How Cars Work
Stopping and Steering
CHAPTER FIVE
The wheels are spinning. The car is moving forward.
Now we have a different problem. Maybe the most important problem on a moving car. The car has to go where the driver wants it to go, and stop when the driver wants it to stop. Otherwise it isn’t a car. It is a runaway shopping cart with an engine.
This chapter is about the two systems that control where a moving car ends up: the brakes and the steering. The physics is completely different from anything in the last four chapters. No more explosions, no more sparks, no more gears. Just energy, friction, and a little geometry.
Stopping is energy
To stop the car, we have to deal with the energy it has.
A moving car has a kind of energy called kinetic energy — the energy of being in motion. Anything that is moving has it. A thrown baseball. A running dog. A speeding car. The faster something is moving, and the heavier it is, the more kinetic energy it has.
The exact amount is given by a simple formula. (One of the few equations in this book, but worth knowing.)
energy = ½ × mass × speed²
The little ² at the end means “multiplied by itself.” Three squared is 3 × 3 = 9. Five squared is 5 × 5 = 25. Sixty squared is 60 × 60 = 3,600.
Here is what this formula tells us. If a car gets twice as heavy but moves at the same speed, its kinetic energy doubles. But if the car stays the same weight and just moves twice as fast, its kinetic energy quadruples — because we squared the speed. Going from 30 miles per hour to 60 miles per hour means four times the kinetic energy, not twice. Going three times as fast means nine times the energy.
This is why high-speed crashes are so much worse than low-speed ones, and why a car going 60 needs four times the braking distance of a car going 30, not twice.
So here is the question. The car has all this energy of motion. To stop the car, you have to get rid of that energy. But energy doesn’t just vanish. Where does it go?