Objective: To identify whether positive, negative, or zero work is being done, to identify the force that is doing the work, and to describe the energy transformation associated with such work.
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Friction, Speed, and Stopping Distance
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Identify the basic relationships between the friction, speed, skid distance, and braking distance.
Use Table 2 to relate the reaction time, car speed, and reaction distance to one another.
Use the model relating the total stopping distance to the variables that affect it to make predictions and explanations.
Utilize Figure 2 to relate the surface, friction coefficient, car speed, and skid distance to one another.
Using the Model
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Friction, Speed and Stopping Distance
The distance that a car will travel to stop depends on the driver's alertness and the roadway conditions. Friction is the force that is developed between roadway surfaces and the locked tires of a skidding car. Friction is responsible for stopping a skidding automobile. Roadway surfaces are typically rated with a friction coefficient (µ) that serves as an indicator of the amount of friction the surface provides. The friction coefficient is the ratio of the friction force to the load (vehicle weight) as measured on a level surface. Friction coefficients are dependent upon the tire and the nature of the roadway surface. Table 1 estimates the friction coefficient for good tires on varying road surfaces.
Knowledge of a friction coefficient allows one to predict the distance that a vehicle skids when the brakes are fully locked. This skid distance is dependent upon the original speed of the vehicle prior to braking. Figure 1 illustrates the dependence of skid distance upon vehicle speed for varying friction coefficients. The distances are based on locked brakes and do not account for anti-lock brakes.
Figure 1 represents the skid distance. The total stopping distance of an vehicle also includes the distance the vehicle travels before the brakes are applied. When the driver perceives a situation that demands braking, the driver must react to the simuli, lift the foot off the gas pedal and fully engage the brake pedal. During this reaction time, the car is traveling at a constant speed and covering distance. The total stopping distance is the sum of this reaction distance plus the skid distance. Table 2 demonstrates the dependence of reaction distance (in feet) upon vehicle speed and reaction time.
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Template Version 1.2 Added Question Scene 4 for Table Completion
Basic Relationships,Skid Distance,Reaction Distance,Using the Model
One aspect of safe driving involves the ability to stop a car readily. This ability depends upon the driver's alertness and readiness to stop, the conditions of the road, the speed of the car, and the braking characteristics of the car. The actual distance it takes to stop a car consists of two parts - the reaction distance and braking distance.
When a driver sees an event in his/her field of view that might warrant braking (for example, a dog running into the street), a collection of actions must be taken before the braking actually begins. First the driver must identify the event and decide if braking is necessary. Then the driver must lift his/her foot off the gas pedal and move it to the brake pedal. And finally, the driver must press the brake down its full distance in order to obtain maximum braking acceleration. The time to do all this is known as the reaction time. The distance traveled during this time is known as the reaction distance. Once the brakes are applied, the car begins to slow to a stop. The distance traveled by the car during this time is known as the braking distance. The braking distance is dependent upon the original speed of the car, the road conditions, and characteristics of the car such as its profile area, mass and tire conditions. Figure 1 shows the stopping distance for a Toyota Prius on dry pavement resulting from a 0.75-second reaction time.
The reaction time of the driver is highly dependent upon the alertness of the driver. Small changes in reaction time can have a large effect upon the total stopping distance. Table 1 shows the reaction distance, braking distance, and total stopping distance for a Toyota Prius with an original speed of 50.0 mi/hr and varying reaction times.