Does the force of kinetic friction depend on the weight of the block? Explain

. 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.

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 question 1 in the Preliminary Questions section?

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?

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 static friction force (y axis) vs. the normal force (x axis). Use either Graphical Analysis or graph paper.

Experiment 5

6. 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?

7. 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?

8. 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.

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.

11. Does the force of kinetic friction depend on the weight of the block? Explain.

12. Does the coefficient of kinetic friction depend on the weight of the block?

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?

EXTENSIONS 1. How is the force of friction or the coefficient of friction affected by the surface area of the

block? Devise an experiment that can test your hypothesis.

2. Examine the force of static friction for an object on an incline. Find the angle that causes a wooden block to start to slide. Calculate the coefficient of friction and compare it to the value you obtain when the angle of the incline is 0°.

3. Try changing the coefficient of friction by using wax or furniture polish on the table. How much does it change?

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