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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

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Analyze the design of three related friction experiments.

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Use provided data and the resulting model to make predictions regarding the amount of static or kinetic friction.

An Investigation of Friction Between Two Surfaces

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Making Predictions

Use the data from three friction experiments to develop a model regarding the variables that affect friction.

Experimental Design

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A Model for Friction

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Friction Friction is a force that resists the motion of objects. A group of students are conducting experiments on friction. Experiment 1 Figure 1 depicts a procedure that is used to measure the friction force. A horizontal force is applied to a block on a table to accelerate it from rest. One observes that the force needed to initiate the motion (accelerate it from rest) is greater than the force needed to sustain the motion at a constant speed. Masses can be added to the block, thus increasing the load (L). The force required to initiate the motion is equal to the maximum value of the static friction force (Fstatic). The force required to sustain the motion at a constant speed is equal to the kinetic friction force (Fkinetic). Table 1 represents typical data from such an experiment. Experiment 2 A student decides to investigate the effect of the surface on the amount of friction. She layers the bottom of the block with coarse sandpaper and then repeats the procedure described in Experiment 1. Her data are shown in Table 2. Experiment 3 Another student makes an effort to determine if the area of contact between the block and the table affects the amount of friction. The student finds a rectangular block with three unequal length sides. The three areas of the sides of the block are shown. The student pulls the block (without the sandpaper) along each of the sides at a constant speed. The data are shown in Table 3.

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Physics-Interactives/Static-Electricity/

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Experimental Design,A Model for Friction,Making Predictions

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Stopping Distance 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.

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