by Chris Woodford. Last updated: June 10, 2016.
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. 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!
Photo: The brake disc on this Porsche sports car is the small, metal wheel just behind the silver spokes of the outer, alloy wheel. When you put the brakes on, a brake pad (red) clamps onto this metal wheel to slow you down.
The science of stopping
If you're moving, you have energy—kinetic energy to be precise. Kinetic energy is simply the energy an object possesses because it has both mass and velocity (speed in a certain direction). The more mass you have (effectively, the heavier you are) and the faster you're going, the more kinetic energy you have.
That's all well and good, but what if you suddenly need to stop? To change from moving quickly to not moving at all, you have to get rid of your kinetic energy.
If you're jumping from an airplane, the best way to lose energy is with a parachute. This giant sack of fabric drags behind you, slowing you down, reducing your velocity, and therefore helping to get rid of your kinetic energy. That means you can land safely. Drag-racing cars and land speed record cars also use parachutes to stop but, in practice, most vehicles simply use brakes.
Photo: Coming in to land: a parachute "brake" reduces your velocity and kinetic energy so you can land more safely. Photo by Senior Airman Micky Bazaldua courtesy of US Air Force.
Different brakes for different machines
From cars and trucks to planes and trains, brakes work in a similar way on most different vehicles. There are even brakes in wind turbines! Here's a quick comparison of some common brake systems.
If you ride a bicycle, you know all about brakes. If you want to stop suddenly, you squeeze the brake levers on the handlebars. Thin metal cables running to the back and front wheels pull on small calipers, forcing thick rubber blocks to press against the wheels. As they do so, friction between the blocks and the metal wheel rims generates heat, reducing your kinetic energy, and bringing you safely to a stop.
The brakes on a steam locomotive work the same way as a car's and are even more obvious. You can see the brake just behind the wheel in this photo. It clamps against the locomotive's driving wheels to slow them down. Since there are no tires on the wheels, the friction that stops the train comes from the immense weight of the locomotive pressing the metal wheels down onto the track.
Motorcycles typically have disc brakes comprising a rotor and a brake block. The rotor is a disc with holes (or slots) in it mounted on the side of the wheel. A brake pad, operated by a cable, jams against the rotor to slow it down by friction. The holes in the rotor help to dissipate the heat generated.
Airplanes have brakes inside their wheels to help bring them to a stop on the runway, but they can also use air brakes to increase drag (air resistance) and slow themselves down—a bit like parachutes. Jet fighters often have a speed brake, which is a large metal plate just behind the cockpit that can be hydraulically raised to increase drag and braking.
Photo by Vincent Parker, US Air Force.
Wind turbines have brakes to stop their rotors (propellers) turning too quickly. The brake is mounted inside the nacelle (the square-shaped casing behind the propeller that contains the gearbox and generator). Most turbines have an anemometer on them to measure the wind-speed. If it rises above a safe level, the brakes come on automatically and bring the rotors to a standstill. It's a shame, because higher wind speeds mean more energy could be produced. But safety always comes first!
Photo by Matthew Bates courtesy of US Air Force.
A closer look at car brakes
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.
Artwork: Early car brakes were amazingly primitive by today's standards. Here's a simple friction braking system from 1910 invented by John Stawartz of Homestead, Pennsylvania. When you pull on the brake lever (yellow), a giant brake "shoe" (blue) drops down under the back wheel (brown). As the car drives onto the shoe, the shoe's teeth (red) bite into the road and the car comes juddering to a halt. Artwork from US Patent 960,426: Automobile Brake by John Stawartz, courtesy of US Patent and Trademark Office.