Web Traffic School
Forces that Influence Driver Control
MOTION CONTROL
Earlier in this program, we discussed man-made laws and how they impact our driving effectiveness.

In this segment, we're going to talk about several other laws that come into play when we're driving a vehicle. They're called natural laws.

GRAVITY

Take gravity, for instance. Gravity is what keeps us from flying off into space--at least that's what it's supposed to do. Just when we get a little cocky, though, gravity has a way of bringing us back down to earth, unless, of course, you're an astronaut.
However, it's not a good idea to defy the laws of gravity when you're driving a car.

CENTRIFUGAL FORCE

Centrifugal force is another natural law we have to contend with when driving. It's the force that pulls an object out from the center. It, too, is a force to be reckoned with when you're driving.

INERTIA

Then, there's a thing called the law of inertia. It says that a body in motion tends to stay in motion; that is, if you're in a moving vehicle that stops suddenly, you'll keep right on going at the same rate of speed you were going before the vehicle stopped, even if you don't want to.
Look, the dashboard!!
Speed


Braking


Steering
There are three basic ways that we control the motion of our cars. They are (1) speed, (2) braking, and (3) steering. Each of these functions is affected by the laws of gravity, centrifugal force, inertia, kinetic energy, and friction.
ENERGY=SPEED

When you accelerate your car, you are increasing the car's energy. Keep in mind, the faster the car is moving, the longer it will take for the vehicle to stop. Your car will respond differently at faster speeds, so you must adjust the way you accelerate, decelerate and steer.
A vehicle's energy of motion doubles when its weight doubles. When a vehicle's weight doubles, it needs about twice the distance to stop.
A vehicle's energy of motion is proportional to the square of its increased speed. When the vehicle's speed doubles, it needs about four times the distance to stop. When the vehicle's speed triples, it needs about nine times the distance to stop.
All of which means, the faster you're driving, the longer it takes you to stop, and the less time you have to react to a situation. You also have increased difficulty steering in the intended path of travel and an overall decrease in car control.
Along with those safety considerations, speeding also causes decreased fuel efficiency, increased engine wear, and increased tire wear.
By reducing speeds, or decelerating, you will have more time to react, shorten your braking distance, have less difficulty steering in your intended path of travel, and have an overall increase in car control. You'll enjoy better fuel efficiency, less engine wear, and less wear and tear on your tires.
FRICTION
If it weren't for friction, you couldn't stop your car in an emergency. You'd have to wait for it to stop itself. Friction is critically affected by your brakes, tires, the road surface, and speed. The brakes on your car have to be strong enough to lock the wheels into a skid. Your tires must have enough tread to create good traction. Traction is the friction of gripping power of the tire as it moves on the roadway.

FACTORS AFFECTING TRACTION

  • Friction
  • Stopping distance
  • Centrifugal force
  • Hydroplaning
Traction is affected in one way or another by friction. For instance, consider the effect of these factors: stopping distance, centrifugal force, and/or hydroplaning. An increase in any or all of these factors will decrease the amount of traction that your car will have because there is a corresponding decrease in the opportunity for friction. By decreasing a car's traction, you are thereby decreasing the amount of control you have over the vehicle.
When your car goes into a full skid, you don't get maximum braking. Sometimes a driver will slam on his brakes and slide right into the object he was trying to avoid.

When you're in a full skid, your front wheels are sliding and you lose control over the steering. The direction of your car won't change, even if you turn the steering wheel.

Sometimes you can regain steering control by releasing the brake enough so the wheels are not skidding, then steering around the object you're trying to avoid hitting.
Tires with good tread are necessary for good traction on our roadways.

The purpose of tread is to increase frictional contact with the road to give us greater stopping power.
Good friction is required for proper traction on the roadway--so is a good road surface. The type of road surface greatly influences the amount of friction our tires create with the roadway. A dry, concrete road provides the best friction, while dirt roads are one of the worst. On dirt roads, our tires are literally rolling across small stones that have a ball bearing effect.

Needless to say, you need a lot more room to stop on a dirt road than you do on dry pavement.
On wet pavement, most of the friction is lost due to hydroplaning. The mixture of oil and water on a concrete or asphalt road causes the tires to "float" on an emulsion. The tires, in effect, lose their contact with the pavement, and thus, lose the benefit of friction.

However, the most difficult surface for maintaining any type of friction is on an icy road. It will take you three times farther to stop than under ordinary conditions.
SPEED AND KINETIC ENERGY
Keep in mind also that every time you stop your car, you have to overcome a force that is known as kinetic energy. This force is the energy developed by an object while it's in motion.

In very simple terms, the law of kinetic energy means that when you double your speed, the kinetic energy of your car is four times as great! And that means the braking distance is four times as great. When you triple or quadruple your speed, the stopping distance is even greater.
Let's say you're driving at 20 mph on dry concrete pavement. At that rate of speed, you'll need 20 feet to stop your car, or one foot for each mile per hour.
Now you've doubled your speed to 40 mph. You'll need four times the stopping distance, or 80 feet, to bring your car to a safe stop.
So you can see, kinetic energy has a direct bearing on speed versus stopping distance.
CENTRIFUGAL FORCE
Another force you need to understand is centrifugal force. A simple definition is the force that pulls an object out from the center of a curve.

Centrifugal force comes into play when you're rounding a curve. It tends to pull your vehicle out away from the inside edge, regardless of which direction you're traveling.
If a car on the "inside" lane of a curve tries to navigate with too much speed, centrifugal force will pull it away from the center and, potentially, into the path of oncoming traffic.
Or, if a car is traveling with too much speed from the other direction--in the "outside" lane of a curve--centrifugal force will pull it out to the right and off the roadway.

If you would like to see the animation again, right click on the animation and choose "Rewind" from the menu list.

INERTIA
Anybody who has ever survived a serious car crash can tell you about this force. If a vehicle is traveling at 40 mph and hits an object that stops its motion, everybody and everything in that vehicle keeps traveling at 40 mph until they hit something that stops them. This is the force called inertia.
Inertia is the force that causes the second and third collisions in an automobile crash. Bodies hitting bodies, and objects in the car hitting bodies.
In this test, a driver is going 40 mph without his safety belt on. His vehicle hits a stationary object and stops.
But he keeps moving at 40 mph--until he hits the windshield, that is.
GRAVITY
Gravity is another force that affects car handling. It is a force that pulls objects downward. When a vehicle is going downhill, gravity is one of the forces pulling it down.

Conversely, when your vehicle is going uphill, gravity helps slow it down.
FORCES AFFECTING CONTROL AND VEHICLE PERFORMANCE
  • INERTIA
  • INERTIA
  • KINETIC ENERGY
  • INERTIA
  • KINETIC ENERGY
  • CENTRIFUGAL FORCE
  • INERTIA
  • KINETIC ENERGY
  • CENTRIFUGAL FORCE
  • GRAVITY
Inertia, kinetic energy, centrifugal force, and gravity all affect driver control and vehicle performance. They affect stopping distance, car control, and the force of an impact.
Stopping Distance

Car Control

Force of Impact
FACTORS AFFECTING IMPACT

SPEED

Speed is one of the factors affecting impact. When you increase the speed at which you hit an object, you directly increase the force of the impact. When you increase the force of impact, you increase the risk of injury or death in a collision.

WEIGHT

When you increase the weight of an object, whether it is the vehicle you are driving or the object you are about to hit, you increase the force of the impact, and conversely, the risk of injury or death.

DISTANCE

When you increase the distance your car has to travel between the time you put on the brakes and the time you reach impact, you increase the ability of your car to slow down before reaching the point of impact. You are, therefore, decreasing the force of impact, which decreases the risk of injury or death in a collision.

GREATER SPEED AND WEIGHT:

  • INCREASE YOUR STOPPING DISTANCE

GREATER SPEED AND WEIGHT:

  • INCREASE YOUR STOPPING DISTANCE
  • REDUCE YOUR CONTROL OF YOUR CAR

GREATER SPEED AND WEIGHT:

  • INCREASE YOUR STOPPING DISTANCE
  • REDUCE YOUR CONTROL OF YOUR CAR
  • INCREASE THE FORCE OF IMPACT
Greater speed and weight increase your stopping distance, reduce your control, and increase the force of impact.

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas
  • Energy-absorbing bumpers

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas
  • Energy-absorbing bumpers
  • Side door beams

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas
  • Energy-absorbing bumpers
  • Side door beams
  • Reinforced windshields

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas
  • Energy-absorbing bumpers
  • Side door beams
  • Reinforced windshields
  • Energy-absorbing steering wheels

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas
  • Energy-absorbing bumpers
  • Side door beams
  • Reinforced windshields
  • Energy-absorbing steering wheels
  • Padded dashboards

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas
  • Energy-absorbing bumpers
  • Side door beams
  • Reinforced windshield
  • Energy-absorbing steering wheel
  • Padded dashboards
  • Head restraints

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas
  • Energy-absorbing bumpers
  • Side door beams
  • Reinforced windshields
  • Energy-absorbing steering wheels
  • Padded dashboards
  • Head restraints
  • Air bags and safety belts

ENERGY-ABSORBING FEATURES

  • Front and rear crash areas
  • Energy-absorbing bumpers
  • Side door beams
  • Reinforced windshields
  • Energy-absorbing steering wheels
  • Padded dashboards
  • Head restraints
  • Air bags and safety belts
  • Anti-lock brakes
Today's automobiles are equipped with a number of features designed to absorb energy and decrease the force of impact. Front and rear crash areas, energy-absorbing bumpers, side door beams, reinforced windshields, energy-absorbing steering wheels, padded dashes, head restraints, air bags, safety belts, and anti-lock brakes help cushion the force of impact, increasing the driver's control and the vehicle's performance in a collision.