Natural Forces and Physics
The Laws of Motion and Inertia

Inertia & The Laws of Motion: Vehicle Occupant Safety

Updated Oct. 31, 2019

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 effect 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 effects 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.

Inertia

Let’s talk a little more about the principle of inertia addressed in Newton’s first law, and how it effects 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 the exerted by the steering wheel, dashboard, windshield or another object outside the vehicle. Safety restraints like seat belts, child seats and air bags are designed to control your body’s sudden drop in speed, to minimize the injury you sustain during a collision.

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