Pressure

If someone steps on your foot with a sneaker, it hurts. If the same happens with a pointed heel, it really hurts, even though the person weighs the same. And in deep snow you sink in, but not with wide snowshoes. Behind all of this is a single idea: pressure.

Pressure = force per area

Pressure tells you how strongly a force is spread over an area: p = F/A, so pressure = force divided by area. You measure the force F in newtons (N) and the area A for example in cm². The pressure then has the unit N/cm². If 100 N press on 10 cm², the pressure is 10 N/cm². The same 100 N on just 1 cm² already gives 100 N/cm².

Large area, small pressure

Snowshoes spread your weight over a large area: the pressure becomes small and you stay on top of the snow. A knife does exactly the opposite: the sharpened blade is extremely narrow, the area tiny and the pressure huge. That is why a sharp knife cuts effortlessly while a blunt one just squashes. Small area means high pressure, large area means low pressure.

Tyre pressure and air pressure

Gases exert pressure too. In your bicycle tyre, compressed air pushes against the rubber from the inside and keeps it firm. With too little pressure, the contact area becomes large and pedalling gets hard. And the air of the atmosphere presses on you all the time, with about 10 N on every cm² of your skin. For pressures like these there is the everyday unit bar: 1 bar is 10 N per cm². So air pressure is roughly 1 bar, and a bicycle tyre is pumped up to about 3 bar. In physics the official unit is the pascal (1 Pa is 1 N per m²). You do not feel the air pressure because it acts equally hard from all sides and your body pushes back from the inside with the same pressure.