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Wednesday, April 24, 2024

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

Why do cars have brake lights on the rear of the vehicle? They are there so that when a car slows down, the drivers behind it know what’s going on. But guess what—electric cars can use a type of braking that doesn't activate the lights! I didn't know about this until I saw this video from Technology Connections about the problem with an electric vehicle operating mode called "one-pedal" driving. Essentially, this allows the driver to control the speed of the car with just the accelerator. When the pressure on the pedal is decreased, the car will switch the electric motor into regenerative braking mode and use this to charge the car's battery. This means that the car slows, but the brake lights don’t activate.

I'm going to explain everything you need to know about regenerative braking, but along the way, this will be a good opportunity to talk about all the different ways you can stop a vehicle and what happens to its energy when you do. Let's get started.

Forces, Energy, and Motion

Imagine a spacecraft out in deep space with no air, no gravitational forces, and obviously no friction. If this spaceship fires its rocket engines, it will speed up. But what happens when the thrusters are shut off and there are no longer any forces acting on the moving vehicle? It might be tempting to say that it will gradually slow down, but it won't. It will just keep moving along in a straight line at a constant speed.

This is a direct consequence of Newton's second law, which says that the net force on an object (Fnet) is equal to the product of the object's mass (m) and its acceleration (a). With zero net force, the acceleration must also be zero. The acceleration tells us the rate of change of velocity—so a zero acceleration means there is no change in velocity.

Well, then how would the rocket stop? Stopping means going from some velocity to a zero velocity. Yes, this means it must accelerate. Accelerate doesn't just mean "to speed up," but rather to change velocity, and that can mean going from a higher velocity to a lower one, including all the way down to zero. In this case, you would need a force to cause this acceleration and the force would have to push on the vehicle in the direction opposite to the velocity. That's how you get things to slow down: with a backwards-pushing force.

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Now let's think about energy. If that same rocket is moving in space, it has energy because of its motion. We call this the kinetic energy, and its value depends on both the rocket’s velocity and its mass. When the rocket slows down, the decrease in velocity means that it also has a decrease in kinetic energy. But energy doesn't just go away. If the spacecraft has a decrease in energy, then something else must increase in energy. In this case, if the spaceship fires a rocket engine to slow down, then the exhaust gas ejected from the thrusters will increase in speed. That means the gases increase in kinetic energy. Energy is conserved, meaning that the total energy before something happens (like the exhaust being shot out of the rocket) is the same as the total energy afterwards.

Now we can use these physics ideas to understand the different ways that ordinary Earth vehicles can slow down.

External Forces

There must be some type of backward-pushing force on a vehicle to get it to stop, and this is going to be true for every braking method we examine. In some cases, this backwards force comes from the car—but it doesn't have to be that way. Have you seen those lines of barrels on highways? Those are sometimes called "crash cushions" or "impact attenuators." They are basically barrels filled with water or sand so that a car can slow down by colliding with them. (Note: Don't slow down using an external force unless you really have no other option.)

These barrels provide the backward-pushing force that slows the car down, but they do it in a smart way. Because they are squishy, they don't push as hard against the car as, say, a tree trunk or concrete barrier. With this lower force, it takes the car a longer time to slow down, which makes it much safer for the people inside. But when the kinetic energy of the car decreases, some type of energy has to increase—right?

If you watch this video of a car running into these barrels, you will notice that the sand or water gets thrown up into the air. Yup, that's where the car’s kinetic energy goes.

Wheel Brakes and Friction

We all know that the proper way to stop a car is by just pushing the brake pedal. But how does this actually stop the car? The answer is friction. We can model the frictional interaction between two surfaces as two separate types of friction. First, there's the static frictional force, which is the case when the two surfaces are stationary relative to each other. Second, there’s the kinetic frictional force, when two surfaces are sliding with respect to each other.

Let's consider a car that stops by sliding its tires on the road (which is also not the recommended way of stopping). In this case, we can draw the following force diagram:

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The kinetic frictional force pushes in the opposite direction as the velocity of the car to make it slow down. But what happens to the kinetic energy of the car as it stops?

Here's a nice illustration of a vehicle with the rear wheel "locked" so that it skids to a stop. This is a view using an infrared camera so that brighter (more orange) parts of the image represent hotter objects.

Notice one wheel slides, leaves a hot streak on the road, and heats up the tire. That's what happens to the kinetic energy: It goes into an increase in thermal energy.

But what about stopping like a normal driver and not locking up the brakes? Since the tire doesn't slide, it's actually a static friction interaction. It turns out that you can get a greater frictional force between two surfaces if the interaction is from static friction instead of kinetic. This is why just about every car has an anti-lock brake system (ABS) to prevent the wheels from sliding and to give the car a better stopping distance.

In both cases, there's something else to consider: If the car stops because the wheels are interacting with the road, then what stops the wheels? That's the purpose of the brakes. Most cars have a disk (called a rotor) attached to the wheel. For each rotor, there are two brake pads that push against the rotor to slow it down. Yes, this is another friction interaction. Here's an infrared image of a car wheel after stopping:

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The brighter (and more orange) rotor shows that it is indeed hot. So when a car stops, the decrease in kinetic energy means an increase in thermal energy of the ground, tire, and rotors. In fact, in cases of extreme braking, like a 747 stopping by only using brakes), the rotors can get so hot that they visibly glow.

Air Drag

What if you are driving along at a constant speed on level ground and you just turn off your car? Unlike a rocket in deep space, it obviously won't keep moving forever; it will eventually slow down and stop.

But doesn't there have to be a backward-pushing force to slow down an object? Yes. In this case, that backward-pushing force would be air drag. As the car moves, there are a bunch of tiny collisions between the vehicle and molecules of air. These collisions push on the car to slow it down. (You already know about air resistance from that time you stuck your hand out of the window of a moving car and could feel that force from the air pushing back on your hand.)

Modern cars have shapes that are designed to minimize air drag to increase fuel efficiency. However, if you really want to use air to stop a rapidly moving vehicle, it's possible to dramatically increase the air drag. All you need to do is to make your vehicle have a larger surface area. That's exactly what happens to a race car when a drag chute—a small parachute that comes out of the back—is deployed. (This isn't a very practical method for stopping a car, since it only works once before you have to repack the chute, but it still counts.)

Where does the energy go? As the car interacts with the air, the air gets pushed so that the molecules move faster and increase in temperature. This change in energy is spread out over such a large volume of air that it's pretty much impossible to measure, but that's indeed what happens to the car’s kinetic energy.

Gravity

You can actually stop a car using gravity. You might have seen this before with runaway ramps on mountain roads. These are offshoots of the road that go up a steep hill. If a vehicle—usually an 18-wheeler—loses the ability to brake, it can just go up the ramp. Yes, there is a backward-pushing force, and that force is gravity. Here's a diagram:

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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.

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This means that if you have an electric motor in a car, it's possible to use it as a generator and charge the car's battery. When the car slows down, that kinetic energy gets converted to energy stored in the battery. Well, at least some of the energy gets stored—there's still some loss because it's not a completely efficient process. Stuff always heats up at least a little bit.

So, what about the brake lights and one-pedal driving mode? In both electric and gas-powered cars, the brake lights come on whenever the driver steps on the brake pedal. But now we see that an EV driver can also slow the car simply by easing off the accelerator–no brake pedal required. In this instance, the car's computer is responsible for switching the motor between driving mode and regenerative mode—and it’s the computer that decides whether or not the brake lights come on. They might not.

(We all knew that one day the computers would take over the world. They have started with brake lights. Weak humans just have to accept that we don't get to decide anymore.)

Is this legal? Yes. Currently, the Federal Motor Vehicle Safety Standard states: “The stop lamps on each vehicle shall be activated upon application of the service brakes. The high-mounted stop lamp on each vehicle shall be activated only upon application of the service brakes.”

Should this rule be changed? If I was in charge—and I'm obviously not—I would create a rule that for electric cars the brake light should come on when the slowing of the car is greater than some specified value, like 1 meter per second per second. That way you would be signaling to cars behind you: "Hey, I'm stopping, so maybe you should too." Really, isn't that the whole reason for a brake light?

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