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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 if any of your experiments needs to be repeated to improve the quality.
- 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.
- 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?
- 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?
- 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 all three Part II data tables.
- Plot graphs of the maximum (peak) static friction force (vertical axis) vs. the normal force (horizontal axis). Use either Logger Pro or graph paper.
- Since Fmaximum static = s N, the slope of the proportional curve fit for this graph is the coefficient of static friction s. For Proportional Curve Fit, click: Analyze > Curve Fit and choose ‘Proportional’. The Proportional Curve Fit passes through the origin.
- In a similar graphical manner, find the coefficient of kinetic friction k. Create plots of the average kinetic friction forces vs. the normal force. Recall that Fkinetic = k N.
- 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,
F = ma. From the mass and acceleration, find the friction force for each trial, and enter it in the data table. - 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.
- Do μs and/or μk depend strongly on the materials of the contacting surfaces? Explain using your experimental data.
- Do μs and/or μk depend strongly on the area of the contacting surfaces? Explain using your experimental data.
- Does the coefficient of kinetic friction depend on speed? Explain, using your experimental data.
- Does the force of kinetic friction depend on the weight of the block? Explain.
- Does the coefficient of kinetic friction depend on the weight of the block?
- 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.
