You’ve got one tonne of air pressing down on you, the same as a small car

Really?

Yes, really. We’ve assumed that you’re at sea level and that the area of the top of your head and shoulders is 0.1 square metres, but that’s not unreasonable. So how could it be possible that you’re carrying around the equivalent of a small car all the time?

The weight of air

Even though they’re too tiny to see, all the molecules of air in the atmosphere above your head weigh something. And the combined weight of these molecules causes a pressure pressing down on your body of 10,000 kg per square metre. This means that the mass of the air above the 0.1 square metre cross section of your body is 1,000 kg, or a tonne.

If you tried to lift a small car, you’d certainly notice it, so why don’t we notice that there’s a tonne of air pressing down on us? Well, the air exerts this force in all directions, so as well as pushing down on us, it also pushes up and balances out the force on our bodies so that we don’t collapse.

Crushing pressure

Human bodies are used to air pressure. The air pressure in our lungs, ears and stomachs is the same as the air pressure outside of our bodies, which ensures that we don’t get crushed. Our bodies are also flexible enough to cope when the internal and external pressures aren’t exactly the same. Aeroplanes need pressurised cabins to compensate for the lower air pressure at high altitudes. Despite this artificial atmosphere, the air pressure inside an aeroplane is not the same as at sea level. You might have noticed that if you drink from a plastic bottle during a flight and put the lid back on, when you land the bottle will be crushed. This is because the air in the bottle is at the lower pressure of the cabin and it can’t withstand the higher air pressure at ground level.

You’ve probably also noticed that your ears pop during the take off or landing of a flight. This is caused by the difference in air pressure on either side of your ear drums and the only way to equalise the pressures is to yawn, suck a sweet or breathing out whilst holding your nose.

Find related websites for atmospheric pressure with physics.org.

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