Natural Forces and Physics
The Laws of Motion and Inertia

Inertia & The Laws of Motion: Vehicle Occupant Safety

Updated Dec. 25, 2020

Following his work on gravitational force, Sir Isaac Newton developed the three laws of motion. These laws explain how and why an object moves or remains stationary. Using the three laws of motion, we can accurately predict how an object will move under different circumstances. Just like gravity, the three laws of motion affect the movement of your car and the way it handles. It is important to familiarize yourself with these laws, as they will be referred to alongside various other concepts across the remainder of this section.

Newton’s first law

Newton’s first law introduces the principle of inertia. According to inertia, an object will not change its course or speed unless acted upon by a counteracting force. As Newton puts it:

  • An object will remain at rest unless acted upon by an outside force.
  • An object in motion will remain in motion unless acted upon by an outside force.

To put it simply, inertia means that an object will move (or not move) according to the last set of “instructions” it was given by an outside force until another outside force intervenes. So, if you were to throw a tennis ball in an environment where no other forces could act on it, that ball would continue moving along the same path, at the same speed, forever. Of course, such environments do not naturally exist. If you were to throw a tennis ball now, gravity would intervene and pull it back to Earth. Even in outer space, a moving object will eventually be slowed or redirected by outside forces.

Newton’s first law affects the way your car starts, stops and is involved in all the other natural processes which govern its movement.

Newton’s second law: acceleration and braking

Newton’s second law is a little more complicated but fortunately, we do not have to delve into it too deeply here. The second law states that:

  • Force (F) is equal to mass (m) multiplied by acceleration (a).

What you need to take from this is that the force an object exerts is affected by its weight and the speed at which it is traveling. The greater the mass and the faster the speed, the more force the object possesses. Consequently, heavier vehicles traveling at higher speeds do more damage than lighter, slower-moving vehicles. When two objects moving toward each other collide (as would be the case with a head-on collision) the speed and mass of both objects will contribute to the force of the impact. Head-on collisions are always more severe than side-on or rear-end collisions, for this reason.

Newton’s third law: action and reaction

Finally, Newton’s third law states that:

  • For every action, there is an equal and opposite reaction.

In a nutshell, this law means that every acting force is met with an equal reaction force moving in the opposite direction. One driving-related example of this law in action would be the force that your tires exert on the road’s surface. The equal and opposite reaction of the road “pushing back” against your tires is what propels your car forward. In a collision, Newton’s third law explains the damage a vehicle sustains. When you hit an object or another vehicle, the force of your vehicle will damage that object. Of course, your vehicle will be damaged too, as the equal and opposite reaction applies force back onto your car at the point of impact.


Let’s talk a little more about the principle of inertia addressed in Newton’s first law, and how it affects your vehicle. To recap, inertia is the tendency of a stationary object to remain stationary, and a moving object to remain in motion.

While your vehicle is moving, it will not stop unless another force intervenes to slow down or stop it. This force could be external, for instance, coming from the surface of the road or an object you collide with. Alternatively, it could be internal, as when you apply the brakes to exert force on the wheels.

Getting your head around the idea of inertia is necessary to understand vehicle-occupant safety. If you were to collide with an object while traveling at 40mph, your car would abruptly stop, but according to inertia, your body would continue moving forward at 40mph until another force stops it. If you are not restrained, that force will be exerted by the steering wheel, dashboard, windshield or another object outside the vehicle. Safety restraints like seat belts, child seats and airbags are designed to control your body’s sudden drop in speed, to minimize the injury you sustain during a collision.

Would you pass a driving test today?

Find out with our free quiz!


Like the article? Give us 5 points!

Click a star to add your vote

4.8 out of 5 stars based on 9 votes.

Read next

The Physics of Braking
Natural Forces and Physics 4 of 5

Potential and Kinetic Energy

Energy is the capacity to do work. The more energy something has, the longer or harder it can work. When slowing down or stopping, your vehicle’s brakes must overcome its kinetic energy. If your vehicle collides with an object, the force of the impact will be equal to its kinetic energy, divided by your stopping distance.

Centrifugal and Centipetal Forces
Natural Forces and Physics 5 of 5

Centrifugal and Centripetal Forces

When an object moves along a curved path, it is acted on by centrifugal and centripetal forces. This will affect your vehicle whenever you drive through a curved stretch of road. These two forces act simultaneously and pull in different directions.

Friction, Traction and Rolling Resistance
Friction and Traction 1 of 4

Friction, Traction and Rolling Resistance

Understanding how your tires interact with the road’s surface is important, as your ability to accelerate, slow down or perform any maneuver depends on maintaining “grip” on the pavement beneath your wheels. This knowledge will also help you to avoid dangerous tire malfunctions like tread separation and blow outs.

Preventing Road Rage 3 of 6

Common Road Rage Behaviors

While scanning the roadway for potential threats, you must watch out for any motorists displaying aggressive behavior. If you encounter a driver experiencing road rage, it is likely that they are the most dangerous hazard in the roadway environment. You must pay attention and act defensively, to protect yourself against the threat such drivers pose.

Preventing Road Rage 4 of 6

How to Avoid Provoking Road Rage

As part of your road safety training, you must learn how to avoid becoming the target of another driver’s road rage. Every day, you will be sharing the road with stressed out and frustrated drivers, many of which are capable of succumbing to road rage if things don’t go their way.

Preventing Road Rage 5 of 6

Dealing with Aggression on the Road

You must know how to deal with aggression on the road just in case you ever find yourself in hot water. It is important not to take an aggressive driver’s actions personally, even if you are the target. When a driver’s aggression develops into rage, there is very little you can do or say to calm them down.

Preventing Road Rage 6 of 6

Controlling Your Emotions

Holding a driver’s license is both a privilege and a responsibility. It is not enough simply to say that you will not become an aggressive driver, you must continually monitor yourself and be proactive about avoiding aggressive behaviors behind the wheel. Most new motorists believe that they will not act aggressively on the road and yet many drivers end up doing it anyway.

Natural Forces and Physics 1 of 5

The Physics of Driving

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. 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.

Natural Forces and Physics 2 of 5

The Force of Gravity

Though we rarely stop to consider its effects, gravity is an ever-present force which acts on you, everything you can see in the room around you and of course, your vehicle. The force of gravity pulling your vehicle toward the center of the Earth will influence your speed when traveling on a hill. It will also affect the way weight is distributed across your vehicle’s four tires.