Exp 12: Static and Kinetic Friction
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 is acting against 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 magnitude of your push. There is a limit
to the magnitude 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 inequality Fstatic  s N
where s is the coefficient of static friction and N the normal force exerted by a surface on the
object. The normal force is defined as the perpendicular component of the force exerted by the
surface. In this case, the normal force is equal to the weight of 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 slide the box with a constant velocity, a force equivalent to the force of
kinetic friction must be 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 Fkinetic = k N, where k is the
coefficient of kinetic friction.
In this experiment, you will use a 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.
OBJECTIVES

Use a Force Sensor to measure the force of static 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
Power Macintosh or Windows PC
LabPro or Universal Laboratory Interface
Logger Pro
Vernier Motion Detector
Vernier Force Sensor
Graphical Analysis or graph paper
string
block of wood with hook
balance or scale
mass set
PRELIMINARY ASSIGNMENT (FIRST, READ THIS LAB)
1. In pushing a heavy box across the floor, is the force you need to apply to start the box moving
greater than, less than, or the same as the force needed to keep the box moving? On what are
you basing your choice? Ok, now actually push a heavy box across a floor. Sketch a plot of
force vs. time for your push.
2. How do you think the force of friction is related to the weight of the box? Explain. Add
weight to the box you pushed in the previous question.
3. Create appropriate data tables in your lab manual.
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Experiment 12
4. Read & complete CYU questions in TPC / Newton’s Laws.
PROCEDURE
Part I Starting Friction
1. Measure the mass of the block and record it in the data table.
2. Connect the Force Sensor to Channel 1 (Student Force Sensor and Dual-Range Force Sensor)
of the LabPro or Universal Lab Interface or to the PORT 1 input (ULI Force Probe) of the
Universal Lab Interface.
3. Open the Experiment 12 folder from Physics with Computers. Then open the experiment file
that matches the force sensor you are using. Set the range switch on the Force Sensor to 50 N.
One graph will appear on the screen. The vertical axis will have force scaled from 0 to
20 Newtons. The horizontal has time scaled from 0 to 5 seconds.
4. Tie one end of a string to the hook on the Force Sensor and the other end to the hook on the
wooden block. Place a total of 1 kg mass on top of the block, fastened so the masses cannot
shift. Practice pulling the block and masses with the Force Sensor using this straight-line
motion: Slowly and gently pull horizontally with a small force. Very gradually, taking one
full second, increase the force until the block starts to slide, and then keep the block moving
at a constant speed for another second.
5. Sketch a graph of force vs. time for the force you felt on your hand. Label the portion of the
graph corresponding to the block at rest, the time when the block just started to move, and the
time when the block was moving at constant speed.
6. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string.
Click
at the top of the graph to set the Force Sensor to zero.
7. Click
to begin collecting data. Pull the block as before, taking care to increase the
force gradually. Repeat the process as needed until you have a graph that reflects the desired
motion, including pulling the block at constant speed once it begins moving. Print or copy the
graph for later reference. Choose Store Latest Run from the Data Menu to store the run as
Run 1 for later analysis. Save the data file to your student folder.
Part II Peak Static Friction and Kinetic Friction
In this section, you will measure the peak static friction force and the kinetic friction force as
a function of the normal force on the block. In each run, you will pull the block as before, but
by changing the masses on the block, you will vary the normal force on the block.
Mass
Wooden block
Dual-Range
Force Sensor
Pull
Figure 1
8. Remove all masses from the block.
9. Click
to begin collecting data and pull as before to gather force vs. time data. Store the
run and resave the data file. Do not hide the stored runs.
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Static and Kinetic Friction
10. Examine the data by clicking the Statistics button, . The maximum value of the force occurs
when the block started to slide. Read this value of the maximum force of static friction from
the floating box and record the number in your data table.
11. Drag across the region of the graph corresponding to the block moving at constant velocity.
Click on the Statistics button again and read the average (or mean) force during the time
interval. This force is the magnitude of the kinetic frictional force.
12. Repeat Steps 9-11 for two more measurements and average the results to determine the
reliability of your measurements. Record the values in the data table.
13. Add masses totaling 250 g to the block. Repeat Steps 9 – 12, recording values in the data
table. Don’t forget to store the runs and resave the data file.
14. Repeat Steps 9 – 12 for additional masses of 500, 750, and 1000 g. Record values in your data
table. Don’t forget to store the runs and resave the data file.
Part III Kinetic Friction Again
In this section, you will measure the coefficient of kinetic friction a second way and compare
it to the measurement in Part II. Using the Motion Detector, you can measure the acceleration
of the block as it slides to a stop. This acceleration can be determined from the velocity vs.
time graph. While sliding, the only force acting on the block in the horizontal direction is that
of friction. From the mass of the block and its acceleration, you can find the frictional force
and finally, the coefficient of kinetic friction.
Wooden block
Push
Figure 2
15. Connect the Motion Detector to DIG/SONIC 2 of the LabPro or PORT 2 of the Universal Lab
Interface. Open the experiment file Exp 12 Motion Detector. Two graphs will appear on the
screen. The vertical axis of the top graph will have distance scaled from 0 to 3 m, and the
lower graph has velocity scaled from – 2 to 2 m/s. The horizontal has time scaled from 0
to 5 s.
16. Place the Motion Detector on the lab table 2 – 3 m from a block of wood, as shown in Figure
2. Position the Motion Detector so that it will detect the motion of the block as it slides
toward the detector.
17. Practice sliding the block toward the Motion Detector so that the block leaves your hand and
slides to a stop. Minimize the rotation of the block. [Why?] After it leaves your hand, the
block should slide about 1 m before it stops and it must not come any closer to the Motion
Detector than 0.4 m.
18. Click
to start collecting data and give the block a push so that it slides toward the
Motion Detector. The velocity graph should have a portion with a linearly decreasing section
corresponding to the freely sliding motion of the block. Repeat if needed.
19. Select a region of the velocity vs. time graph that shows the decreasing speed of the block.
Choose the linear section. The slope of this section of the velocity graph is the acceleration.
Drag the mouse over this section and determine the slope by clicking the Linear Regression
button, . Record this value of acceleration in your data table.
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Experiment 12
20. Repeat Steps 18 – 19 four more times.
21. Place masses totaling 500 g on the block. Fasten the masses so they will not move. Repeat
Steps 18 – 19 five times for the block with masses. Record acceleration values in your data
table.
DATA TABLE
Part I Starting Friction
Mass of block
kg
Part II Peak Static Friction and Kinetic Friction
Total
mass
(m)
Normal
force
(N)
Trial 1
Peak static friction
Trial 2
Trial 3
Average
peak static
friction
(N)
You create the table in your lab manual.
Total
mass
(m)
Normal
force
(N)
Trial 1
Kinetic friction
Trial 2
Trial 3
Average
kinetic
friction
(N)
You create the table in your lab manual.
Part III Kinetic Friction
Data: Block with no additional mass
Trial
Acceleration
2
(m/s )
Kinetic friction force
(N)
k
You create the table in your lab manual.
Data: Block with 500 g additional mass
Trial
Acceleration
2
(m/s )
Kinetic friction force
(N)
k
You create the table in your lab manual.
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Static and Kinetic Friction
ANALYSIS
1. Sketch the force vs. time graph drawn in Part I (stored as Run 1). Label the portion of the
graph corresponding to the block at rest, the time when the block just started to move, and the
time when the block was moving at constant speed.
2. Still using the force vs. time graph you created in Part I, compare the force necessary to keep
the block sliding compared to the force necessary to start the slide. How does your answer
compare to your answer to question 1 in the Preliminary Questions section?
3. The coefficient of friction is a constant that relates the normal force between two objects
(blocks and table) and the force of friction. Based on your graph (Run 1) from Part I, would
you expect the coefficient of static friction to be greater than, less than, or the same as the
coefficient of kinetic friction?
4. For Part II, calculate the normal force of the table on the block alone and with each
combination of added masses. Since the block is on a horizontal surface, the normal force
will be equal in magnitude and opposite in direction to the weight of the block and any
masses it carries. Fill in the Normal Force entries for both Part II data tables.
5. Plot a graph of the maximum static friction force (y axis) vs. the normal force (x axis). Use
graph paper. See Graphing handout.
6. Since Fmaximum static = s N, the slope of this graph is the coefficient of static friction s. Find
the numeric value of the slope, including any units. Should a line fitted to these data pass
through the origin?
7. In a similar graphical manner, find the coefficient of kinetic friction k. Use a plot of the
average kinetic friction forces vs. the normal force. Recall that Fkinetic = k N. Should a line
fitted to these data pass through the origin?
8. Your data from Part III also allow you to determine k. Draw a free-body diagram for the
sliding block. The kinetic friction force can be determined from Newton’s second law, or
F = ma. From the mass and acceleration, find the friction force for each trial, and enter it in
the data table.
9. From the friction force, determine the coefficient of kinetic friction for each trial and enter the
values in the data table. Also, calculate an average value for the coefficient of kinetic friction
for the block and for the block with added mass.
10. Does the coefficient of kinetic friction depend on speed? Explain, using your experimental
data.
11. Does the force of kinetic friction depend on the weight of the block? Explain.
12. Does the coefficient of kinetic friction depend on the weight of the block?
13. Compare your coefficients of kinetic friction determined in Part III to that determined in
Part II. Discuss the values. Do you expect them to be the same or different?
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Experiment 12
EXTENSIONS
1. How is the force of friction or the coefficient of friction affected by the surface area of the
block? Devise an experiment that can test your hypothesis.
2. Examine the force of static friction for an object on an incline. Find the angle that causes a
wooden block to start to slide. Calculate the coefficient of friction and compare it to the value
you obtain when the angle of the incline is 0°.
3. Try changing the coefficient of friction by using wax or furniture polish on the table. How
much does it change?
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Static and Kinetic Friction
Kinematics Review and Force Problems:
Work the following problems. The number in parenthesis preceding each problem is a reference
for the teacher. The number(s) following some problems are answers. These answers do not
necessarily have the correct number of significant digits. Drawing a free-body [force] diagram
may help.
1. (5.33) You drive a car for 2.0 h at 40 km/h, then for another 2.0 h at 60 km/h.
a. What is your average velocity? (50)
b. Do you get the same answer if you drive 100 km at each of the two speeds?
Show your work. (No.)
2. (5.49) A plane uniformly accelerates from 66 m/s to 88 m/s in 12.0 s. Determine:
a. The displacement of the plane while accelerating. (920)
b. The acceleration of the plane. (2)
c. The force exerted on a 72.5 kg passenger. (130)
3. (5.58) Rocket-powered sleds are used to test the responses of humans to acceleration.
Starting from rest, one sled can reach a speed of 444 m/s in 1.80 s and can be brought to a
stop again in 2.15 s. Assume the sled has a passenger with mass of 65 kg.
a. Calculate the acceleration while the sled is speeding up. (250)
b. Compare this starting acceleration to the acceleration due to gravity by
determining the ratio [as a decimal, not a fraction] R = a/g. (25)
c. Calculate the force exerted on the passenger by the sled. (2x104)
d. Calculate the force exerted on the sled by the passenger.
e. Repeat the above calculations for the period when the sled is slowing down. (-200,
20)
f. Determine the displacements of the sled during both intervals. (400, 500)
4. (6.23) A 65 kg swimmer jumps off of a 10.0 m tower. The swimmer comes to a stop 2.0
m below the surface. Determine the:
a. Swimmer’s velocity on hitting the water. (-14)
b. Acceleration of the swimmer while slowing in the water. (+50)
c. Net force on the swimmer while slowing. (+3185)
d. Force exerted on the swimmer by the water while slowing. (+4000)
e. Time intervals during both accelerations. (1.4, 0.3)
5. (6.30) A horizontal force of 30.0 N is required to slide a 12.0 kg wooden crate across the
floor at a constant velocity.
Hint: Review http://www.physicsclassroom.com/Class/newtlaws/U2L2b.html
Determine the magnitude and direction of the:
a. Gravitational force on the crate. (-120)
b. Normal force exerted by the floor on the crate. (+120)
c. Applied force on the crate. (30)
d. Net force on the crate. (0)
Hint: http://www.physicsclassroom.com/Class/newtlaws/U2L2d.html
e. Frictional force exerted by the floor on the crate.
Calculate the:
f. Acceleration of the box while moving? (0)
g. Coefficient of kinetic friction between the crate and floor. (0.255)
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Experiment 12
6. (6.37) A sled of mass 50.0 kg is pulled along flat, snow-covered ground. The static
friction coefficient is 0.30, and the kinetic friction coefficient is 0.10. Calculate or
determine the following:
a. The weight of the sled. (500)
b. The minimum force required to start the sled moving. (150)
c. The minimum force required to keep the sled moving at a constant velocity. (50)
d. Once moving, what total force must be applied to the sled to accelerate it at a rate
of 3.0 m/s/s? (200)
e. At what rate will the sled accelerate if the force required to start the sled moving
[static] is applied while the sled is moving [kinetic]? (2)
7. (6.39) A 65 kg boy and a 45 kg girl use an elastic rope while engaged in a tug-of-war on
an icy, frictionless surface. The girl accelerates toward the boy at 3.0 m/s/s [relative to the
ground]. Determine the:
a. Force exerted by the girl on the rope. (135)
b. Force exerted by the rope on the girl.
c. Force exerted by the boy on the rope.
d. Force exerted by the rope on the boy. (135)
e. Magnitude of the acceleration [relative to the ground] of the boy toward the girl.
(2.0)
f. Magnitude of the acceleration [relative to the girl] of the boy toward the girl. (5.0)
g. Weight of the girl. (440)
h. Weight of the boy.
i. Normal force on the girl.
j. Normal force on the boy. (600)
The smooth, icy surface is replaced with a rough, icy surface. The coefficient of static
friction is 0.24. Assume that the force(s) applied by the girl and boy remain constant.
k. Calculate the friction force exerted on the girl’s feet by the icy surface.
l. Will the girl move? Explain. Work is required.
m. Will the boy move? Explain. Work is required.
The coefficient of kinetic friction is one-half the value of the coefficient of static friction.
Assume that the force(s) applied by the girl and boy remain constant.
n. Calculate the coefficient of kinetic friction. (0.12)
o. Calculate the kinetic frictional force on both the boy and girl. (76, 53)
p. Calculate the acceleration of the girl, relative to the ground. (1.8)
q. Calculate the acceleration of the boy, relative to the ground. (0)
r. A passing skater nudges the boy with enough force to break him free from the ice.
Calculate his acceleration. (< 1)
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