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Friction Factors We are constantly aware of the frictional force that opposes the motion of one surface in contact with another. When there is a sheet of ice on a sidewalk, friction is reduced, and it is difficult to walk. The lack of friction is an inconvenience. However, machines are lubricated to reduce friction where it is not an advantage. If you pull an object horizontally at constant velocity, the applied force just balances the frictional force. You will perform a series of short experiments in which you vary one property of the two surfaces in contact and measure the force necessary to move an object at constant velocity. Objective: Study the effect of various conditions on the force of friction. Equipment: string several blocks paper wax paper sandpaper plastic wrap spring scale Al foil Cloth Halls carriage masking tape Procedure: Record your observations on your data sheet. Record several trials and average results. A. Nature of the Surface 1. Tie a string around a block so that the string does not interfere with the sliding surface. 2. Practice until you can pull the block with constant velocity. Have a lab partner read the spring scale while the block is in motion. Record the force in Table A1. Make sure you pull the scale in a horizontal position each time. 3. Pull horizontally from rest and measure the minimum force required to set the block in motion. It occurs just before the block starts moving, and may be hard to catch. This force is equal in magnitude and opposite in direction to the force of static friction. Record this force in Table A2. 4. Vary the surface interaction by pulling the block over a variety of surfaces. Be sure to hold the surfaces still and taut during the pull. B. Area of Surface 1. Position the block with its largest surface in contact with the table. Measure the force required to pull the block across the table at constant velocity. Record the force in Table B1. Be sure to make your readings while the block is moving uniformly. 2. Turn the block on one of its narrow edges and measure the force as in step one. (You may have to relocate the string) Try to pull the block with the same velocity as before. Record this force in Table B1 3. Turn the block once more to measure the force necessary to overcome friction on the third side. C. Speed of motion 1. Measure and record in Table C1 the force necessary to move a block uniformly at different velocities. D. Rolling and Sliding Motion 1. Place a piece of masking tape around a Halls Carriage so that it will slide rather than roll when pulled. Place a 500g mass in the carriage. Measure the force required to make it move at a constant velocity. Record your measurements in Table D1. 2. Remove the tape and measure the force necessary to roll the carriage at approximately the same constant velocity. Record your observations. E. Force Pressing the Surfaces Together 1. Measure and record in Table E1 the force needed to pull a single block uniformly. 2. Place another block on top of the first and repeat the procedure, recording your observations. 3. Repeat with a third block. Calculating the Coefficient of Friction: 1. Using the data from Table A1, determine µk (kinetic) for each of the surfaces tested. Make any additional measurements needed for your calculations. (Ff = µFN) 2. Using the data from Table A2, determine µs (static) for each of the surfaces tested. Name: Period: Friction Factors Data Sheet Prior to recording data, please predict the outcome based on your prior knowledge of friction. Table A1: Nature of the Surface - Kinetic Friction Prediction: Which surface will exhibit the greatest kinetic friction? _________________________________________ Results: Which surface exhibited the greatest kinetic friction? ___________________________________________ Surface Trial 1 Trial 2 Trial 3 Average Wood-wood Wood-paper Wood-wax paper Wood-plastic Wood-foil Wood-cloth Wood-sandpaper fine Wood-sandpaper rough Table A2: Nature of the Surface – Static Friction Prediction: Which surface will exhibit the greatest static friction? _________________________________________ Results: Which surface exhibited the greatest static friction? ________________________________________ Surface Trial 1 Trial 2 Trial 3 Average Wood-wood Wood-paper Wood-wax paper Wood-plastic Wood-foil Wood-cloth Wood-sandpaper fine Wood-sandpaper rough Table B1: Area of Surface Prediction: Will more or less surface exhibit the greatest friction? ________________________________________ Results: Surface area (cm²) Trial 1 Trial 2 Trial 3 Average Table C1: Speed of Motion Prediction: Which speed will result in the most friction? _________________________________________________ Results: Relative speed Trial 1 Trial 2 Trial 3 Average Slow Moderate Fast Table D1: Rolling and Sliding Motion Prediction: Will rolling or sliding motion exhibit more friction? ____________________________________________ Results: Motion Trial 1 Trial 2 Trial 3 Average Sliding Rolling Table E1: Force Pressing the Surfaces Together Prediction: Will increased weight cause greater friction? __________________________________________ Results: Number of blocks Trial 1 Trial 2 Trial 3 Average Calculating the Coefficient of Friction: Surfaces Ff = µk FN Wood-wood Wood-paper Wood-wax paper Wood-plastic Wood-foil Wood-cloth Wood-sand paper µ kinetic Ff = µs FN µ static Analysis: 1. Which factors influence the force of friction? 2. Which material had the greatest static friction? The least? 3. Which material had the greatest kinetic friction? The least? 4. Leather, rather than rubber, is generally recommended for the soles of toddler’s shoes. Why? 5. Sand is often scattered on roads during snowstorms and icing. Why? 6. If there were no friction would it be possible to tie a knot in a piece of string or rope? Would the knot hold? Explain. 7. Knowing that the static friction for a rubber tire on a concrete road is 1.0 while the kinetic (sliding) friction is only 0.7, explain the benefit of anti-lock braking systems. Write a brief paragraph summarizing your findings and relating several interesting or surprising findings.