metal can. A can containing a small amount of water is heated until the water boils. Then the can is capped, the hot plate is switched off, and the can is allowed to cool. As the can cools, the pressure inside the can becomes less than the pressure outside the can. The difference in pressure crushes the can.

Have your ears ever popped while you were riding in an elevator, flying in a plane, or driving up to the mountains? In situations such as these, the outside air pressure changes more quickly than your ears can adjust. This creates a difference in pressure between the inside of your ear and the air outside. These unbalanced pressures equalize when air passes through a small tube within your ear. This is the popping sensation you feel.

You may be surprised to learn that as you read this, the atmosphere is exerting more than 1000 newtons of force on the top of your head. As shown in Figure 4, this is enough force to crush a can! Why then aren't you crushed by air pressure? Fortunately, the inside of your body also exerts pressure. You are not crushed like the can in Figure 4 because the pressure inside your body balances the air pressure outside. The balanced forces cancel, resulting in a net force of zero.

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