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How do brakes work?

You're driving along quite happily when, all of a sudden, a dog runs out into the road just in front of you. You have a split second to react to what's happened. When you stamp on the brakes, you confidently expect they'll bring you to a halt in time.

But, how can you be so sure?  Because brakes use the power of science and thankfully, for the most part, science doesn't let us down!

Most cars have two or three different types of braking systems. Peer through the hubcap of a car's front wheels and you can usually see a shiny metal disc just inside. This is called a disc brake. When the driver steps on the brake pedal, a pad of hard-wearing material clamps onto the brake disc and rubs it to make it slow down—in a similar way to bicycle brakes.

Some cars have disc brakes on all four wheels, but many have drum brakes on the back wheels, which work in a slightly different way. Instead of the disc and brake block, they have shoes inside the hollow wheel hub that press outwards. As the shoes push into the wheel, friction slows you down.

A car's handbrake applies the two rear brakes (disc or drum) in a slower, less forceful way when you pull on a lever located between the front seats.

A speeding car has loads of energy and, when you stop, virtually all of it is converted into heat in the brake pads. The brakes can heat to temperatures of 500°C (950°F) or more! That's why brakes have to be made of materials that won't melt, such as alloys, ceramics, or composites.

Imagine how much force you need to stop a fast-moving car. Simply pressing with your foot would not generate enough force to apply all four brakes hard enough to bring you quickly to a stop. That's why brakes use hydraulics: a system of fluid-filled pipes that can multiply force and transmit it easily from one place to another.

When you press on the brake pedal, your foot moves a lever that forces a piston into a long, narrow cylinder filled with hydraulic fluid. As the piston plunges into the cylinder, it squirts hydraulic fluid out through a long and narrow pipe at the end (much like squirting a syringe). The narrow pipe feeds into much wider cylinders positioned next to the car's four brakes. Because the cylinders near the brakes are much wider than the one near the brake pedal, the force you originally applied is multiplied greatly, clamping the brakes hard to the wheels.

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