Forces & Friction
Lab 5
One can measure the coefficient
of kinetic friction between your
iOLab device and a surface by
measuring the acceleration of the
object as it comes to rest after a
push.
Part 1:
1. Cali
ate your device.
2. In Experiment 4 you measured the actual mass
of your iOLab device by hanging the mass by the
metal loop. You may use that value for this first
part of the experiment. Otherwise see the
appendix for directions on measuring mass and
do that before the next step.
3. Attach the plate on the force probe.
4. Select the Accelerometer and Force sensor.
5. Place the device with the wheels up (not in
contact with the table).
6. Begin recording and give
the device a push from the
plate.
7. Use Analysis mode to
find the acceleration of the
device after your push by
highlighting the motion
after your push and finding
the average value. (Note:
You only need the Ay
direction). Include this
value and a screen grab of
this graph in your report.
8. When you stop pushing
on the device, it still has an
acceleration. Why?
9. From Newton’s 2nd Law in the x-direction (Equation 1) you can find the kinetic force on the
object. Notice this is after the push, so the only force on the object is kinetic friction. Write
down your value for fk based on the mass of the object and the average acceleration you
measured in 7 using Equation 1.
10. From Newton’s 2nd Law in the y-direction you can find the Normal Force (Equation 2).
Write down your value for the normal force using the mass of the object, g, and Equation 2.
m
(1)
(2)
(3)
+y
+x
11. Repeat steps 3-7,9 for 3 additional pushes (you only need the screen grab for one
of the pushes in your report). Calculate an average kinetic friction.
12. Using Equation (3), calculate an experimental coefficient of kinetic friction
m
(1)
(2)
(3)
+y
+x
Part 2:
1. Cali
ate your device.
2. Attach a mass (such as your phone or wallet,
do not use anything heavier than your phone) to
your iOLab device using scotch or masking tape
(wheel side). Measure the new mass of the
system (see appendix for directions.)
3. Repeat steps #3-7,10 from Part 1 of the
experiment with the new mass and record your
kinetic friction and normal force (you only need to
do with with one push *however if you have the
time find the average values for bonus credit).
4. Repeat steps #1-3 of (Part 2) for another new added mass
(again, nothing heavier than your phone).
5. Create a table of your normal force values and your friction
values and include this in your report. You can use the average
value from Part 1, and the measured values for Part 2 (unless you
also calculated averages, then use them instead).
6. Using a spreadsheet program (Microsoft Excel or Google Sheets
etc.) create a graph of your kinetic friction force (y-axis) versus the
normal force (x-axis). Include this in your report.
7. Use a linear fit to the graph to find the slope. Since
(3)
small
negligible
7. Use a linear fit to the graph to find the slope. Since
(3)
small
negligible
the slope of your graph will return the coefficient of kinetic
friction.
8. As this measurement includes several different masses this
coefficient of friction should be more robust than any one push or
any one mass.
9. Discuss the quality of the fit line to the data (does the line
appear to represent the data?)
10. Is your value of b small compared to the y-values (kinetic
friction) on your graph?
11. Compare this value of the coefficient of friction to that you found in part 1. Which
value do you trust more, and why.
12. As your lab partners will have used different surfaces, your coefficients of kinetic
friction will not agree. Compare your graphs from Part 2 #6-7. Did anyone’s data
etter match the best fit line than yours?
13. Discuss with your partners ways you may reduce uncertainty in your
measurements. Include the key ideas in your report.
Appendix – Finding Mass of iOLa
Demonstration Video
https:
www.youtube.com/watch?v=ELLk7WrYuHg
1. Attach the hook to your iOlab. We will now measure the actual mass of the device. We
will do this by picking the mass up and holding it steady, then measuring the applied force
on the object as it balances out gravity.
m
2. As always, first Cali
ate both the
accelerometer and the force sensor.
3. Choose the Accelerometer Sensor
and select Ay.
4. Select the Force Sensor.
5. Click Record, then pick up the device,
hold it for a second or two, then set it
ack down and stop recording.
6. Zoom in on the constant non-zero
applied force after you have picked up
the mass and before you set it down.
7. After zooming in, highlight a section of
the graph. Your graphs should now look
like...
...this.
8. Write down the average acceleration As your object is at rest, your device is
directly measuring the acceleration due to
gravity and not the actual acceleration.
9. Write down the average force.
10. Now calculate the actual mass of the device by...
Include this value in your report.
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