Gravity
 Gravity (Natural Law)– force that pulls all things to
earth
 Driving Up Hill – acts against gravity
 Naturally loose speed
 Must accelerate to maintain speed
 Braking distance shortened
 Down Hill
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Naturally gain speed
It will take longer to stop
Braking distance will be longer
Steeper the incline – longer the stopping distance
Gravity
 Center of Gravity
 Point around which an objects weight is evenly
distributed
 Automobile makers attempt to lower car’s center of
gravity so it handles better
 Higher the center of gravity = more unstable (see pg 92)
 Kinetic Energy – energy in motion
 Object moves it picks up energy
 Faster = More energy
 More Weight = More energy of motion (kinetic energy)
Energy of Motion
 As speed & weight of a vehicle increases…
 More speed = longer distance to stop
 Speed doubles = 4x more stopping distance (x2)
 Speed triples = ___ more stopping distance
 Speed quadruples = _____ more stopping distance
 More weight = longer distance to stop
 Weight doubles = Stopping distance doubles
 See picture pg 93
Friction & Traction
 Friction – force that keeps each tire from sliding on the
road
 Traction – friction created by tires
 Makes it possible for tires to grip the road
 Traction allows the car to move, change speed, direction, etc.
 Tread –created by grooves - grooved surface on tire
 Wet Road Conditions – allows water to flow through the
grooves away from the tire
 Hydroplaning - car floats on water – looses traction
 More tread on the road = more gripping = more control
 So… Tires play the most important role in maintaining
control
Tires
 Bald Tires
 Little tread = little gripping = little control
 Dangerous especially on icy or wet roads
 Increase risk of blowouts - rapid loss of air pressure
 Tire Inflation
 Each tire works best at certain air pressures
 Too much or too little changes the amount of tread on
the road
 Remember: More tread on the road = more control
 Also: Correct tire pressure = Better gas mileage
 Tires roll easier at correct pressure
Tires & Inflation
 Under Inflation – only outside edge of tire provides
traction
 Outside edges wear first
 Will fail quicker
 Over Inflation – only the center of the tire provides
traction
 Center will wear out first
 Temperature
 Colder = lower tire pressure
 Hotter = increase tire pressure
 Check Tire pressure regularly before driving
Traction
 Split Traction
 When braking & turning or braking & accelerating
 Some traction is used for braking or accelerating
 Some traction is used for turning
 2 Things Needed for Ideal Traction
 #1. Good Vehicle Condition
 Tires, shock absorbers, steering system
 Worn shocks will cause car to bounce off the road
 Tire Treads – must have at least 1/16th of an inch
 Penny Test – shouldn’t be able to see all of Lincoln’s head
 #2. Good Road Conditions (Concern = Ice!)
Curves
 Energy of Motion (Kinetic Energy) will try to make
you go straight around a curve
 Faster = greater force to go straight
 Vehicle Control in a Curve (4 factors)
 #1 Speed
 Higher speed = less control
 Lower speed = more control
 Best Practice: Lower speed before entering a
curve
 #2 Sharpness of Curves
 Sharper curve = more needed traction
 Best Practice: Sharper curve = slower speed
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Curves
Vehicle Control in a Curve
(4 factors continued)
 #3 Banked Curves – higher on the outside vs inside
 Helps to reduce vehicle’s tendency to move to the
outside
 #4 Load of the Vehicle
 Higher load = more kinetic energy
 Best Practice: Higher load , slower around
curves
 Vehicles of Different Sizes & Power Handle
Differently
 Smaller vehicle = generally stop & accelerate quicker
Stopping Distance
 Total Stopping Distance
 Distance the Car travels while you make a stop
 Must do 3 things to stop
 #1 Perceive the hazard
 #2 React
 #3 Brake
 Perception Time
 Time it takes to Identify, predict & decide
 Varies based on
 Visibility, the hazard & driver abilities
 Perception Distance
 Distance vehicle travels during this time
Stopping Distance
 Reaction Time
 Length of time it takes to execute action
 Average driver’s reaction time = ¾ of a second
 Reaction Distance
 Distance vehicle travels while you react
 Braking Distance
 Distance vehicle travels from the time you apply the
brakes until the vehicle stops
 Remember: Faster = longer stopping distance (x2)
 Example: 40 mph will have a 4x longer braking distance
than 20mph
Stopping Distance
 Also see page 98
Estimating Stopping
Distance
 4 Second Rule – used to estimate stopping distance
 Pick an object on the road ahead where you think you can
stop
 Count 4 seconds
 Check vehicle position
 Note: It will take you 300’ (football field) to stop going 65
mph
 Factors That Affect Braking
 Speed – faster = longer braking distance
 Vehicle Condition -worn tires/shocks = longer braking distance
Factors Affect Braking
 Roadway Surface – rain/snow/gravel = longer BD
 Driver Ability – distracted or impaired = longer BD
 ABS - Can better stop your vehicle while turning
 Hills – Uphill = shorter / Downhill = longer
 Loads – Heavy Loads = longer BD
Controlling Force of
Impact
 Force of Impact – force generated when one moving
object hits another
 3 Factors Determine “Force of Impact”
 Speed – increase or decrease in speed squares the
force of the impact
 Weight of Vehicle – heavy vehicle = greater force of
impact
 Distance between Impact and stopping – Hit
something that doesn’t “give” = greater force of
impact
Seat Belts & Air Bags
 3 Collisions Occur in a Crash
 Vehicle hits object
 Occupants hit the inside of the vehicle
 Body organs (brain, heart, liver) hit skeleton
 Restraint Devices - holds occupant in place
 Passive Restraint – works automatically – occupant
doesn’t have to do anything (Ex. Airbag)
 Active Restraint – device you have to engage
(Ex. Seatbelt)
 Wear it properly
Air Bags
 Deploy at 200 mph
 Designed to work with seat belts
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Most air bags are only effective with front collisions
Seat Belts help in side/ rollover etc.
Keep hands at 9 & 3
Sit Back – chest should be 10 “ away
Children in child seats and children under 12 must sit in back
seat
 Advancements
 Sensors for weight and position in seat
 Switch to turn off
Supplemental Devices
 Front or Rear End crushes on impact
 Energy absorbing bumpers
 Side door beams
 Reinforced Windshields
 Energy absorbing steering columns
 Padded Dash
 Child Safety Seats – back seat – required in all states
 Head Restraints