Inertia and Energy

Inertia & Potential Energy

The effects of inertia and energy on driving

Law of Inertia

Objects that are moving tend to continue moving and objects that are at rest tend to remain at rest, unless acted upon by some other force. This is called the law of inertia.

For example, vessels in space can move great distances without much spent energy because there is very little air acting to slow them down.

When you are waiting for a green signal on flat pavement at an intersection, you would not move unless you engage the engine or are otherwise pushed ((to provide the force).

On earth you have the force of gravity acting downwards from the mass of the car and tires and a similar force upward onto the tires from the roadway providing the friction to hold your car. The air, on the other hand, also acts as a force to slow your car down when in motion.

Inertia While Driving

While driving, inertia keeps your vehicle moving, unless it is acted upon by something (a force), such as your brakes, the road surface (friction), a fixed object (such as a tree or a cement road divider as in the image to the extreme right), or another vehicle (in a collision).

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

When you are hit from behind while stopped, your head tends to stay in place due to inertia while the seat pushes the rest of your body forward. This causes whiplash. Using your head restraint mitigates injuries due to whiplash.


Wearing a seatbelt while driving will stop the forward momentum of your body if you have a collision.  It will prevent you from hitting the windshield, steering wheel, and other parts of the interior of your vehicle.

Potential Energy

Potential energy is the energy that an object possesses because of its position or form.  For example, a book on a table has the “potential” to fall to the floor, whereas a book that is already on the floor does not have this potential energy.

When you are parked on a hill, gravity causes your car to have potential energy.  This energy is converted to kinetic energy (motion) if it breaks loose and rolls down the hill.

There is also potential energy built up in the components of your car’s suspension system that may cause you to swerve when you come out of a turn.

Kinetic Energy

Kinetic energy is the energy an object possesses because the object is in motion.  For example, the potential energy had by a book on a table is converted to kinetic energy (motion) when it falls.  The book lying flat on the floor does not have this same potential or kinetic energy.

As you increase your driving speed, both your body and you vehicle acquire kinetic energy which eventually must be:

(a) absorbed by your brakes, engine compression forces, or other friction in a controlled stop, or

(b) absorbed by your body, your vehicle’s body, and the objects you hit if you are involved in a crash. 

The kinetic energy of your body while it is in motion, of loose objects in the car and of the car itself, all increase with weight and the square of your speed so that:

(a) if you increase your speed from 10MPH to 20 MPH, you are dealing with four times the amount of kinetic energy, and

(b) if you increase your speed from 10MPH to 50MPH, you are dealing with 25 times the amount of kinetic energy

The kinetic energy of your moving vehicle determines your ability to stop the car. Assuming you have good tires, brakes, and dry pavement:

(a) at 55MPH, it takes about 400ft to react and bring your car to a complete stop.

(b) at 35MPH, it takes about 210ft to react and bring your car to a compete stop.

Gravity decreases your kinetic energy when you are driving uphill and increases it when you are driving downhill.  Therefore, the force of gravity will make it easier for you to stop the car if you are going uphill and more difficult to stop the car if you are going downhill.

Braking to a stop converts kinetic energy into heat energy your brakes through friction.  If you and your vehicle are involved in a collision, the kinetic energy is still converted into heat through friction, but not in your brakes, but instead at the contact point between your vehicle and the object of impact.