Chapter 6

Chapter 6
The Speed Factor: Negotiating
Curves and Braking
6.1 Physical Laws That Affect Driving
 Speed
 Refers to how fast something is moving/How quickly position is changing over time
 Measured in m/h or km/h
 Acceleration
 Measures the rate of change of an objects speed
 Increasing speed over a given period=acceleration
 Decreasing speed over a given period=deceleration
 A car is traveling from point A to point E. At the beginning of point A, you
are traveling 0mph. At the end of point C you are traveling 60mph. When
you reach point D you are still traveling 60mph. When you stop at point E,
you are traveling 0mph.
Starting Point
 Force exerted by massive objects on another object to “attract”, or “pull”
toward its center
 In terms of driving:
 When traveling uphill, gravity pulls your car towards the ground causing you to use more
power for the engine to maintain speed
 When traveling downhill, it requires less power because you are moving in the direction
that gravity is pulling you
 Friction between your vehicle’s wheels and the surface of the road
 Necessary to move, change direction, and stop
 Keeps control of your vehicle
 Traction increases when the amount of tire tread touching the road
 The grooved surface of the tire that grips the road.
 Water flows through the grooves
 1/16 inch tread
 Police used a penny to test tread depth
Inflation and Traction
 Check owner’s manual for best pressure
 When pressure is right, you get your best control, better gas mileage, and
tire wear.
 Under-inflation
 Over-inflation
 Split Traction
Reduced Traction
 Two things to maintain ideal levels of traction
 Vehicle must be in good condition
Tires, shock absorbers, steering system
 Road must be smooth
Snow, Ice, Rain, Gravel
Reduced Traction
 Ability of a force to cause an object to rotate
 In driving terms, it is the engine’s ability to apply more force to turn your
 When the force applied to your wheels exceeds the force resisting(traction),
your wheels WILL spin
 An object in motion stays in motion
 An object at rest will remain at rest
 Unless acted upon by a force
 Examples of inertia:
 Accelerating
 Decelerating/Braking
 Turning
 Crashing
Kinetic Energy and force of Impact
 Kinetic Energy
The energy an object in motion has
For a vehicle to stop, it must lose kinetic energy
Force of Impact
The force of a collision as determined by the magnitude of kinetic energy of the objects that
collide and the distance over which the kinetic energy is lost
Softer objects spread have more “give” and will spread the force of impact over a greater distance
3 Factors
Distance between impact and stopping
Reducing Force of Impact
 Guard Rail
 Dashboards
 Bumpers
 Body of Car
 Air Bags
6.2 Negotiating Curves
 Centrifugal Force
 The outward force or “pull” experienced when traveling in a circular path
 In terms of driving, the inertia of your body attempting to continue traveling in a straight
line as the vehicle changes direction
Center of Gravity (COG)
 Point at which an object’s weight is centered
 Vehicles that are wide have a lower COG
 SUVs and Jeeps have higher COG
 Vehicles with higher COG have higher potential to roll or flip on sharp turns
 Adding weight to your vehicle’s weight as measured when it is empty
 Can affect a vehicle’s COG
 You are loading your vehicle anytime you get in the car
 Increased loading will cause your vehicle to work harder
Road Slopes
 Level
 Banked
 Crowned
Banked Road
 Dips down in one direction so that one side is higher than the other
 Dips in the direction of the curve to reduce loss of traction
 Reduces incidence of rollover
 Some are banked the wrong way, must be aware
Crowned Roads
 Higher in the center than the sides
 Promote runoff to reduce hydroplaning
 Can help or harm you, depending on the direction of travel
Entering a Curve
 SPEED is only thing we can control
 Must reduce speed before entering the curve
 Reducing during the turn is extremely dangerous
 Road conditions
 Should be a last resort
 Maintain lane position
 Do not understeer or oversteer
 Slow in, Fast out
 Brake going into the turn and accelerate out
6.3 Braking
 How to brake
 Get to know how “touchy” the brakes are before you begin driving at high speeds
 Keep heel of right foot on the floor
 Move from accelerator to brake without lifting foot
Anitlock Braking Systems (ABS)
 Sensors mounted on each wheel that determines whether the wheel locks
or stops rotating during braking
 Sensor sends a signal to the microprocessor that controls the brakes
 Microprocessor tells the brakes to release enough pressure on that
particular wheel to allow it to skid
 The microprocessor then tells the brake to increase pressure
 This is repeated many times a second
 You may feel a “pulsing” sensation
Emergency Braking
 E-Brake
 Pumping the brakes
 For vehicles without ABS
 If done on a vehicle with ABS, it can send false information to the sensors and can cause
the car to skid
Stopping Distance
 Strong correlation between the speed you are traveling and the time
required for you to stop.
 Your ability to stop smoothly, accurately, and rapidly depend on two things:
 Reaction distance
Distance traveled by your vehicle during the time it takes you to identify the need to stop
Depends on speed, visibility, and your mental and/or physical state
Faster you are moving and the harder it is to see, the longer it takes to react
 Braking distance
Distance needed to come to a complete stop once the brakes have been applied
Depends on vehicle type and weight, type and wear of tires, and condition of brakes
Reaction distance + Braking distance =
 Stopping distance
 Total distance required to stop from the first time you recognize the need to brake to the
time the vehicle is no longer moving
 SPEED is the most important factor
The faster you go, the longer it takes to stop
6.4 Speed Limits
 What is a safe driving speed?
 Factors that affect “safeness”:
Posted speed limits, weather conditions, visibility, surface condition of the roadway, traffic
conditions, lane width, and any special speed laws (depends on the type of vehicle you are
• Posted Speed Limits
Maximum speed limit
• Not there to recommend a driving speed
Minimum speed limit
• Prevent drivers from moving at such slow speeds that they impede or block the normal flow of
Basic Speed Law
 Everyone has one
 It says that a driver should operate his/her vehicle at a speed that is
reasonable for existing conditions
 Examples would include inclement weather, poor visibility, poor traffic
conditions, or poor roadway surface
Other Speed Laws
 Uniform Speed Zones
Provide speed limits in commonly encountered locations even where no max. or min. speed
limits are posted
School zones
Railroad crossings
Business districts
Residential districts
Rural highways
Interstate freeways
Special Speed Laws
Slow moving vehicles
Trucks or tractors with three or more axles

similar documents