# Drivers Ed Lesson 3

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Natural forces that act on your vehicle include the following:
Gravity. Inertia. Momentum. Kinetic energy. Potential energy. Friction. Centrifugal force.
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Misjudging Natural Forces
By misjudging natural forces, you might lose control while making a turn or lose traction on wet pavement and be unable to stop.
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Gravity
Gravity is the force that pulls all objects toward the center of the earth. Gravity affects your speed when you drive uphill and downhill. Traction relates to gravity in that it is the result of friction between the road and your tires caused by the weight of the car (the pull of gravity). Traction is a vital component of driving because you need traction to steer.
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Driving Uphill
When you're driving uphill, the force of gravity is working against you to slow you down, and you may need to accelerate or shift to a lower gear to maintain your speed.
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Driving Downhill
When you drive downhill, the reverse is true. Gravity will cause you to go faster and increase your stopping distance. You may need to shift to a lower gear or smoothly apply your brakes to slow to a safe speed and control your vehicle.
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Parking on an Incline
When you leave a vehicle parked on an incline, gravity pulls your vehicle downhill. To keep your vehicle from rolling away, leave your vehicle in a low gear or in "Park" if it has an automatic transmission. You should always engage your parking brake and may need to block your wheels by placing an object in front of or behind the tires. In case your parking brake fails, always turn the front wheels of your vehicle so that the vehicle won't roll into the traffic lane.
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Parking Downhill
If you're parking facing downhill, turn your front wheels toward the curb or side of the road.
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Parking Uphill
If you're parking facing uphill next to a curb, turn your front wheels toward the middle of the road and allow the vehicle to roll back against the curb. If you're parking facing uphill and there is no curb, turn your wheels toward the side of the road.
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Inertia
Objects that are moving tend to continue moving, while objects that aren't moving tend to remain at rest unless they're acted upon by some other force. This is called the law of inertia and it applies to driving. For example, when you're waiting for a green light on flat pavement at an intersection, your vehicle won't move unless you engage the engine or are otherwise pushed.
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Effect of Inertia on Driving
While driving, inertia keeps your vehicle moving unless the vehicle is acted upon by something, such as your brakes, the road surface, a fixed object (such as a tree), or another vehicle. Inertia also causes your body and loose objects in the car to keep moving forward if your vehicle comes to a sudden stop.
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Inertia in Crashes
You may be injured because of the inertia and momentum of loose objects in your car which can fly through the air during a sudden stop. When you're stopped and hit from behind, your head tends to stay in place due to inertia while the rest of your body is pushed forward by the seat. This causes whiplash. Properly adjusting your headrest can reduce injury due to whiplash.
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Potential Energy
Potential energy is the energy that an object possesses because of its position or form. A book on a table has the "potential" energy to fall to the floor, whereas a book that is already on the floor doesn't have this potential energy.
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Effect of Potential Energy on Driving
When you're parked on a hill, gravity causes your car to have potential energy. This energy can be converted to kinetic energy (motion) if the vehicle breaks loose and rolls down the hill. There is also potential energy built up in the components of your car's suspension system that can cause you to swerve when coming out of a turn.
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Kinetic Energy
Kinetic energy is the energy a body possesses because it's in motion. For example, the potential energy of a book on a table is converted to kinetic energy (motion) when it falls. A book lying flat on the floor doesn't have this same potential or kinetic energy.
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As you increase your driving speed, both your body and your vehicle acquire kinetic energy that eventually must be
Absorbed by your brakes, engine-compression forces, or other friction in a controlled stop. Absorbed by your body, your vehicle's body, and the objects you hit if you're involved in a crash.
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The kinetic energy of your body when 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
If you increase your speed from 10 mph to 20 mph, you're dealing with 4 times the amount of kinetic energy. If you increase you speed from 10 mph to 50 mph, you're dealing with 25 times the amount of kinetic energy.
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kinetic energy of your moving vehicle determines
your ability to stop the car
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The amount of stopping distance required will
increase with an increase of your kinetic energy.
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Gravity and Kinetic Energy
Gravity decreases your kinetic energy when driving uphill and increases it when driving downhill. Therefore, the force of gravity will make it easier to stop your car if you're going uphill but more difficult to stop your car if you're going downhill.
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Braking and Energy
Braking to a stop converts kinetic energy into heat energy in 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.
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Friction
Friction is a force caused by the contact of one surface on another. It results in the resistance of an object moving over a surface. For example, it's easier to move your hand over fine sandpaper than rough sandpaper because less friction is produced by the surface of the fine sandpaper
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Friction is
Increased by the weight of your vehicle. Decreased if tires are overinflated or worn smooth. Increased if tires are underinflated. Affected by materials used to construct the road. Affected by road-surface conditions, including factors due to weather.
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Stopping Power
Because friction is increased by the weight of your vehicle, a fully loaded truck has more stopping power than an empty one, as it's heavier. The additional weight helps to stop the vehicle by producing more friction between the tires and the road surface.
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Worn or overinflated tires will decrease the friction between your tires and the road surface, reducing the traction needed to Start smoothly. Stop quickly. Pull out of a turn or curve without losing control. Road surface changes due to ice, rain, snow, oil and diesel fuel buildup, or sand and dirt will reduce the friction and traction of your tires and could result in you losing control of your vehicle.
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Skidding
Skidding happens when the friction of your brakes is stronger than the friction between your tires and the road; this causes you to lose traction. The kinetic energy of your vehicle can't be converted into friction in your brakes (heat energy) when your brakes are locked, because they're in a locked-wheel skid. If the components of your brakes become too hot, they can't release any additional heat and will fail to absorb any more kinetic energy (such as when traveling down a steep hill and using your brakes often).
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Anti-Lock Brakes
The purpose of anti-lock brakes is to prevent your wheels from locking during a quick stop so that your brakes can continue to absorb energy and you can maintain traction.
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Loss of Traction Can be Avoided By
driving more slowly and using your brakes properly.
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The friction that the clutch creates when released can:
Cause a loss of traction if released improperly (peeling out). Slow your vehicle by engaging a lower gear. Allow the vehicle to move regularly.
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Friction wears out your brakes and clutch. If used improperly, excessive wear to the brake pads and clutch can occur. To avoid this:
Don't "ride" your brakes. Don't drive with your clutch partially engaged (depressed). Use lower gears to slow your vehicle on downhill grades (downshift).
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Lubrication
Friction can be reduced through lubrication. Improper lubrication of your engine and transmission will cause excessive friction that can result in mechanical failure. You could be left stranded or lose control of your vehicle as a result.
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Centrifugal Force
Centrifugal force is the tendency of objects to be pulled outward when rotating around a center.
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Centripetal Force
Centripetal force is always directed toward the center of the circle. Without this force, an object will simply continue moving in straight-line motion.
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Turning and Centrifugal Force
When you turn, your car is subject to centrifugal force that pulls it away from the direction you want to turn and into a straight line. Traction is necessary to keep from you from losing control of your car during a turn.
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Banked roadways help in overcoming the centrifugal force that pulls your car away from the direction in which you want to turn
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To keep a vehicle in a turn without allowing centrifugal force to pull the car out, you should do the following:
Decelerate before the curve. Brake gently and gradually while turning. Downshift (if your car has manual transmission).
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Factors that Determine the Force of Impact
Kinetic energy, or the speed and weight of the vehicle and its occupants. The characteristics of a vehicle, including its body rigidity and crush zones.
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Kinetic Energy and Impact
The greater a vehicle's kinetic energy, the greater the force of impact.
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Crush Zones
I-beam construction in the vehicle's frame and doors provides greater rigidity and more protection against the force of impact. Crush zones are areas of the vehicle designed to absorb the force of impact by collapsing to protect passengers.
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During a Crash
The kinetic energy of your vehicle and body must be dissipated. The force needed to dissipate this energy is lower if the time period over which your vehicle moves during the crash is longer. If the vehicle stops suddenly, the force will be very high.
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Force and Collisions
The force that stops your car during a crash will be greatest if you have a head-on collision with another vehicle or a large immovable object, such as a bridge abutment, wall, or tree. This is true because the momentum and kinetic energy of your car must be absorbed almost instantaneously.
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Weight and Collision
If two vehicles moving at the same speed are involved in a collision, the vehicle that weighs less will take the greater impact. The larger and heavier the vehicle, the greater the energy and momentum. The smaller and lighter vehicle will have greater deceleration and may even be pushed in the reverse direction of travel.
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In the following instances, the smaller and lighter vehicle may be crushed:
A semi-truck against a sedan. A train against a semi-truck.
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You can reduce the force on you and your car during an unavoidable crash if you can redirect your path toward objects that will cause your car to stop over a greater distance, such as
Bushes rather than trees. Snow. Soft dirt. Sand barrels placed in front of freeway abutments.
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Modern vehicles have features that reduce the forces on your body during a crash by absorbing energy and increasing the distance over which the impact occurs. These include:
Crush zones, or areas of the vehicle designed to absorb impact by collapsing without harming the passenger area of the vehicle. I-beam construction in the frame and doors to give the vehicle more rigidity. Air bags, which allow the momentum of your body to be absorbed over a slightly greater distance than would occur if your body hit the steering wheel or windshield. Energy-absorbing bumpers that reduce the effect of kinetic energy during a front or rear-end collision. Padded dashboards. Safety-glass windows and windshields designed to crumble into small, dull-edged pieces to reduce lacerations.
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Gravity
Gravity affects the speed of travel when you're heading uphill and downhill.
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Inertia and Whiplash
If your vehicle is hit from behind while stopped, your head tends to stay in place due to inertia, while the rest of your body is pushed forward by the seat. This causes whiplash. Properly adjusting your headrestreduces injuries due to whiplash.
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Seat belts