Lab 7

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Florida International University *

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LAE4405

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Mechanical Engineering

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Dec 6, 2023

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docx

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Uploaded by agriz002

Lab #07 Lab partner-1:_________________________ID:_________________ Lab partner-2:_________________________ID:_________________ Title: Static and Kinetic Friction Preliminary Question Answers: 1) Friction is the main factor that effects how easy objects can slide. Things like the carpet of your bedroom floor or the roughness of sandpaper like surfaces inhibit object’s ability to slide across other surfaces. 2) One way to reduce the force needed to move the box across the table would be to push the box at a slight angle or applying a lubricate to the table. 3) In pushing a heavy box across the floor, the force that you apply would have to be greater than the force needed to keep the box moving since the weight of the box would have a direct effect on the friction force. Using the friction force formula, we know that f r =  N. Analysis: 1. A reliable indication that the experiments are of good quality is that results are consistent meaning that different Trials of the same experiment give consistent results. Inspect your Data Tables to see it any of your experiments needs to be repeated to improve the quality. Trials were repeated when they yielded inconsistent results. 2. 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. See Graph below. 3. 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? As the block was at rest there was no force acting on it therefore the friction force was equal to the normal force. An external force was then applied to the block that was greater than the friction force causing the block to move and eventually reach a constant speed.

4. 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 he coefficient of static friction to be greater than, less than, or the same as the coefficient of kinetic friction? The same. 5. For Part IL, 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 all three- Part Il data tables. See data tables. 6. Plot graphs of the maximum (peak) static friction force (vertical axis) vs. the normal force (horizontal axis). Use either Logger Pro or graph paper. See graphs 7. Since F maximum static = u,N, the slope of the proportional curve fit for this graph is the coefficient of static friction u.. For Proportional Curve Fit, click: Analyze>Curve Fit and choose Proportional'. The Proportional Curve Fit passes through the origin. See graphs. 8. In a similar graphical manner, find the coefficient of kinetic friction H. Create plots of the average kinetic friction forces vs. the normal force. Recall that Friner=4,1. See graphs. 9. Your data from Part Ill also allow you to determine u. Draw a free-body diagram for the sliding block. The kinetic friction force can be determined from Newton's second law. IF= ma. From the mass and acceleration, find the friction force for\ each trial, and enter it in the data table. See data table. 10. 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. See data table Part III. 11. Do  s , and/or  k depend strongly on the materials of the contacting surfaces? Explain using your experimental data. The materials of the contacting surface greatly affected the coefficients of kinetic and static friction. As the block was pulled over a rougher surface, like in 2C, the average kinetic and static friction increased.

12. Do  s , and/or  k depend strongly on area of the contacting surface? Explain using your experimental data. The area of the contacting surface had no effect on the coefficients of kinetic and static friction because all of the data was consistent with each other. 13. Does the coefficient of kinetic friction depend on speed? Explain, using your experimental data. 14. Does the force of kinetic friction depend on the weight of the block? Explain. 15. Does the coefficient of kinetic friction depend on the weight of the block? No, the coefficient of kinetic friction is related to the surface on which the block has encountered. 16. 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? Which one do you think is more precise (with smaller uncertainty)? Justify your answer using your experimental data. The average coefficient of kinetic friction for Part III was calculated to be 0.348 and in Part II was the slope of each trial are: a) 0.3042 b) 0.232 c) 0.139. The expected values of the coefficient should be similar because each of the three trials used the same total mass and normal force. Data Tables: Submitted separated. Graphs:

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