Friction and drag

Friction can be a help or a nuisance.

Helpful friction

Walking or running
Brake blocks on a bike and the rim of the bike wheel
Soles on your shoes
Disc brakes on a car.
Cornering in a car or on a bike
Friction between the tyres and the road
Friction keeping knots in shoelaces or string tied together.
Air brakes for aircraft and in parachutes
Gloves for catching balls and goalkeeping
Sandpaper

Problems with friction

In all moving parts in machinery making heat
Spacecraft re-entering the Earth's atmosphere making heat
Friction between an aircraft and the air making heat
Friction between a boat and the water
Friction between a car and the road making a static electric charge
Friction causing car tyres to become bald

Reducing friction

Engineers always try to make the friction in their machines less to make them more efficient.
They do this by putting something between the parts to stop them rubbing together. They can use:
(i) oil
(ii) ball bearings
(iii) rollers
(iv) a gas
The two surfaces are kept apart by:
(a) blowing air between them (hovercraft)
(b) using strong magnets to push them apart

Air friction is made less by streamlining the object.
Cars are designed to give low drag and speed skaters wear tight fitting suits to reduce air friction