How brakes stop a vehicle
A vehicle can be brought to a stop by applying the brakes only because of friction. Two systems, both built on the same idea.
A bicycle
The brakes of a bicycle work by causing brake pads made of rubber to press tightly against the metal rim of the wheel. When the surface of the wheel and the rubber surface are pressed against each other, the frictional force between those surfaces increases and the bicycle stops.
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Squeeze harder, stop sooner
Pulling the lever harder presses the pads more tightly against the rim. That raises the normal reaction between pad and rim, and by activity 04 the friction rises with it.
A motor car
Modern motor cars use a system called disc brakes for braking. Here a disc fixed to the wheel is gripped by brake pads, and the frictional force produced is used to stop the wheel from turning.
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Where the energy goes
Braking is the one place where friction turning energy into heat is the point rather than the problem. The moving vehicle's energy is carried away as heat in the discs and pads — which is why they get hot enough to glow on a racing car.
The chapter, joined up
Brakes need friction to be large at the disc and the tyres need it large at the road — but the wheel bearings on the same vehicle need it small. Friction is neither good nor bad in itself; it is managed, place by place.