The Natural Laws 1.INERTIA A. A body in motion continues moving in a straight line and at a constant speed until some force acts upon it. B. A body at rest tends to stay at rest until some outside force acts upon it.

2. FRICTION A. Friction is defined as the force between two surfaces that resists the movement of one surface across the other. B. Friction between the road and the tires is called TRACTION or ADHESION. 1. This is what keeps the car on the road through a turn or curve.

C. Tire pressure effects traction. 1. Recommended pressure can be found in the owner’s manual. 2. Underinflated tire profile 3. Overinflated profile D. Tire condition effects traction. 1. New tires vs. worn tires 2. Legal tread depth must be greater than 2/32 inch (Lincoln’s head on a cent)

E. Rain effects traction. 1. Hydroplaning can begin at speeds as low as 35 mph in only 1/12 inch of water. F. Ice and Snow effect traction. 1. Traction is poorest at 32º F because slush is more slippery than packed snow. 2. Snow tires are not very effective on ice. 3. Studded snow tires (where and when they are permitted) are better. a. Studs are metal and their use may be restricted because they cause damage to road surfaces. 4. Tire chains are better still.

3. MOMENTUM A. It is a product of weight and speed ( weight X speed = Momentum ). B. More momentum means more damage in a collision. C. Lighter vehicles cause less damage but may be damaged more easily because there is less protection.

4. KINETIC ENERGY A. Also known as the energy of motion. B. More speed = more energy C. It requires more time and a greater distance to stop when traveling at a higher speed because there is more energy to overcome.

D. Stopping distance increases as the square of the difference in the speed. Examples: Speed Stopping Distance = 10 mph = 50 ft = 20 mph = 200 ft (2² = 4 x 50 = 200) = 30 mph = 450 ft (3² = 9 x 50 = 450) = 40 mph = 800 ft (4² = 16 x 50 = 800)

E. Acceleration when entering a highway: A heavier vehicle needs more gas to accelerate up to speed. If the weight of the vehicle is such that even the increased pressure on the gas pedal won’t work, then more space is needed in order to complete the maneuver. F. Braking: A heavier vehicle needs more stopping distance due to its increased momentum and energy.

5. GRAVITY A. It will increase the speed of a vehicle going downhill. B. It will slow down a vehicle going uphill. C. Center of gravity effects the vehicle 1. This is the point within the vehicle around which the vehicle’s weight is distributed evenly in all directions. 2. A low center of gravity means good handling. 3. An overloaded vehicle, or one with a higher profile (SUV) will have a higher center of gravity and will not handle as well on curves.