How Cars Work · Stopping and Steering
It turns into heat
The answer: it turns into heat.
This is one of the deepest rules in physics. Energy can change from one form to another, but it cannot be created or destroyed. If a car has 100 units of kinetic energy at full speed and 0 units when it has stopped, that 100 units had to go somewhere.
Brakes work by deliberately creating friction between a part that spins with the wheel and a part that doesn’t move. The friction slows the wheel down. It also makes the parts very, very hot. All the kinetic energy the car had a moment ago ends up as heat in the brake parts.
In a typical stop from highway speed, the brakes can heat up to several hundred degrees Fahrenheit — hot enough to fry an egg, several times over. In racing, they can hit far more than that. (See the sidebar.) That heat doesn’t come from anywhere magic. It is the energy of motion the car had a moment ago, converted by friction into heat.
Try It: Hands as Brakes
You will need: your hands.
Rub your palms together as fast and hard as you can for fifteen seconds. Then immediately put them against your cheeks or your forehead.
Your hands are warm. Maybe noticeably hot.
You did some work to rub them together — your muscles burned a little energy. That energy didn’t disappear. It went into heat in your palms, through friction.
That is what brakes do. Except instead of two hands, it is a metal disc and two pads of friction material, with the kinetic energy of a 3,000-pound car moving at 60 miles per hour being shed as heat in just a few seconds. The brakes get extremely hot. They have to. There is nowhere else for the car’s energy to go.
Two kinds of brakes
Two designs of brakes show up in old cars: drum brakes and disc brakes.
A disc brake has a flat metal disc, called the rotor, bolted to the wheel. The rotor spins with the wheel, all the time. Around the edge of the rotor sits a clamp called a caliper that doesn’t move. Inside the caliper are two flat pads of friction material, one on each side of the rotor, with a small gap between them. When the driver presses the brake pedal, the caliper squeezes the two pads against the spinning rotor, pinching it from both sides. The rotor slows. The wheel slows with it. The pads get hot.
A drum brake is the opposite shape. Instead of a flat disc, there is a hollow drum bolted to the wheel — picture an upside-down cake pan. Inside the drum are two curved pads, called brake shoes. When the driver presses the pedal, the shoes get pushed outward, pressing against the inside surface of the spinning drum. Same idea, just inside-out.
By the late 1960s, most American cars had disc brakes on the front wheels and drum brakes on the back. (Disc brakes handle heat better, but cost more to make. The front brakes work harder than the rear, so cars got the better brakes where they mattered most.)
Either way, the principle is the same. A spinning part gets pinched or pushed against by a non-spinning part. Friction. Heat. The car slows down.