## BSL Physics Glossary - air friction, water friction, streamline - definition

I'm going to discuss two kinds of friction: air and water. As air rushes past a moving car, the air friction will slow down the car. If there were no air friction the car would travel much faster.

You can see the same thing with water friction. Water moves past a moving boat. If there wasn't any water friction, the boat would travel much faster.

An example of where we see air friction is in a vehicle. A sports car is long and smooth - streamlined. The air can slide over it quickly. In a more flat-fronted vehicle like a truck there is much more air friction. It slows the vehicle down. The air pushes against the front of the vehicle.

We can see an example of water friction in boats. A smooth, long hull will be perfect for letting the water rush past easily. The boat will speed easily through the waves. But if the front of the boat is flat, like on a barge, the water friction will slow it down. The water will push against the boat. The boat won't be able to go quickly.

So, with both air and water, a streamlined shape reduces friction and helps the boat or car go quickly. A flat front increases air or water friction and slows the boat or car down.

We can also see air friction when a person jumps out of an aeroplane. If the person is lying face down they have more air pushing up on the whole of their body as they fall. But if the person tilts upwards then suddenly the amount of air friction is reduced, because it is only the feet that are pushing against the air. Suddenly the person drops quickly! There is still some air friction. If there were no air friction, the person would drop much more quickly!