PHYS 2401 LAB: Static Friction
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Purpose:
In this lab, you will be determining the coefficient of static friction between pairs of surfaces.
Materials: cork cart, felt cart, Force Sensor, set of masses, angle indicator, dynamics cart track,
string, and PASCO data acquisition system
Static Friction on a Cart Pulled Horizontally
1) Connect the Force Sensor to channel A of the PASCO interface. Open Data Studio and create
new experiment. On the selection window, select Force Sensor. Click and drag the Force icon to
graph. Check if the Force Sensor is working by clicking on the Start button on the Data Studio
toolbar. Dots should be plotted along the y=0 axis. If this is not the case, press the tare button on
the side of the sensor. If nothing is plotted on the graph, close Data Studio and repeat the set up.
Note that the force sensor gives a negative reading if it is pulled and a positive reading
when pushed.
Prediction
Friction Force
? In this experiment, the Force Sensor measures the
friction force between the cart and track. Predict how
the graph of friction force vs time would look like if
you pull the cart slowly until it starts to move.
2) Now verify your prediction. Place the cart with
cork surface on the dynamics track. Use the bubble
Time
level to make sure the track is horizontal. Add a 500g mass on the cart. Connect the cart to the Force
Sensor using a string (about 15 cm long). Hold on
the Force Sensor parallel to the track and not touching it (see Figure below). Click Start on the
Data Studio toolbar. Using the Force Sensor, slowly pull the cart until it starts to move. Make
sure you pull on the Force Sensor horizontally.
?
How does the graph you got compare
with your prediction? So, how does the
friction force vary with time as you pulled
the cart (initially at rest) until it starts to
move?
String
Cart + Mass
Force Sensor (pulled
slowly and horizontally)
? What does the peak reading on the Force Sensor correspond to? (Ask your instructor if you are
not sure of your answer)
© Physics & Geology Department, University of Texas-Pan American
PHYS 2401 LAB: Static Friction
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3 ) Make a table similar to Table 1 in Excel. Name the worksheet Static Friction. You will record
all your data on this worksheet.
4) Measure the peak reading on the graph generated. Record the reading on the Static Friction
worksheet.
Table 1: Static Friction (Cart Pulled Horizontally)
Mass of Cork Cart = _______ Mass of Felt Cart =________
Maximum Static Friction
Normal Force (FN)
Added
Force (fs.max)
Mass
Cork Cart
Felt Cart
Cork Cart
Felt Cart
(N)
(N)
(N)
(N)
500 g
1000 g
1500 g
2000 g
2500 g
5) Determine the maximum force for the different masses indicated in Table 1.
6) Determine the maximum static friction for the felt cart for the different added masses shown
in Table 1.
The ratio of the maximum static friction (fs) and the normal
force (FN) is the coefficient of friction between the two surfaces that rub
against each other.
7) Complete Table 1 in the Static worksheet. Make sure you include the mass of the cart in
calculating the normal force.
8) Using Data Studio, plot the maximum static friction (fs) versus the normal force (FN) for
each of the carts. From the plots, determine the coefficient of friction (and its corresponding
standard deviation) for the pairs of surfaces.
µs (cork cart & track) =_________
σμ
s ,cork +track
µs (felt cart & track) =_________
= _____ σ μ s , felt+track = _____
© Physics & Geology Department, University of Texas-Pan American
PHYS 2401 LAB: Static Friction
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?
How would your computed µ be affected if you have pulled the cart at an angle instead of
along the horizontal? Explain your answer. (You will verify your prediction in the next part of
the Lab.)
Static Friction on a Cart Pulled at an Angle
9) On the Static worksheet create another table similar to Table 2.
Angle at which you pulled the cart (in degrees): ___10____
Added
Mass
Maximum Static Friction
Force (Fs.max)
(N)
Normal Force FN
(N)
500 g
1000 g
1500 g
2000 g
2500 g
10) Insert the angle indicator in the slot on the Force Sensor
(see Figure at right).
11) Place the 500-g mass on the cart. Attach the string to the
Force Sensor and the cart. Slowly pull the felt cart
(through the Force Sensor) at angle of 10° until it moves.
Make sure that the Force Sensor and string are aligned
(see Figure at right) and that you don’t change the angle
as you pull.
Force Sensor & String
Should be aligned
11) Repeat step 10 using the different masses indicated in Table 2.
12) Complete the Static worksheet.
© Physics & Geology Department, University of Texas-Pan American
PHYS 2401 LAB: Static Friction
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13) Plot fs,max and the normal force, FN. From your plot, determine the coefficient of static
friction, µs in this case.
14) On the Static worksheet, create a comparison table for the coefficient of static friction
(between the felt cart and track) when you pulled the cart horizontally and when you pulled it
an angle.
? How does the value of the coefficient of friction, µ (for the felt) compare when you have it
pulled horizontally and pulled at an angle? Justify your answer.
? What was affected when you pulled the cart at an angle?
Think about these!
Given just the following materials: felt cart, meter stick, and a board, how would you determine
the coefficient of static friction? Describe your experimental set-up and explain your
methodology.
© Physics & Geology Department, University of Texas-Pan American