PWV 12 Static Kinetic Friction (Force)

Name: Christian Gavidia Date: 11/6/2023 Lab Answer LabQuest App 12 Static and Kinetic Friction (ForceSensor) If you try to slide a heavy box resting on the floor, you may find it difficult to get the box moving. Static friction is the force that counters your force on the box. If you apply a light horizontal push that does not move the box, the static friction force is also small and directly opposite to your push. If you push harder, the friction force increases to match the magnitude of your push. There is a limit to the magnitude of static friction, so eventually you may be able to apply a force larger than the maximum static force, and the box will move. The maximum static friction force is sometimes referred to as starting friction . We model static friction, F static , with the inequality F static ≤ μ s N where μ s is the coefficient of static friction and N is the normal force exerted by a surface on the object. The normal force is defined as the perpendicular component of the force exerted by the surface. In this case, the normal force is equal to the weight of the object. Once the box starts to slide, you must continue to exert a force to keep the object moving, or friction will slow it to a stop. The friction acting on the box while it is moving is called kinetic friction . In order to slide the box with a constant velocity, a force equivalent to the force of kinetic friction must be applied. Kinetic friction is sometimes referred to as sliding friction . Both static and kinetic friction depend on the surfaces of the box and the floor, and on how hard the box and floor are pressed together. We model kinetic friction with F kinetic = μ k N , where μ k is the coefficient of kinetic friction. In this experiment, you will use a force sensor to study static friction and kinetic friction on a wooden block. A motion detector will also be used to analyze the kinetic friction acting on a sliding block. Figure 1 OBJECTIVES l Use a force sensor to measure the force of static and kinetic friction. l Determine the relationship between force of static friction and the weight of an object. l Measure the coefficients of static and kinetic friction for a particular block and track. Physics with Vernier 12 -1

Experiment 12 l Use a motion detector to independently measure the coefficient of kinetic friction and compare it to the previously measured value. l Determine if the coefficient of kinetic friction depends on weight MATERIALS LabQuest LabQuest App Motion Detector Dual-Range Force Sensor string block of wood with hook balance or scale mass set (optional) Graphical Analysis, Logger Pro , or graph paper PRELIMINARY QUESTIONS 1. In everyday life, you often experience one object sliding against another. Sometimes they slip easily and other times they do not. List some things that seem to affect how easily objects slide. 2. Consider a box sitting on a table. It takes a large force to move it at constant speed. List at least two ways you could reduce the force needed to move the box at constant speed. 3. In pushing a heavy box across the floor, is the force you need to apply to start the box moving greater than, less than, or the same as the force needed to keep the box moving? On what are you basing your answer? PROCEDURE Part I Starting Friction 1. Measure the mass of the block and record it in the data table. 2. Set the range switch on the Dual-Range Force Sensor to 10 N. Connect the force sensor to LabQuest. Choose New from the File menu. 3. Tie one end of a string to the hook on the force sensor and the other end to the hook on the wooden block. Place a total of 1 kg mass on top of the block, fastened so the masses cannot shift. Before you collect data, practice pulling the block and masses with the force sensor using a straight-line motion. Slowly and gently pull horizontally with a small force. Very gradually , taking one full second, increase the force until the block starts to slide, and then keep the block moving at a constant speed for another second. 4. Sketch a graph of force vs. time for the force you felt on your hand. Label the portion of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed. 12 -2 Physics with Vernier

Experiment 12 Figure 2 15. Place the motion detector on the lab table 1–2 m from a block of wood, as shown in Figure 2. Use the same surface you used in Part II. Position the motion detector so that it will detect the motion of the block as it slides toward the detector. 16. Tap Meter. Disconnect the force sensor. Set the motion detector sensitivity switch to Cart. Connect the motion detector to a digital (DIG) port on LabQuest. Choose New from the File menu. 17. Practice sliding the block toward the motion detector so that the block leaves your hand and slides to a stop. Minimize the rotation of the block. After it leaves your hand, the block should slide about 1 m before it stops and it must not come any closer to the motion detector than 0.25 m. 18. Collect data for the sliding block. a. Start data collection. b. After a moment, give the block a brief push so that it slides toward the motion detector. 19. Inspect your graph. a. Examine the graph of velocity vs. time. b. The velocity graph should have a portion with a linearly decreasing section corresponding to the freely sliding motion of the block. Repeat data collection if needed. 20. Fit a straight line to this portion of the data, the slope of which is the block’s acceleration. a. Tap and drag across the region of linear decrease to select the data points. b. Choose Curve Fit ► Velocity from the Analyze menu. c. Select Linear as the Fit Equation to fit a straight line to the velocity data. d. Record the magnitude of the slope of the fitted line, which is the block’s acceleration, in your data table. e. Select OK. 21. Repeat Steps 18–20 two more times. 22. Place masses totaling 500 g on the block. Fasten the masses so they will not separate from the block. Repeat Steps 18–20 three times for the block with masses. Record acceleration values in your data table. DATA TABLE Part I Starting Friction Mass of block 0.243kg 12 -4 Physics with Vernier

Static and Kinetic Friction (Force Sensor) Part II Peak Static Friction and Kinetic Friction Total mass (kg) Normal force (N) Peak static friction Average peak static friction (N) Trial 1 Trial 2 Trial 3 0.243 2.38 0.45 0.50 0.76 0.57 1.243 12.15 3.34 2.86 2.84 3.01 0.743 7.28 1.74 1.47 1.32 1.51 Total mass (kg) Normal force (N) Kinetic friction Average kinetic friction (N) Trial 1 Trial 2 Trial 3 0.243 2.38 0.41 0.43 0.62 0.48 1.243 12.15 2.81 2.30 2.21 2.44 0.743 7.28 1.65 1.53 1.32 1.50 Physics with Vernier 12 -5

Static and Kinetic Friction (Force Sensor) Part III Kinetic Friction Cancel Data: Block with no additional mass Trial Acceleration (m/s 2 ) Kinetic friction force (N) μ k 1 2 3 Average coefficient of kinetic friction: Data: Block with 500 g additional mass Trial Acceleration (m/s 2 ) Kinetic friction force (N) μ k 1 2 3 Average coefficient of kinetic friction: Physics with Vernier 12 -7

Experiment 12 ANALYSIS 1. Inspect your force vs. time graph from Part I. Label the portion of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed. The portion of graph from time, t = 0 seconds to t \\small \\leq2 seconds, (upto the highest peak of the graph), corresponds to block at rest. The portion of the graph at time, t \\small \\leq2 seconds, (the sudden dip after the highest peak) depicts the block just beginning to move. The portion of graph from time, t = 2 seconds to t = 5 seconds, corresponds to constant speed motion of the block. 2. Still using the force vs. time graph you created in Part I, compare the force necessary to keep the block sliding compared to the force necessary to start the slide. How does your answer compare to your answer to Preliminary Question 3? Force necessary to start the slide is F1. Force necessary to keep block sliding F2 compared from newton third law F1=-F2 3. The coefficient of friction is a constant that relates the normal force between two objects (blocks and table) and the force of friction. Based on your graph from Part I, would you expect the coefficient of static friction to be greater than, less than, or the same as the coefficient of kinetic friction? 4. For Part II, calculate the normal force of the table on the block alone and with each combination of added masses. Since the block is on a horizontal surface, the normal force will be equal in magnitude and opposite in direction to the weight of the block and any masses it carries. Fill in the Normal Force entries for both Part II data tables. 5. Plot a graph of the maximum (peak) static friction force (vertical axis) vs. the normal force (horizontal axis). Use LabQuest, Logger Pro , or graph paper. 6. Since F peak static = μ s N , the slope of this graph is the coefficient of static friction μ s . Find the numeric value of the slope, including any units, by adding a Proportional Curve Fit. The Proportional Curve Fit passes through the origin. 7. In a similar graphical manner, find the coefficient of kinetic friction μ k . Plot the average kinetic friction forces vs. the normal force. Recall that F kinetic = μ k N and the Proportional Curve Fit passes through the origin. 8. Your data from Part III will also allow you to determine μ k . Draw a free-body diagram for the sliding block. The kinetic friction force can be determined from Newton’s second law, or Σ F = ma . From the mass and acceleration, find the friction force for each trial, and enter it in the data table. 9. From the friction force, determine the coefficient of kinetic friction for each trial and enter the values in the data table. Also, calculate an average value for the coefficient of kinetic friction for the block and for the block with added mass. 10. Does the coefficient of kinetic friction depend on speed? Explain, using your experimental data. 12 -8 Physics with Vernier