Friction forces act against the movement of one surface over another such as tennis shoes on a grass court. Friction is the force which prevents the player slipping and sliding. When air passes over a surface, particularly at speed then a frictional force called air resistance occurs.

## Friction

Some examples of friction acting in sport are:

• A sprinter accelerating on a track. The foot applies a force downwards and backwards into the track. The friction forces of the track resist this causing forward movement (otherwise the sprinter would slip). Friction acts in the opposite direction to the force the foot applies on the track but in the same direction as the movement.

• A downhill skier. As the skis glide over the snow friction forces resists the movement of the ski on the surface of the snow. These forces are much less than experienced on the track or the skier would be unable to move down the hill.

Friction is increased when:

• One or both of the surfaces are rough.
• The temperature of some surfaces is increased for example rubber tyres on a racing car need to be warmed up to create more friction.

## Air resistance

Air resistance is a friction force applied by the air on bodies that are moving through it. The amount of air resistance on a body depends on:

• The velocity it is traveling at (the faster it goes, the more air resistance).
• The cross sectional area (the larger the area, the more air the body has to displace so the higher the air resistance forces).
• The shape and surface of the body (a pointy shape is going to 'cut' through the air more easily than a flat one and if the surfaces are smooth then the air flows over more easily).