All the Ways to Slow a Car (Even Some Bad Ways)

Since the vehicle is moving up the incline and gravity only pulls straight down, there is a component of this force that pulls in the opposite direction as the velocity to make the vehicle slow down. As it moves up the incline, there is an increase in gravitational potential energy. The higher it goes, the greater the potential energy.

Of course, the same thing can happen in reverse. If you let an object move down a ramp, there would be a decrease in gravitational potential energy and a resulting increase in kinetic energy. So you still need some brakes or some type of friction to prevent the vehicle from ultimately slipping backwards. Most of these runaway ramps are made of very soft gravel to cause a large frictional force so that a stopped truck stays stopped.

Downshifting

Manual transmission, or stick shift, cars aren’t as popular as automatic ones—but they still exist. With stick shift, the driver has to manually change from one gear to another while increasing speed. But they can also use this same process to decrease the speed of the car.

Let’s say they are in fourth gear moving along at 40 miles per hour. If they shift down to third gear and take their foot off the gas pedal, the car will slow down. They don’t have to touch the brake pedal, which means that the car’s brake lights won’t come on even though it is slowing down. Of course, if a driver needs to stop in a very short distance, this downshifting isn’t going to be enough, and they are going to have to use traditional braking.

How does this work? I’m only going to give you a superficial description of the internal combustion engine, but it’s all that we need to understand the downshift. An engine provides power by adding gasoline to a compressed space in the cylinders. When the fuel is ignited, the gas expands and pushes the pistons down. The pistons moving up and down rotate the crankshaft, which (with a few more connections) turns the wheels. Boom, you are driving! To get this to work, you need fuel, a spark to ignite the fuel, and compression.

What if you remove the spark and the fuel? If the wheels are engaged with the engine through the transmission, there’s still the compression of a gas in the cylinders. This compression of a gas adds resistance to the rotating engine and can be used to slow down the car. (Of course, you still need the friction between the tires and the road.)

In terms of energy, we still need an increase in energy to correspond to the decrease in kinetic energy. It shouldn’t be a surprise that you get an increase in thermal energy. When a gas is compressed, it gets hotter—and there’s your energy.

Regenerative Braking

What if there was a way to slow down a car and decrease the kinetic energy, but to also save that energy? Well, that’s exactly what happens in regenerative braking.

This all starts with an electric motor, which is essentially just a loop of wire on a rotating shaft near a magnet. When electric current flows through the loop, there is an interaction between the current and the magnet, and this makes the loop rotate on the shaft. This actually works backwards, too. If you move a wire in the presence of a magnetic field, it will create an electric current. This means that an electric motor and an electric generator are the same thing. For the motor, you give it current and it moves stuff. As a generator, you rotate the shaft and you get an electric current.