Static and Kinetic Friction Experiment in Phy 201: General Physics I Lab, Lab Reports of Physics

Download Static and Kinetic Friction Experiment in Phy 201: General Physics I Lab and more Lab Reports Physics in PDF only on Docsity! Phy 201: General Physics I Lab Summer 2006 Instructor: Tony Zable Experiment #4: Static and Kinetic Friction 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 is acting against 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, Fstatic, with the inequality Fstatic £ s N where s is the coefficient of static friction and N 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 Fkinetic = k N, where 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. OBJECTIVES · Use a Force Sensor to measure the force of static friction. · Determine the relationship between force of static friction and the weight of an object. · Measure the coefficients of static and kinetic friction for a particular block and track. · Use a Motion Detector to independently measure the coefficient of kinetic friction and compare it to the previously measured value. · Determine if the coefficient of kinetic friction depends on weight. MATERIALS · Windows PC · Motion Detector · LabPro Interface · Force Sensor · Logger Pro software · block of wood with hook · string · Digital scale · Graphical Analysis software · Gram mass set PCC Cascade Campus 4 - 1 Experiment #4: Static & Kinetic Friction Summer 2006 PRELIMINARY QUESTIONS 1. 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 choice? 2. How do you think the force of friction is related to the weight of the box? Explain. PROCEDURE Part I: Starting Friction 3. Measure the mass of the block and record it in the data table. 4. Connect the Force Sensor (Dual Range Force Sensor) to the CH 1 input of the LabPro Interface. 5. Open “12a Static Kinetic Fric” from the Physics with Computers experiment files of Logger Pro. Set the range switch on the Force Sensor to 50 N. One graph will appear on the screen. The vertical axis will have force scaled from 0 to 20 Newtons. The horizontal has time scaled from 0 to 5 seconds. 6. 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. Practice pulling the block and masses with the Force Sensor using this 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. 7. 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. 8. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string. Click on U se Wor d 6. 0c or lat er to v iew M acin tosh pi ct ur e. at the top of the graph to set the Force Sensor to zero. 9. Click U se Word 6.0 c or lat er to vi ew M acin tosh pi ct ur e. to begin collecting data. Pull the block as before, taking care to increase the force gradually. Repeat the process as needed until you have a graph that reflects the desired motion, Phy 201: General Physics I Lab 4 - 2 Experiment #4: Static & Kinetic Friction Summer 2006 DATA TABLE Part I Starting Friction Mass of block kg PART II PEAK STATIC FRICTION AND KINETIC FRICTION Peak static friction Average Total mass (m) Normal force (N) Trial 1 Trial 2 Trial 3 peak static friction (N) 27628 Kinetic friction Average Total mass (m) Normal force (N) Trial 1 Trial 2 Trial 3 kinetic friction (N) Part III Kinetic Friction Data: Block with no additional mass Trial Acceleration (m/s2) Kinetic friction force (N) k 1 2 3 4 5 Average coefficient of kinetic friction: Phy 201: General Physics I Lab 4 - 5 Experiment #4: Static & Kinetic Friction Summer 2006 Data: Block with 500 g additional mass Trial Acceleration (m/s2) Kinetic friction force (N) k 1 2 3 4 5 Average coefficient of kinetic friction: ANALYSIS 12. Print out or sketch the force vs. time graph drawn in Part I (stored as Run 1). 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. 13. 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 question 1 in the Preliminary Questions section? 14. 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 (Run 1) 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? 15. 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. 16. Plot a graph of the maximum static friction force (y axis) vs. the normal force (x axis). Use either Graphical Analysis or graph paper. Phy 201: General Physics I Lab 4 - 6 Experiment #4: Static & Kinetic Friction Summer 2006 17. Since Fmaximum static = s N, the slope of this graph is the coefficient of static friction s. Find the numeric value of the slope, including any units. Should a line fitted to these data pass through the origin? 18. In a similar graphical manner, find the coefficient of kinetic friction k. Use a plot of the average kinetic friction forces vs. the normal force. Recall that Fkinetic = k N. Should a line fitted to these data pass through the origin? 19. Your data from Part III also allow you to determine k. Draw a free-body diagram for the sliding block. The kinetic friction force can be determined from Newton’s second law, or F = ma. From the mass and acceleration, find the friction force for each trial, and enter it in the data table. 20. 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. 27629 10. Does the coefficient of kinetic friction depend on speed? Explain, using your experimental data. 27630 11. Does the force of kinetic friction depend on the weight of the block? Explain. 27631 12. Does the coefficient of kinetic friction depend on the weight of the block? 27632 13. Compare your coefficients of kinetic friction determined in Part III to that determined in Part II. Discuss the values. Do you expect them to be the same or different? Phy 201: General Physics I Lab 4 - 7