3.2 – Inertia and Energy

The law of inertia means that objects that are moving tend to continue moving and objects that are not moving tend to remain at rest, unless acted upon by some other force.

When driving, inertia keeps your vehicle moving, unless it is acted upon by something, such as your brakes, the road surface, a fixed object (such as a tree), or another vehicle.

Inertia causes your body and loose objects in your car to keep moving forward when your vehicle stops suddenly. You may be injured because of the inertia and momentum of loose objects in your car that fly through the air during a sudden stop.

Counteracting Inertia’s Effects with Seat Belts

img_01Using safety seat belts prevents or limits the driver and passengers’ injuries that could occur from the effects of inertia. For instance, if the driver of a motor vehicle hits their brakes suddenly to avoid a collision, the force of inertia will propel the driver’s body forward because it is still in motion. By the driver “buckling up,” the driver’s body will not continue to propel forward. This is also true for passengers. Passengers who are wearing their safety belts will be held in position more securely.


Potential Energy
Potential energy is the energy of a body or a system with respect to the position of the body or the arrangement of the particles of the system.

Kinetic Energy
Kinetic energy is the energy of motion. The motor vehicle becomes a force of motion that cannot be stopped without the inclusion of other forces. This force may be brakes or another object.

Effects of Kinetic Energy on Driving

  • Doubling speed quadruples kinetic energy. The faster an object goes, the longer it will take that object to come to a stop. Never drive faster than weather and road conditions allow and do not go faster than the posted speed limit.
  • The force of gravity decreases the effect of kinetic energy as a car moves uphill. Since gravity is pulling the vehicle downhill, the kinetic or motion is decreased. That is why lower gears are needed.
  • The force of gravity increases the effect of kinetic energy as a car moves downhill. When a vehicle is traveling downhill, gravity is also pulling the vehicle downhill. This causes the force of gravity, inertia and kinetic energy to work together and create a powerful force that the driver must keep under control. The driver may need the use of brakes and gears to keep the vehicle within their control.
  • Applying the brakes can stop a moving automobile by transferring kinetic energy into heat energy. Braking to overcome the kinetic energy turns the energy into friction when braking.
  • The kinetic energy of your moving vehicle determines your ability to stop the car. Incorporating your reaction time, your stopping distance will be:
    • Five times further if you increase your speed from 10 MPH to 20 MPH
    • 25 times further if you increase your speed from 10 MPH to 50 MPH.
  • In a collision, kinetic energy is dissipated into energy and heat. When involved in a collision, motion is transferred into friction. For instance, the vehicle’s energy loses its kinetic energy once it collides with another vehicle. It usually becomes motionless.

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