The Physics of Driving: Natural Forces, Friction, Traction and Balance

Everything in the known universe is subject to natural forces like inertia, gravity, friction and energy. Your car is no exception, in fact, it relies on the laws of physics to operate. Without natural forces, your car would be unable to start, move, stop or change direction. The way in which your vehicle interacts with these forces is somewhat determined by its design. As the laws of physics are constant and unchangeable, car manufacturers can use them to create stable, safer and more efficient vehicles.

Even more so than design, the way your car behaves in response to the invisible, natural forces acting upon it is determined by the way you drive it. As part of your driver’s training, you must learn how different forces and natural laws affect your car, in order to maintain control and respond appropriately in emergency situations.

This may seem like an enormous undertaking but remember that you are already a skilled driver of another, quite different vehicle which is also at the mercy of natural forces: your body. Just like your car, your body is powered by energy, resisted by friction, subject to inertia and pulled toward the center of the earth by gravity. Despite constant interference from these forces, you can walk, run, jump, crouch and engage in countless other physical activities. Eventually, you will have a similar level of intuitive control over your car.

Natural forces are always working on your vehicle, while it is stationary and in motion. In an ideal driving situation on a flat, straight, level and well-surfaced roadway, you may not pause to consider the effect these forces have. In more complex environments, you will need an awareness and an understanding of natural forces in order to make safe driving decisions and avoid collisions. If you are driving on a hill, through a curve, executing a turn, or negotiating bad weather, your knowledge of natural laws could easily mean the difference between an uneventful trip or a catastrophic crash.

If you are feeling a little intimidated by the prospect of studying the laws of physics as part of your driver’s ed course – don’t panic. Learning how to control your car does not require knowledge of complex mathematical equations or abstract theories. All you need is a general understanding of the natural forces working on your car and how you can work with them to maintain control and stay safe on the roads. Let’s begin with a quick run-down of the topics covered in this section.

Gravitational force

Our exploration of the science of driving begins with a look at gravity. This force exists to some degree between any two objects in the universe, though we generally only notice the gravitational pull of the Earth. Gravity attracts objects; the force with which it pulls them together is determined by their mass and the distance between them. Larger objects (like the Earth) have a stronger gravitational pull. We will delve into the theory of gravity, its history and effects in greater detail in the next module of this section. For now, all you need to know is that gravity pulls your vehicle toward the center of the earth and will affect it in different ways, depending on its weight distribution, the gradient of the road you are driving on and various other natural forces.

The downward pull of gravity will affect your vehicle’s speed when you are driving or parking on a slope. While driving uphill, your car will be pulled backward by gravity and effectively slowed. On a downhill slope, it will be pulled forward, resulting in an increase in speed. This section will teach you how to adjust your driving behavior to compensate for the effects of gravity in hill driving situations.

Newton’s laws of motion

Before we can go any further in our quest to understand how natural forces affect the movement of a vehicle, we must first look at the universal rules which govern motion. This module covers Newton’s three laws of motion, which together, can predict and explain the way any object moves (or doesn’t move) in response to outside forces.

Newton’s first law concerns inertia. Inertia is the reason that changing a moving object’s speed or direction always takes more effort than keeping them constant. In driving, inertia makes maneuvering a vehicle around a bend or bringing it to a stop physically challenging – particularly if you do not have power steering or brakes!

His second law relates to the force which a moving object exerts. Ever wondered why a car colliding with a wall will do more damage at 30mph than at 15mph? Our exploration of force will show you how it is calculated and therefore, why some objects exert greater force than others. Newton’s third law deals with action and reaction. This rule explains why a car colliding with a wall not only damages the wall but sustains damage itself. The information covered in this module will help you to understand the concepts discussed across the rest of the section; make sure you take the time to read it!

Energy and potential energy

The amount of energy an object has determines its ability to do work. Objects with more energy can work harder - or for longer - than objects with less energy. While energy comes in many forms, our focus during this course will be movement energy or kinetic energy. Kinetic energy is produced by an object in motion. Stationary objects still have energy, though it is known as potential energy. An object is more likely to begin moving, the more potential energy it has. When it comes to driving, the amount of kinetic energy your car has will affect how easy it is to slow down or stop; the greater the energy, the harder it will be.

Speed has a significant effect on energy, and consequently, how long it will take your vehicle to come to a complete stop in an emergency. Learn how energy affects braking distance and the force of impact in collisions, in this essential module.

Centrifugal and centripetal forces

Centrifugal and centripetal forces work in opposition to each other and affect objects traveling on a curved path. Unless you spend all your time driving on straight, high-speed expressways, these forces will regularly play a part in the way your vehicle behaves on the road. To understand centrifugal and centripetal forces, it helps to imagine any curve you are driving on as part of a complete circle. The centripetal force pulls your vehicle towards the center of this circle, while centrifugal force pulls it directly away from the center. While driving on a curved road your car will always be acted on by both these forces. You must learn to find a balance between them to avoid understeer (not turning enough) and oversteer (turning too much), while driving along a curved pathway. The skills and techniques required to achieve this are explained in full here.

Friction and your tires

Next up, we look at the resisting forces which influence the internal moving components of your car and the way its tires grip the surface of the road. Central to this discussion is friction. Friction describes the resistance between any two contacting surfaces as they slide against each other. Low friction results in ease of movement, while high friction makes movement difficult. The friction which occurs between car tires and the road is known as traction.

The friction between your vehicle’s tires and the road must be high for the vehicle to move and for you to steer effectively. Without friction, the tires would slide across the surface of the asphalt rather than gripping and rolling over it. Car tires and road surfaces are designed to maximize friction but there is also a lot you can do while driving to improve grip on the road or regain friction if your wheels have begun to skid. Learn about friction, why it is so important and what you can do to maintain it, in this part of the section.

We also deal with a concept known as rolling resistance here. Rolling resistance refers to how much your tires are compressed as they roll over the surface of the road and is largely determined by tire pressure. This is an important issue to get to grips with, as excessive rolling resistance can result in overheating and eventually, tire failure.

Friction and braking

In addition to the connection between the tires and the road, friction plays a part in various other essential vehicle systems. Perhaps most important among them is the brakes! When you depress the brake pedal, pads are pressed against the turning wheels to create friction, which will slow down and eventually stop the vehicle. Friction will ultimately lead to wear, which is why it is so important to observe proper braking techniques. All brake systems will wear out eventually, but it will happen far sooner than it should if you ride the brakes while driving.

Braking correctly is essential for your safety as well as the health of your vehicle. To slow down or stop, the brakes must absorb the vehicle’s kinetic energy. There may not be enough time for this to occur if you slam your foot down on the brakes suddenly. When the vehicle’s movement energy has not been fully absorbed, the wheels will lock, and you will continue to move forward. Unless you drive an ABS-equipped vehicle, gradual braking is key.

What affects traction?

At this point in the section, you will have learned about friction and the vital role it plays in allowing your tires to grip the surface of the road. This resistance between tire and tarmac is known as traction. Creating good traction is a primary goal behind vehicle, tire and roadway designs. However, these are not the only factors that influence your grip on the road. Others include:

• Weather conditions
• The weight distribution of your vehicle
• The gradient of the road (whether it slopes uphill or downhill)
• The material the roadway is surfaced with
• Upcoming maneuvers such as turns, braking and acceleration

In this module, you will learn how to adjust your driving behavior to suit the roadway environment and maintain traction.

How to correct traction loss

Despite your best efforts, you may one day find yourself in a frightening situation where your tires lose traction and begin to skid across the surface of the road. The key to regaining control when this happens is to remain calm and apply the traction correction techniques discussed in this module.

When traction loss occurs in your vehicle’s front wheels, steering may become totally ineffective. On a corner or bend, this will result in a phenomenon known as understeer. The opposite effect, known as oversteer, occurs when traction is lost in the rear wheels while driving through a curve in the road. Both situations can result in serious crashes if not handled correctly. Oversteer can cause your rear wheels to spin out and leave the road or enter a lane of opposing traffic. When understeer occurs, it can cause drivers to plow directly off the side of the road. If you ever find yourself dealing with either of these situations, recalling the information covered in this module could save your life.

Vehicle balance

The next three modules of this section deal with the distribution of weight across your vehicle’s four wheels and why this should matter to you, as a driver. Ideal vehicle balance would be an equal amount of weight pushing each tire against the surface of the road. Whenever weight is shifted toward the front, back, left or right of the vehicle, the tires bearing less weight will suffer from reduced traction. The condition of your vehicle, any maneuvers you perform, adjustments in speed and gravity will all determine how weight is distributed across the car’s wheels. We explore these ideas fully in this first vehicle balance module. Here is a quick summary to get you started:

• Acceleration will shift weight into the rear tires.
• Braking will shift weight into the front tires.
• Driving uphill will shift weight into the rear tires.
• Driving downhill will shift weight into the front tires.
• Steering left will shift weight into the right tires.
• Steering right will shift weight into the left tires.

As you become more comfortable behind the wheel, you will learn to sense shifts in your vehicle’s balance as you speed up, slow down or perform maneuvers. Ultimately, your goal will be to maximize traction by adjusting your driving behavior to keep the vehicle as balanced as possible.

Maintaining vehicle balance

Our next module covers the four types of weight imbalance that can occur in a moving vehicle. These are:

1. Forward pitch: weight moving toward the front of the vehicle.
2. Backward pitch: weight moving toward the back of the vehicle.
3. Roll: weight moving from one side of the vehicle to the other.
4. Yaw: weight spinning around the vehicle’s center of gravity.

Often, you will encounter driving environments that create pitch and roll weight shifts simultaneously. Maintaining control of your vehicle will require predicting and compensating for these balance shifts with appropriately timed speed adjustments. You must also ensure your tires are in good condition and can grip the surface of the road, as the effects of vehicle imbalance are harder to manage when traction is already poor.

Vehicle balance in complex environments

Changes in your vehicle’s balance are easy to predict and manage when you are driving in ideal conditions on a straight, level road. In challenging driving environments, maintaining vehicle balance is a slightly more complex task. As we have already discussed, a vehicle’s center of gravity (and therefore, its weight distribution) are influenced by the gradient and shape of a road, the road’s surface, weather conditions and other environmental factors.

This module will walk you through common driving scenarios, teaching you how to compensate for shifts in weight and optimize traction. The secret to executing weight adjustments successfully is giving yourself enough time to plan and perform them. As a defensive driver, you must constantly scan the roadway ahead to look for changes in gradient, road surface and curves in the road that will affect your vehicle’s balance.

The physics of collisions

Our science module rounds off with a look at how natural forces interact and determine severity in traffic collisions. Here’s what you need to know:

• The amount of kinetic energy an object has determines the force it exerts and sustains during a collision.
• When hitting a stationary object, the severity of the crash will be determined by how fast you are traveling and how well the object you collide with can absorb your kinetic energy.
• In a head-on collision between two vehicles, heavier, faster-moving vehicles always do more damage.

When a driver understands how energy and force influence the severity of traffic collisions, they can take steps to minimize severity when faced with an imminent crash. Learn how to protect yourself in an emergency and get familiar with your vehicle’s in-built safety features here.

Working with natural forces

The rules governing movement in our universe cannot be changed, bent or broken. Gravity, resistance, inertia, energy, force and other concepts discussed in this module are constant and as such, they are predictable. By understanding how your car moves and the forces which act upon it, you can work with natural forces to get the best performance from your vehicle. Disregard these laws and controlling your vehicle will always be an uphill struggle – sometimes quite literally! On that note, let’s get started with our first important concept: gravity.