Lab deck: Multiple
Johann Alcaraz
Mary Regacho
Mariah Barrera
Lab #20: Smooth operators
Purpose
Apparti
Theory
Methods
Data Table
Analysis
Conclusions
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To use the Datastudio program to record data and analyze data
To gain a better understanding of kinetic and sliding friction
To find the coefficient of friction for different materials
To find out what happens to the coefficient of friction when the normal force gets
changed
Force of friction is catagorized in two types: static friction and sliding (kinetic) friction. It
is caused by electromagnetic forces between atoms and molecules at an objects surface.
Force is put on an object to move along a surface so that static friction can build at its
max before the object moves.
Sliding (kinetic) friction denies the motion of objects as it moves at constant velocity
over a surface.
Setup
1.
2.
3.
4.
collect all lab materials
situate group at lab deck
assign jobs
read through lab and make sure everyone understands
5. Set up the PASCO Interface and computer and start DataStudio. Connect the
Force Sensor into the interface.
6. Open the DataStudio file: 20 Friction Forces.ds
o
The DataStudio file has three Graph displays (Ffriction vs Time, Static vs
Normal, Sliding vs Normal) and four Table displays (two of Sliding
Friction for felt and cork and two of Static Friction for felt and cork). Data
recording is set at 1000 Hz (100
0
measurements per second) and the force sensor is calibrated so a pull is a
positive force.
7. Set up the equipment as shown in the figure. ======>
8. Place the track on a flat, horizontal surface.
9. Place the felt friction accessory tray on the track with the felt down.
10. Tie a piece of string between the friction tray and the hook on the Force Sensor.
The string should be about 5 cm (2 in).
Procedure
The lab group followed this provided set of procedures as accurately as they could with a
few minor changes in that they repeated a few of the steps. All in all, these procedures
were their guides.
Record Data: Felt Friction Accessory
o
NOTE: The procedure is easier if one person handles the apparatus and a
second person handles the computer. It is very important to Zero the
Force Sensor prior to each data run.
1. Make sure you can see the graph labeled ‘Frictional Forces’. Hold the Force
Sensor so no slack is in the string. Zero the sensor and click ‘Start’. Slowly and
gently pull the Force Sensor/Friction Accessory down the track. (Note: It is very
important to pull so the tray moves with constant velocity.)
2. Click ‘Stop’ after taking 5 seconds of acceptable data.
3. For a second run, add a mass bar to the friction tray and perform the above
procedure again.
4. For a third run, add another mass bar to the friction tray and perform the above
procedure again.
Record Data: Cork Friction Accessory
5. Replace the felt friction tray with the cork friction tray. Perform the same
procedure above starting with no mass. For a second run, add a mass bar. For the
third run, add another mass bar.
o
When completed you should have six runs of data.
Analyze
Find the normal force, the maximum static friction force, and the sliding friction force for
both the felt and the cork. Combine these values onto other graphs and find the
coefficients of friction.
Part 1: Complete the Graph of Normal vs. Static
Felt Friction Tray
6. Use your data to find the normal force and the static friction force for each of the
felt data runs. Enter your results in DataStudio in ‘Table 1 – Felt’.
7. Enter the data values of “0,0” in the first row of Table 1. Weigh the empty felt
friction tray and enter its weight as the Normal Force value in the second row of
Table 1. (Note: Remember the Normal Force is the mass, in kilograms, multiplied
by “g”.)
8. Select the Frictional Forces graph. Select Run #1 from the Data menu.
Highlight the largest peak. The maximum value will appear in the Legend box.
Enter this maximum value as the Static Friction Force in the second row of Table
1.
9. Repeat the process to get the Normal Force and the Static Friction Force for Run
#2 (one mass bar added to tray) and Run #3 (a second mass bar added to tray).
Record your results in Table 1.
o
Table 1 should have four values in each column. Look at the graph of
Normal vs. Static. Your points should be plotted on the graph.
10. Click the Scale to Fit button if necessary. Select the Fit button. From the menu
choose Linear and insert the slope, m, in the Data Table in the Lab Section.
o
You will now perform the same procedure above for the data for the cork
friction tray.
Cork Friction Tray
11. Choose ‘Table 2 – Cork’. Insert the data values of “0,0” in the first row of Table
2. Weigh the felt friction accessory and insert the Normal Force value in the table.
(Note: Remember the Normal Force is the mass, in kilograms, multiplied by the
acceleration of gravity.)
12. Select the Frictional Forces graph. Select Run #4 from the Data menu.
Highlight the largest peak. The maximum value will appear in the Legend box.
Enter this value as the Static Friction Force in the second row of Table 2.
13. Repeat the process to get the Normal Force and the Static Friction Force for Run
#5 (one mass bar added to tray) and Run #6 (a second mass bar added to tray).
Record your results in Table 2.
o
Table 2 should have four values in each column. Look at the graph of
Normal vs. Static. Your points should be plotted on the graph.
14. Click the Scale to Fit button if necessary. Select the Fit button. From the menu
choose Linear and insert the slope, m, in the table in the Lab Report.
Part 2: Complete the Graph of Normal vs. Sliding
Felt Friction Tray
15. Choose ‘Table 3 – Felt’. Insert the data values of “0,0” in the first row. Enter the
Normal Force values in the first column. These will be the same values from the
first column of ‘Table 1 – Felt’.
16. Select Run #1 as the only run for your graph. Highlight the flat part after the
largest peak and before the drop off. The mean value will appear in the Legend
box. Enter this mean value as the Sliding Friction Force in the second row of
Table 3.
17. Repeat the process to get the Normal Force and the Sliding Friction Force for
Run #2 (250-gram mass bar added to tray) and Run #3 (a second 250-gram mass
bar added to tray). Record your results in Table 3.
o
Table 3 should have four values in each column. Look at the graph of
Normal vs. Sliding. Your points should be plotted on the graph.
18. Click the Scale to Fit button if necessary. Select the Fit button. From the menu
choose Linear and insert the slope, m, in the table below.
Cork Friction Tray
19. Choose ‘Table 4 – Cork’. Repeat the process to get the Normal Force and the
Sliding Friction Force for Run #4, no mass added, Run #5 (one mass bar added
to tray) and Run #6 (a second mass bar added to tray). Record your results in
Table 4.
20. Table 4 should have four values in each column. Look at the graph of Normal vs.
Sliding. Your points should be plotted on the graph. Click the Scale to Fit button
if necessary. Select the Fit button. From the menu choose Linear and insert the
slope, m, in the table below.
Material
Coefficient of Static Friction, s
Coefficient of Sliding Friction, k
Felt
Cork
TO SEE ASSOCIATED TABLE TO GRAPH, PLEASE REFER TO
--> 10.21.08. Lab 5 (20).Exc03
NORMAL FORCE = mass x acc. due to gravity = |WEIGHT|
- Empty Felt = (.083 kg)( 9.81 m/s/s) = .814 N.
- 1 Mass Felt = (.327 kg)( 9.81 m/s/s) = 3.21 N.
- 2 Mass Felt = (.572 kg)( 9.81 m/s/s) = 5.61 N.
- Empty Cork = (.091 kg)( 9.81 m/s/s) = .893 N.
- 1 Mass Cork = (.335 kg)( 9.81 m/s/s) = 3.29 N.
- 2 Mass Cork = (.58 kg)( 9.81 m/s/s) = 5.69 N.
PREDICTIONS:
1. When an object is in contact with a surface, how does the object’s material affect the
frictional forces?
--An object’s material affects the frictional forces because the smoother it is the smaller
the coefficient for static friction will be.
2. How does the normal force acting on an object affect the force of friction?
--The normal force on an object affects the force of friction by slowing it down. The
greater the normal force the slower the object moves. Thus making the coefficient for
static friction greater.
QUESTIONS:
1. What is the physical quantity represented by the slopes of friction versus normal force?
-- Static Friction force.
2. What pattern do you notice about the coefficients of friction?
-- Through Runs 1 - 3 and 4-6, it increases.
3. What happens to the static friction force as the mass is increased?
-- It increases.
4. What happens to the sliding friction force as the mass is increased?
-- The coeficiant for sliding friction is increased when mass is increased.
5. What are the units for the slope of each graph? Explain.
--The units for the graphs are newtons/ seconds
6. Think about your experience with the friction trays. When a car skids to a stop (with
wheels "locked"), which friction force is at work? How do you know?
-- Sliding friction force is at work. I know because as the car moves, it comes to an
evetual stop due to the opposing friction force of sliding friction.
7. Do your results support your predictions?
--
Dear Mary and Mariah,
We have to finish analyzation of these graphs tomorrow.
--Johann