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Today’s Wonder of the Day was inspired by Nya. Nya Wonders, “How do car brakes work” Thanks for WONDERing with us, Nya!
What do you think about when you're riding in the car with friends or family members? On your way to school, you might be thinking about the test you have that day or what's for lunch. On the way home from soccer practice, you might be WONDERing what's for dinner or how you're going to get all that homework finished before tomorrow.
You probably don't give much thought to the car itself. Does it need an oil change? How does the engine work? Do the tires need to be rotated? Unless something breaks down, we tend to take cars and how they work for granted. They simply get you from one place to another quickly.
If a deer runs out in front of your car, though, you might start thinking about how one part of the car works: the brakes. When the driver slams on the brakes, you'll be glad that they quickly bring the car to a stop, preventing you from a nasty collision.
If you give it some thought, brakes are an amazing invention. If you're riding a scooter and you need to slow down, you can put out your feet and drag them along the ground. But what about whizzing down the highway at 55 miles per hour? Putting your feet out on the highway wouldn't do much good, would it?
So how can a light push on a car's brake pedal slow a speeding car to an abrupt stop? Is it magic? Of course not! It's science.
A car in motion has a lot of kinetic energy, which is energy of motion. To stop a car, the brakes have to get rid of that kinetic energy. They do so by using the force of friction to convert that kinetic energy into heat.
When you press your foot down on the brake pedal, a connected lever pushes a piston into the master cylinder, which is filled with hydraulic fluid. That hydraulic fluid gets squirted along a system of pipes into other, wider cylinders positioned next to the brakes on each wheel.
This hydraulic system multiplies the force of your foot on the brake pedal into enough force to apply the brakes and make the car stop. The brakes themselves are usually one of two types: disc brakes or drum brakes.
Many modern cars have disc brakes on the front wheels and drum brakes on the rear wheels. More expensive models may have disc brakes on all four wheels. Only very old or very small cars tend to have drum brakes on all four wheels.
Disc brakes consist of a brake disc, a brake caliper, and a brake pad. When the brake pedal is depressed, the hydraulic fluid causes the brake caliper to press the brake pad against the brake disc. The rubbing of the brake pad against the brake disc generates friction, which converts kinetic energy into heat in the brake pad.
How much heat? A lot! Stopping a speeding car can heat the brakes to 950º F or more! To withstand such heat, brake pads must be made of special materials that won't melt at such high temperatures. Some of those special materials include composites, alloys, and ceramics.
Drum brakes also use friction but in a slightly different way. Drum brakes consist of a brake drum and brake shoes. The hollow drum turns with the wheel. When the brake pedal is depressed, a hydraulic cylinder pushes brake shoes with friction linings against the inner surface of the brake drum, creating friction and thereby slowing the wheel.
