2 Lab 2: Friction and Slope

Acknowledgment

This project is made possible with funding by the Government of Ontario and through eCampusOntario’s support of the Virtual Learning Strategy. To learn more about the Virtual Learning Strategy visit: https://vls.ecampusontario.ca.

PHY136 Stay-At-home Experiment 2

Friction and Slope

What you will do:

Let your phone slide down a slope. Using the accelerometer that is built in the phone to measure the coefficients of static friction and kinetic friction between the phone and the slope.

What you will need:

• A smart phone installed with the Phyphox app
• A flat solid board acting as a slope (such as a cardboard box, a hardcover book, a cut board, etc.)
• A bag that can hold your phone
• A ruler or a measuring tape

Theoretical Preparation

The “acceleration with g” sensor of the cell phone includes the gravitational acceleration of the Earth in the accelerometer readings. When the phone is tilted relative to the vertical direction, the acceleration readings on the 3 axes of the phone are the vector components of the gravitational acceleration vector. (The gravitational acceleration vector on the surface of the Earth is always pointing vertically down, with an approximate value of 9.81 m/s2.)

Demonstration Video: https://youtu.be/9PdVDBwTneA

Procedures

Part 1: Static Friction

1. In the Phyphox app, open the “acceleration with g” sensor.
2. Place the board on the floor. Rest the phone on top of the board with one edge of the phone parallel to the edge of the board. Start an acceleration with g measurement.
3. Put soft padding on one side of the board (to catch the phone and stop the slope from skidding). Slowly lift the other side of the board until the phone starts to slide down. Stop the measurement.
4. On the accelerometer readings, you should see the acceleration component that is parallel to the direction of the slope increase gradually before a small drop. (In the example screenshot below, the y axis was gradually increasing before a small drop at about 4 seconds mark.) Note down the time point and the acceleration immediately before the drop. This time point is the time point when the parallel component of gravity exceeds the maximum static friction.
5. On the other graphs showing the other components of the gravitational acceleration vector, Note down the values of acceleration at the time when the parallel component of gravity exceeds the maximum static friction.
6. Repeat the steps 2-5 above for a total of three times.
7. Show that the vector sum of the components of the measured acceleration is indeed similar to the accepted value of gravitational acceleration on the surface of the Earth.
8. Use the values of the components of acceleration that you read to calculate the tilt angle of the board at which the phone starts to slide down.
9. Use the values of the components of acceleration that you read to calculate the coefficient of static friction between the back of your phone and the board. (Question: Do you need to know the mass of the phone? Why or why not?)
10. Put the phone into a bag (if you bought the experiment kit, you should use the velvet drawstring bag, alternatively you can wrap your phone in a t-shirt or towel), and measure the coefficient of static friction between the bag and board using the same method described above. If you lose access to the phone screen, you can use remote-control to start and stop the measurement. Make note in your lab report of what material you wrapped your phone in.
11. Is the coefficient of static friction that you get in Step 10 larger or smaller than the one in Step 9? Considering the materials of the back of your phone, the bag, and the board, do the results make sense?

Part 2: Kinetic Friction

1. Use another object to prop up your board at a fixed tilt angle so that the phone slides down the slope spontaneously.
2. Use a ruler or a tape measure to measure the height and the length of the slope, and then calculate the tilt angle using trigonometry.
3. In the Phyphox app, open the “acceleration without g” sensor, navigate to the “absolute” acceleration tab.
4. Hold the phone at the top of the slope, start an acceleration without g measurement, then release the phone and allow the phone to accelerate down the slope. Stop the measurement.
5. Identify the start time and the end time of the sliding motion on the absolute acceleration record.
6. Take the average value of the absolute acceleration during the sliding motion. This is the acceleration of the phone along the parallel direction of the slope.
7. Draw a free body diagram of the phone, and calculate the coefficient of kinetic friction between the back of the phone and the board. Include the free body diagram in your report.
8. Put the phone into the same bag that you used for Part 1. Repeat the Steps 3-6 above to find the coefficient of kinetic friction between the bag and the board. If you lose access to the phone screen, you can use remote-control to start and stop the measurement.
9. Are the coefficients of kinetic friction larger or smaller than the corresponding coefficients of static friction? Do these results make sense? Why might one be smaller than the other?

Tips for data analysis:

• Complete all calculations that are required in the procedures above. Show sample calculations in the appendix.
• Answer all questions in the procedures section above.
• Attach graphs or tables when necessary. (Do NOT attach the raw data!)
• If you write a program for calculations, attach the code in the appendix.

Suggested topics for discussion:

• Find the uncertainty values for each calculation in the results section. Show a sample calculation of your error propagation in the appendix.
• Identify the sources of error in your measurements.
• Feel free to do more tests to verify your claims if possible.
• Make a reasonable conclusion based on your results.
• Compare your results with literature values (and add citations). You may reference the textbook.
• What other quantities can be determined with the same experimental design?
• What can be done to reduce the uncertainties at no cost while staying at home?

We value your feedback!

Did you enjoy this experiment? What are the aspects that you dislike the most about this experiment? – Let me know in the “Student Feedback Survey for Lab 2” on the discussion board on Quercus! We may award the most helpful inputs with +1 bonus mark for the lab report!

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