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Air has weight

Air and fluids · level 3, lesson 1 of 6

A question to start

Air feels like nothing at all. But the air in your classroom weighs about as much as four grown-ups. Can that be true?

The idea

Air is made of tiny bits, far too small to see, and each one has a little mass. Put enough of them together and the air has real weight.

You can show it with a balance. Hang a blown-up balloon on each end of a stick, so the stick balances. Then pop one balloon. The side with the full balloon drops, because the air squeezed inside it has weight.

full balloon: heavierpopped
A room 10 m long, 8 m wide and 3 m high Mars air Earth air
Mass of 1 cubic metre of air about 0.016 kg about 1.2 kg
Mass of all the air in the room about 4 kg about 290 kg

Worked examples

1. The room. It holds \(10 \times 8 \times 3 = 240\) cubic metres of air. Each cubic metre of Earth air is about 1.2 kg, so the room holds about \(240 \times 1.2 = 288\) kg: about 290 kg.

2. A litre of air. A cubic metre holds 1,000 litres. So a litre of air is about \(1.2 \div 1000\) kg, which is 1.2 g, about the mass of a paper clip.

3. The same room full of Mars air would hold \(240 \times 0.016 = 3.84\) kg, about 4 kg. Mars air is very thin.

Try it

  1. How can you show that air has weight?
  2. A cupboard holds 2 cubic metres of air. About what mass of air is that, on Earth?
  3. Why does the full balloon on the balance hang lower?
  4. A room holds 100 cubic metres. About what mass of Mars air would fill it?
Answers
  1. Balance two blown-up balloons on a stick, then pop one: the full one drops.
  2. \(2 \times 1.2 = 2.4\) kg.
  3. The air squeezed inside it has weight.
  4. \(100 \times 0.016 = 1.6\) kg.

Common mistake

"Air weighs nothing." Air is light, but not weightless. A big room's air weighs hundreds of kilograms.

Where it leads