Air resistance - how can it work?

Air getting in the way

Wrong Track: Air is so light, how can it be strong enough to exert a force?

Right Lines: Air is not nothing. It has a mass and it does get in the way of moving objects.

Particles colliding

Thinking about the learning

For many learners air is seen as nothing. Comparing the mass of air in a small domestic room to the mass of a child might help to undermine this distinction (both can be 40 kg).

It is invisible and so doesn't really amount to much, if anything. It certainly is not the same sort of thing as a car engine, which we can see is strong enough to exert a force. The fact that air pressure at the surface of the Earth is about 100,000 newton metre^-2 of surface is also not at all obvious.

Thinking about the teaching

There are many cases where we need air to get in the way. Parachutes rely on air colliding with the large area of the canopy. Wind generators move because air exerts a force on the blades. Sail boats depend on a force from moving air. Although air does have a low density, if we can bump into enough of it (via a large area like a sail) and if the air is moving fast enough it will exert a considerable force. The moving air is not in itself a force. The air doesn't carry a force simply by moving. However, when the air collides with a surface, both the air and the surface experience a force. A force from the surface acts on the air to slow it down. There is a force in the opposite direction acting on the surface. It is this force on the surface which drives the sail boat through the water.

Motion is relative. Air colliding with a surface happens when either the moving air hits the surface (e.g. a sail boat) or the moving surface hits the air (e.g. a parachute or a sprint cyclist).