The D.E.P. Academy Driver Education Course


Chapter 3 Natural Forces That Affect Your Vehicle

Natural Laws And Driving An Automobile

  • Natural laws determine how an automobile may be operated safely.  If you're in a vehicle and the driver applies the brakes, the vehicle stops, but your books in the back seat of the car continues moving forward onto the floor:  Why did this happen?  A natural law is why.  Natural laws are always at work. Some of these laws are inertia, friction, momentum, kinetic energy and gravity. Natural laws enforce themselves.

The Force Of Gravity

  • Gravity is the force that pulls objects towards the center of the earth.  If you toss a ball into the air, it comes down.  The ball falls because of gravity.  Gravity is a force that pulls objects toward the center of the Earth.  Because gravity affects all objects, it can make a vehicle speed up or slow down.
  • Gravity gives objects their weight.  The weight of an object, such as your car, is distributed evenly about a point.  This point is called the object's center of gravity.  The lower an object's center of gravity, the more stable the object.  Most cars are designed to have a low center of gravity in order to handle well in turns and during quick maneuvers.  Changes in a vehicle's center of gravity affects how well the vehicle handles.  A roof carrier loaded with heavy objects raises the vehicle's center of gravity, making it less stable and difficult to control on turns and curves and during sudden changes in speed. Vehicles that have a high center of gravity, such as sport utility vehicles, some types of vans and pickup trucks, also have these problems.  Gravity in relation to the contour of the road affects how well a vehicle can handle the road.
  • When you drive uphill, gravity acts to slow your vehicle.  To maintain speed, accelerate just before the vehicle begins to climb the hill.  When you drive downhill, gravity acts with your vehicle, so your speed increases.  To keep the vehicle from moving too fast, ease up on the accelerator.  You may also have to use your brakes or downshift.

Inertia And Energy

  • Definition of inertia:  Matter, if at rest, tends to remain at rest, or if in motion, tends to remain in motion, unless it is acted upon by an outside force. 
  • All things have inertia.  It was inertia that caused your books on the backseat to continue moving forward after the driver braked.  Two properties govern inertia.  One is that objects at rest do not move unless some force acts on them. The other is that moving objects continue to move in a straight line unless some force acts on them. 
  • As the vehicle was moving, so were your books.  When the driver braked, a force was exerted to make the vehicle stop, but unrestrained, your books kept moving forward in a straight line.  Then they fell to the floor.

Counteracting Inertia's Effects With Seat Belts

  • You have to understand inertia when you drive because you and your passengers have inertia.  If you brake a vehicle hard, everyone in it will tend to keep moving forward.  Drivers must manage this risk by anticipating how to reduce inertia's effects.
  • One way to do this is to wear seat belts.  These belts provide a force that acts against inertia.  If you brake hard and are not wearing a seat belt, you may be thrown forward against the windshield or dashboard.  Another way to manage risk is to be sure to secure all loose objects, such as your books, luggage, or boxes.

Potential Energy

  • Potential energy is energy that a body possess because of its position or form, like a car parked on a hill.

Kinetic Energy

  • Kinetic energy is the energy a body possesses because of it's motion.
  • Doubling speed quadruples kinetic energy
  • The force of gravity decreases the effect of kinetic energy as a car moves uphill.
  • The force of gravity increases the effect of kinetic energy as a car moves downhill.
  • A moving automobile can be stopped by applying the brakes, thus transferring kinetic energy into heat energy.
  • In a collision, kinetic energy is dissipated into energy and heat.

    The Force Of Momentum


  • The force of a moving object is called momentum. The momentum of an object is determined by its weight and speed. A baseball traveling at 40 mph has more momentum than a tennis ball traveling at the same speed.
  • When you are driving, both you and your car have momentum. This is because both you and your car have weight and are moving. The faster you move, the more momentum you have. Your momentum at 20 mph is double what it would be at 10 mph. At 70 it's 7 times greater.
  • When you stop, the friction of your brakes on your tires, and your tires on the road overcomes your momentum. When it's raining your tires have less friction (traction), requiring you to slow more gradually (overcoming momentum at a slower rate).
  • When you are in a crash, the momentum of your car and body are absorbed by your car and seatbelt.

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