Air presses on everything, and it presses hard

Air is all around us. The layer of air round the Earth is the atmosphere. It contains nitrogen, oxygen, argon, carbon dioxide and small amounts of other gases, and it reaches up many kilometres above the ground.

Does air press on things? (Activity 6.3). Turn a paper plate upside down, fix a stick to it as a handle, and set it on a flat surface. Take two identical sheets of chart paper, each about 70 cm × 56 cm. Fold one twice, make a hole in the middle for the stick, lay it over the plate and lift by the stick. Now use the second sheet unfolded, also with a hole for the stick, and lift again.

The unfolded sheet is harder to lift. Its weight hasn't changed at all: only the area it covers has grown. So the extra effort can't come from its weight. It comes from air pushing down on the sheet, and a bigger sheet means a bigger push. Air presses on every object. The pressure of the air around us is called atmospheric pressure.

Blow up a balloon and it swells all round, not just in one direction. So air presses in all directions.

A sucker (Activity 6.4). Press a rubber sucker firmly onto a smooth flat surface. It sticks and is hard to pull off. Pressing it squeezes most of the air out from under the cup, so the pressure inside drops. The higher pressure of the outside air then holds it on. (Phone holders that stick to glass work this way.)

Atmospheric pressure is large. The air above a patch just 15 cm × 15 cm pushes down with a force of about 2250 N, like the weight of a 225 kg mass. Why aren't we crushed? Because the pressure inside our bodies, from the fluids and gases in our tissues and organs, is equal to it and balances it from within.

The SI unit is the pascal, but weather reports use the millibar (mb) or the hectopascal (hPa). Each is equal to 100 Pa.