How do brakes work?

We all know that brakes are for slowing and, ultimately, stopping a car. But how do they work? Well, there are plenty of different types of brakes that have been tried in the last century, but they all work on the same principle of physics. That is, they use friction to convert kinetic energy into heat (and often a bit of noise) to slow the car.

When you step on the brake pedal, you cause two friction surfaces to come together. That friction converts the car’s momentum into heat and it slows down. Most modern cars have what’s called a disc brake on each wheel and in that case, when the brake pedal is pushed, fluid (which is non-compressible, remember) pushes stationary brake pads onto a disc, or rotor, that’s turning with the car’s wheels.

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The harder you push the brake pedal, the harder the brake pads squeeze the rotors and the faster the car slows down. And the more heat is generated. Ever spun a pushbike wheel and grabbed the tyre to stop it? The car tyre is the rotor and your hand is the brake pads.

The conversion of kinetic energy to heat is why your hand gets hot when you do this. The big change to braking over the last few years has been in hybrid cars which start to brake by switching their electric motor to a generator and converting kinetic energy into electricity to recharge the on-board batteries.

But again, this conversion from kinetic to another type of energy is what slows the vehicle down. And when a hybrid has to stop fast, conventional disc brakes take over.