Friction Lab
Background Information:
If you try to slide a heavy box resting on the floor, you may find it difficult to get the box moving. Static friction
is the force that counters your force on the box. If you apply a light horizontal push that does not move the
box, the static friction force is also small and directly opposite to your push. If you push harder, the friction
force increases to match the strength of your push. There is a limit to the size of static friction, so eventually
you may be able to apply a force larger than the maximum static force, and the box will move.
The maximum static friction force is sometimes referred to as starting friction. We model static friction, Fstatic,
with the equation fs = µs n where µs is the coefficient of static friction and n is the normal force exerted by a
surface on the object. If an object is resting on a horizontal surface, the normal force is equal to the weight of
the object as long as no one is pushing down on the object.
Once the box starts to slide, you must continue to exert a force to keep the object moving, or friction will slow
it to a stop. The friction acting on the box while it is moving is called kinetic friction. In order to keep the box
moving at a constant speed, a force the same size as kinetic friction applied. Kinetic friction is sometimes
referred to as sliding friction. Both static and kinetic friction depend on the surfaces of the box and the floor,
and on how hard the box and floor are pressed together. We model kinetic friction with fk= µk n, where µk is
the coefficient of kinetic friction.
In this experiment, you will use a Dual-Range Force Sensor to study static friction and kinetic friction on a
wooden block. A Motion Detector will also be used to analyze the kinetic friction acting on a sliding block.
Objective:
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Use a Dual-Range Force Sensor to measure the force of static and kinetic friction.
Determine the relationship between force of static friction and the weight of an object.
Measure the coefficients of static and kinetic friction for a particular block and track.
Use a Motion Detector to independently measure the coefficient of kinetic friction and compare it to
the previously measured value.
Determine if the coefficient of kinetic friction depends on weight.
Materials
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LabQuest 2
Wireless Dynamics System (WDSS)
Wooden Block
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Different amount of Mass
Triple Beam Balance
String
Assign each member of the group to the following tasks.
a. Lab Captain: In charge of Reading the Procedure, Asking the Teacher Questions and Making
sure all directions are followed correctly.
b. Data Recorder: Writes down all the data collected during the lab.
c. LabQuest Operator: Will run the lab quest and set up the sensors.
d. Force Applier: This person will pull the sensor and block system.
Name: _______________
BLOCK: ___
Pre Lab Questions: To be answered individually
A student is trying to slide a heavy box across the floor. Answer the pre-lab questions based on that scenario.
1. When you push lightly on a heavy box on the floor, why won’t the box start to move?
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2. How hard must you push on a heavy box to get it to move?
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3. How hard must you push on the heavy box to keep it moving once it has started to slide?
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4. Draw a free body diagram of the forces acting on the box.
5. What is the equation a student would need to use to calculate the frictional forces acting on the box?
Procedure: To Be Read by Lab Captain
PART ONE: Investigating static and kinetic friction.
1. Collect materials from Instructor and clear off a space on the floor or your lab table.
2. Turn on the LabQuest 2 and turn on the Wireless Dynamics Systems (WDSS).
3. Connect the WDSS to the LabQuest by completing the following steps.
a. Select the Sensors tab at the top of the screen and choose “WDSS Setup.”
b. When the screen prompts you, select “Scan.”
c. Choose the name of the WDSS that is printed on side of the WDSS then select to enable the
sensor “Force.”
4. Place the sensor on a flat surface to calibrate the WDSS.
a. Select the Sensor tab at the top of the screen and choose to “Zero WDSS: WDSS Force.” This
will reset the sensor. You should only need to do this once throughout the lab.
5. Using the Triple Beam Balance, measure the mass of the block and record it in the data table.
6. Place the wooden block on a flat surface.
7. Attach the WDSS hook to the hook on the block using the string provided by the instructor.
8. On the LabQuest, change the screen to the graph (by selecting the small graph at the top of the screen.
9. When the LabQuest operator hits play, the force applier SLOWLY pulls the block until it begins to move
and will continue to pull it until the collection stops.
10. Sketch the graph in the Data collection portion of the graph. LABEL when the block was at rest, and
when it was moving on that sketch.
11. Collect the Data for each trial by completing the following steps.
a. Using the stylus, drag across the graph where the maximum force is present (the peak).
b. Select the Analyze tab at the top of the screen and choose “StatisticForce.”
c. Record the maximum force needed to make the block begin to slide.
d. Turn of the statistics by repeating step b.
e. Now select the flat region of the graph when the block started to move.
f. Repeat step b.
g. Record the Mean force. This is the average force needed to keep the block moving at a
constant speed.
12. Repeat Steps 10-12 and add 200 grams each time you repeat the experiment.
PART 2: Investigating how your shoe uses friction.
1. In this portion of the lab, each student will determine the coefficient of kinetic friction between their
shoes and the floor.
2. Each person must get the mass of their shoe using the triple beam balance. Record those values in the
Data table.
3. Connect the WDSS to your shoe (you may need to use a string and some tape).
4. Repeat the experiment from Part One.
5. Record the force needed to keep your shoe moving at a constant speed (See Part One for how to find
this number.)
6. Repeat this experiment for each person’s shoe.
Data Table: To be filled in by Data Recorder
Part One: Sketch of Graph.
Force (N)
Time (s)
PART ONE TABLE
Mass of Block and Weights (kg)
Maximum Static Friction Force (N)
Average Kinetic Friction Force (N)
Mass of Shoe
Average Kinetic Friction Force (N)
PART TWO TABLE
Student’s Name
Data Analysis: To be complete together with your lab group.
Part One
1. Calculate the weight of the block of wood and masses for each trial of the experiment. Record these
values in the table below.
2. Record the Normal force the block experiences in each trial of the experiment. Record these values in
the table below.
3. Using the normal force and the frictional forces recorded in the experiment, determine the coefficient
of static and kinetic friction. Record them in the table below.
Weight (N)
Normal Force (N)
s
k
Part Two:
1. Calculate the weight of each person shoe in your lab group.
2. Record the Normal force each shoe experiences when resting on the floor.
3. Using the normal force and the frictional forces recorded in the experiment, determine the coefficient
of kinetic friction for each shoe. Record them in the table below.
Student’s Name
Weight (N)
Normal Force (N)
k
Data Analysis: To be complete together with your lab group.
Part One:
1. Which coefficient of friction is greater, the static or the kinetic? ______________________________________
2. Did changing the weight of the block affect the coefficient of friction? _________________________________
a. Why do you think that happened? ________________________________________________________
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3. Use what you have learned in part one of the lab to answer the following question.
a. Winter time is coming and vehicles will begin to slip and slide on the snowy roads. If you drove a pick-up
truck, what could you do to improve the friction between the truck tires and the road?
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Part Two:
1. Which student had the greatest coefficient of friction for their shoe? ___________________________________
2. Which student had the smallest coefficient of friction for their shoe? ___________________________________
3. Compare the bottoms of the two shoes with the most and least friction. How are they different?
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4. Use what you learned from part two of the lab to answer the following question.
a. Explain why it is illegal to drive with tires that have all the tread worn off and why would it be
dangerous? __________________________________________________________________________
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