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Natural Laws and Vehicle Natural Laws and Vehicle ControlControl

Chapter 9Chapter 9

Driver EducationDriver Education

Legacy High SchoolLegacy High School

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Gravity and the Energy of MotionGravity and the Energy of Motion

• There are many natural laws such as gravity, speed and momentum that exert forces on a vehicle.

• Many times these forces are great enough to cause a driver to lose control.

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Gravity and the Energy of MotionGravity and the Energy of Motion

• To be a safe driver, you need to know about these natural laws and the effect they can have on your ability to safely control a vehicle.

• You cannot change the natural laws, you can have better control of your vehicle with knowledge of these laws.

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Gravity and the Energy of MotionGravity and the Energy of Motion

• Gravity – gravity keeps your vehicle on the ground.

• Gravity makes going up hill difficult and increases downhill speed.

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Center of GravityCenter of Gravity

• The center of gravity is usually located at the center of your vehicle and is the point where the vehicle’s weight is evenly distributed.

• When a vehicle “rolls” it rotates around the center of gravity.

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• Raising a vehicles center of gravity could make it unstable on a steep hill or on a sharp turn.

• High profile vehicles have a higher center of gravity and may be involved in rollover accidents.

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Energy of MotionEnergy of Motion

• Once a vehicle is moving, it takes energy to stop.

• The heavier or faster a vehicle is, the longer the stopping distance.

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Energy of Motion – Stopping Energy of Motion – Stopping DistanceDistance

• A vehicles stopping distance doubles when its weight doubles.

• When a vehicle’s speed doubles, the stopping distance increases 4 times.– When a vehicle’s speed triples, the stopping

distance increases nine times. (the square rule)

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Centrifugal forceCentrifugal force

• Centrifugal force is the tendency for an object to continue to move in a straight line when the object changes direction.– A vehicle turning left

wants to continue forward, this is why passengers are “forced” to lean to the right.

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Centrifugal forceCentrifugal force

• If a vehicle loses traction (tire grip on the road), centrifugal forces can cause the vehicle to run off the road.

Questions on Gravity and the Energy of Motion

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Friction and TractionFriction and Traction

• The tires and their contact with the road surface control the direction and speed of a vehicle.– It is these four small areas of contact that give

vehicle control.

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Friction and TractionFriction and Traction

• Friction is the force that keeps each tire from sliding on the road.– Friction gives your vehicle control.

• Another term used for friction when speaking about vehicles and tires is traction.

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Tires and TreadTires and Tread

• Tread – the grooved surface on a tire that allows water or snow to flow through the grooves and away from the tire.

• Traction is increased as is vehicle control when water is channeled away from the tire.

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Tires and TreadTires and Tread

• A worn or bald tire is dangerous and will not grip a wet or icy road.

• Bald tires have no tread and may suddenly deflate if the tire is punctured – a blowout.

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Tires – Inflation and TractionTires – Inflation and Traction

• Each tire has inflation and other specifications.

• Use tires designed for your vehicle and type of driving you may do.

• Specific tread types may function better in rain or snow.

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Tires Underinflation Tires Underinflation

• When driving on an underinflated tire, only the outside edges of the tire are in contact with the road surface.

• The outside edges wear first.

• Will not perform properly in an emergency and will heat and fail prematurely.

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Tires -- Tires -- OverinflationOverinflation

• If a tire has too much air pressure, only the center of the tire will grip the road and will wear out first.

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Tires -- InflationTires -- Inflation

– Outside air temperature can change the air pressure in tires.

• Cold temperatures decrease pressures• Hot temperatures increase pressures.

– All tires “leak” a small amount of air.– Check tire pressures on a monthly basis.– Inspect tires for tread wear and inflation every

time you enter the vehicle.• Inspect for cracks or tire damage.

– Check tire pressures when the tires are “cold” – before you start driving.

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Tread depthTread depth

– Ideal levels of traction are maintained when tires have good tread depth and are properly inflated.

– When using a tread depth gauge, tires need to have at least 1/16-in. of tread or more (this is the minimum amount of tread allowed by law).

– By using a penny as a quick reference, insert the penny into the tread groove with Lincoln’s face showing, but with his head upside-down. If you are able to see all of Lincoln’s head, the tire needs replacement.

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HydroplaningHydroplaning

• A wedge of water in front of the tires may pass under the tires and the tires will ride on a cushion of water.

• Traction will be lost completely.

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Road surfaceRoad surface

• Wet or icy road surfaces will reduce traction, giving the driver less control and increasing stopping distances.

• Rough, gravel, or dirt road surfaces have less traction than paved surfaces.

• Oil on the road surface can mix with water or rain causing decreased traction.

• Always slow down when traction is decreased.

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CurvesCurves

• The vehicle’s speed, the sharpness of the curve, the bank of the curve, the road condition and the vehicle’s load will affect the control you have in a curve.

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CurvesCurves

• You have no control over how sharp a curve is, or road condition, but you can control your vehicle’s speed.

• Reduce your speed before entering the curve.– If you cut your speed in

half, the force pushing you off the road will be cut four times.

Friction and Traction Questions