Download Static and Kinetic Friction Lab Handout

Static and Kinetic Friction
OBJECTIVES:
1. To determine the factors affecting kinetic friction.
2. To compare forces of static and kinetic friction.
3. To calculate the coefficient of friction between surfaces.
INTRODUCTION:
Friction is a force that resists motion between an object and its surroundings.
Whenever an object interacts with its surroundings, friction is unavoidable. In this
experiment, we will consider only frictional forces between two flat surfaces that are in
contact with each other. Frictional forces between two surfaces are always parallel to the
surfaces and in the opposite direction of motion. Friction that prevents an object from
moving is called static friction, and friction that resists motion that’s already occurring is
called kinetic friction.
What determines the magnitude of these frictional forces between two surfaces?
In this experiment, you will investigate the variables that affect friction and determine a
way to predict the frictional force between two surfaces. You’ll also determine how the
magnitudes of the forces of static and kinetic friction compare to each other.
METHOD:
Part I: Nature of Surfaces
1. Answer interpretation question #1 before beginning the procedure.
2. Navigate to the honors physics shared files folder (from either the school web site
physics page or the Honors Physics I moodle page). Open the “Class Materials,”
“LoggerPro” folders. Click on the file “frictionlab” and open the file.
3. Connect the dual-range force sensor to channel 1 of the LabPro interface. Make
sure the switch is set to the “50 N” range.
4. Connect the force sensor to the friction block as shown below
(felt side facing down). You may need to use a paper clip to
make the connection.
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5. Make sure that the force sensor is not pulling on the block. Then zero the sensor
by clicking on the “Set Zero Point”
button in the LoggerPro toolbar. The
displayed force reading should now be close to 0.00.
6. Place a brick on top of the friction block. Make sure the block is located at a spot
on the table where it can be pulled for at least a meter without running off the
table’s edge.
7. Click on “Collect” in LoggerPro while slowly dragging the block across the table
at constant speed. Make sure to pull horizontally on the force sensor (not at an
upward angle). Continue pulling until a constant, horizontal plot of force vs. time
is displayed on your graph.
8. Click and drag over the region of your graph where the force remains fairly
constant. Then click on the “Statistics”
button in the LoggerPro toolbar.
Note the mean value for the frictional force between the felt and table and record
it in your data table.
9. Click on “Store Latest Run” in the “Experiment” menu. Place two bricks end to
end on the table. Then place the friction block wood side down on top of the
bricks and connect it to the force sensor, as shown below.
10. Place a brick on top of the friction block and repeat step 6 (zero the force sensor
again if necessary.
11. Repeat step 7 (choose “Latest” when prompted). Note the mean value for the
frictional force between the wood and brick surfaces and record it in your data
table.
12. Click on “Store Latest Run” in the “Experiment” menu. This time, place the
friction block felt side down on top of the bricks. Place a brick on top of the
friction block, zero the force sensor if necessary, and repeat steps 6 and 7 (choose
“Latest” when prompted). Note the mean value for the frictional force between
the felt and brick surfaces and record it in your data table.
13. Print a copy of your graph (make sure the statistics boxes don’t cover the plotted
curves) for each lab partner and answer interpretation question #2.
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Part II: Normal Force
1. Select “Clear All Data” from the “Data” menu and close the statistics box. You
will now investigate how the force pressing the two surfaces together affects
friction. This force is called the normal force, and it represents the surface
pushing back on the object. Its direction is always perpendicular to the surfaces.
2. Connect the force sensor to the friction block (felt side facing down on the lab
table). Zero the sensor if necessary and place a brick on top of the block. Click
on “Collect” in LoggerPro while slowly dragging the block across the table at
constant speed. Make sure to pull horizontally on the force sensor (not at an
upward angle). Continue pulling until a constant, horizontal plot of force vs. time
is displayed on your graph.
3. Click and drag over the region of your graph where the force remains fairly
constant. Then click on the “Statistics”
button in the LoggerPro toolbar.
Note the mean value for the frictional force between the felt and table and record
it in your data table.
4. Click on “Store Latest Run” in the “Experiment” menu. Then add a second brick
to the top of the friction block and repeat steps 2 and 3. Repeat for three and four
bricks. Print a copy of your graph (make sure the statistics boxes don’t cover the
plotted curves) for each lab partner. Then answer interpretation question #3.
Part III: Surface Area
1. Select “Clear All Data” from the “Data” menu and close the statistics box. You
will now investigate how an object’s surface area affects friction.
2. Connect the force sensor to the friction block (felt side facing down on the lab
table, as before). Zero the sensor if necessary and place a single brick on top of
the block. Click on “Collect” in LoggerPro while slowly dragging the block
across the table at constant speed. Make sure to pull horizontally on the force
sensor (not at an upward angle). Continue pulling until a constant, horizontal plot
of force vs. time is displayed on your graph.
3.
Click and drag over the region of your graph where the force remains fairly
constant. Then click on the “Statistics”
button in the LoggerPro toolbar.
Note the mean value for the frictional force between the large surface of the felt
and the table and record it in your data table.
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4. Click on “Store Latest Run” in the “Experiment” menu. Then turn the friction
block onto its side so that the small felt surface is touching the table (as shown
below). Repeat steps 2 and 3. Answer interpretation question #4.
Part IV: Velocity
1. Select “Clear All Data” from the “Data” menu and close the statistics box. You
will now investigate how the speed at which an object is moving affects friction.
2. Connect the force sensor to the friction block (felt side facing down on the lab
table, as before). Zero the sensor if necessary and place a single brick on top of
the block. Click on “Collect” in LoggerPro while slowly dragging the block
across the table at constant speed. Make sure to pull horizontally on the force
sensor (not at an upward angle). Continue pulling until a constant, horizontal plot
of force vs. time is displayed on your graph.
3.
Click and drag over the region of your graph where the force remains fairly
constant. Then click on the “Statistics”
button in the LoggerPro toolbar.
Note the mean value for the frictional force between the felt and the table and
record it in your data table.
4. Click on “Store Latest Run” in the “Experiment” menu. Repeat steps 2 and 3
while dragging the block at a faster constant speed. Be sure to analyze only the
portion of the graph where the block was moving at constant speed. Then answer
interpretation questions 5-6.
Part V: Static vs. Kinetic Friction
1. Select “Clear All Data” from the “Data” menu and close the statistics box. You
will now investigate how static friction compares to kinetic. Static friction is the
force that prevents the block from initially moving whereas kinetic friction is the
force that opposes it while it’s moving.
2. Place two bricks end to end on the table. Then place the friction block felt side
down on top of the bricks and connect it to the force sensor. Place a brick on top
of the friction block and zero the force sensor again if necessary.
3. Click on “Collect” in LoggerPro while very slowly increasing the applied force
until the block just begins to move. Make sure to pull horizontally on the force
sensor (not at an upward angle). Continue pulling until a constant, horizontal plot
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of force vs. time is displayed on your graph. If the edge of the block gets caught
in the crack between the bricks, try starting with the front edge of the block just
beyond the crack.
4. Click and drag over the region of the graph that was plotted before the block
started to move. Then click on the “Statistics”
button in the LoggerPro
toolbar. Note the maximum value for the frictional force between the felt and the
table. This was the value of the frictional force just before the block began to
move (static friction). Record this in your data table.
5. Click and drag over the region of your graph where the block was moving at a
constant speed. Then click on the “Statistics”
button in the LoggerPro
toolbar. Note the mean value for the frictional force between the felt and the
table. This was the frictional force after the block began to move (kinetic
friction). Record this in your data table.
6. Print a copy of your graph (make sure the statistics boxes don’t cover the plotted
curves) for each lab partner. Then answer interpretation questions 7-8.
Part VI: Coefficients of Friction
1. The coefficient of friction between two surfaces, is defined as the ratio between
𝑓
the frictional force, f, and the normal force, N, or 𝜇 = . Since the table was
𝑁
pushing back with enough force to keep the block in equilibrium, the normal force
was equal to the weight of the block plus brick(s).
2. Use the high capacity balance to measure the mass of your block and a single
brick. Record your results in the data table. Assuming that all of the bricks have
the same mass, determine the normal force for each of the scenarios in Part II and
complete that column of the data table. Don’t forget to compute the weight of
each mass combination.
3. Using the measured frictional forces from Part II, compute the coefficient of
friction for each mass combination and complete that column of the data table.
4. Using the measured frictional forces from Part I, compute the coefficient of
friction for each of the surface combinations. Recall that a single brick was used
along with the friction block in Part I. Then answer interpretation questions 9-10.
5. When police investigate an accident scene, one of the things they need to know is
the coefficient of friction between the rubber tires and the road surface. To
determine this, they use a device called a drag sled. The drag sled consists of a
block with attached rubber tire tread that can be dragged across the road surface.
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Measure the mass of the drag sled on the high capacity balance and enter it in the
data table.
6. Place the drag sled on the lab table (tire tread facing
the table) and place a brick on top of it. Select “Clear
All Data” from the “Data” menu and close the
statistics box. Click on “Collect” in LoggerPro while
slowly dragging the drag sled across the table at
constant speed. Make sure to pull horizontally on
the force sensor (not at an upward angle). Continue
pulling until a constant, horizontal plot of force vs.
time is displayed on your graph.
7. Click and drag over the region of your graph where the force remains fairly
constant. Then click on the “Statistics”
button in the LoggerPro toolbar.
Note the mean value for the frictional force between the drag sled and the table
and record it in your data table.
8. Calculate the coefficient of kinetic friction between the drag sled and the lab table
and enter it in the data table. Finally, answer interpretation questions 11-14.
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DATA:
Part I: Nature of Surfaces
Surfaces
Average
Frictional
Force (N)
Felt on table
Wood on brick
Felt on brick
Part II: Normal Force
Number of
Bricks
Average
Frictional
Force (N)
1
2
3
4
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Part III: Surface Area
Area
Average
Frictional
Force (N)
Large Felt
Small Felt
Part IV: Velocity
Speed
Average
Frictional
Force (N)
Low
High
Part V: Static vs. Kinetic Friction
Type of
Friction
Static
Frictional
Force (N)
Kinetic
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Part VI: Coefficients of Friction
Mass of Friction Block: _______________ g
Mass of Brick: _______________ g
Number of
Bricks
Normal Force
(N)
Frictional
Force (N)
Coefficient of
Kinetic
Friction
Normal Force
(N)
Frictional
Force (N)
Coefficient of
Kinetic
Friction
1
2
3
4
Surfaces
Felt on table
Wood on brick
Felt on brick
Mass of Empty Drag Sled: _______________ g
Mass of Drag Sled with Brick: _______________ g
Normal Force: _______________ N
Coefficient of Friction: _______________
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DATA TREATMENT:



Normal force in Part VI
Coefficient of friction in Part VI
Be sure to include graphs for Parts I, II, V, and interpretation question #2.
INTERPRETATIONS:
1. What are some factors that you think would affect the magnitude of the kinetic
frictional force between two surfaces?
2. Does frictional force seem to depend upon the nature of the surfaces that are in
contact with each other? What might account for this?
3. Plot a graph of average frictional force vs. number of bricks (you’ll want to start a
second copy of LoggerPro in order to do this). How does frictional force appear
to be related to normal force? Based upon your graph (and the equation for the
best-fit line), predict the frictional force if 10 bricks were to be stacked on the
friction block.
4. How did the frictional force for the larger surface compare to the smaller for each
material? Based upon your data, does friction seem to vary appreciably with
surface area as long as the surface and normal force remain constant? What might
account for this?
5. How did the frictional force for the lower speed compare to the higher speed?
Based upon your data, does friction seem to vary appreciably with velocity as
long as the surface and normal force remain constant?
6. Based upon everything you’ve done so far, upon what factors does the frictional
force between two surfaces appear to depend?
7. How does the magnitude of static friction appear to compare to kinetic friction?
Suppose you wish to slide your living room couch across the floor. Based upon
this experiment, would it be more difficult to initially set the couch into motion or
to keep it moving?
8. Did the force of static friction remain constant from when you first pulled on the
block up until the time it started to move? As you began to pull harder and
harder, explain what the static frictional force was doing up until the point the
block began to move.
9. Based upon your experimental results, does coefficient of friction appear to vary
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appreciably with normal force when the surfaces are held constant?
10. Based upon your experimental results, does coefficient of friction appear to vary
appreciably for different surfaces when the normal force is held constant?
11. Suppose a 2000 kg car whose tires are have the same composition as the drag sled
is driving on a level surface who frictional characteristics are similar to that of the
lab table. The driver loses control of the car while going around a turn and skids
to a stop. Draw a free-body diagram showing all forces acting on the car and use
Newton’s second law of motion to determine its acceleration while skidding.
Show all of your work!
12. If the police measure the length of the skid marks to be 175 m, how fast was the
driver going when he started to skid? Show all of your work! Express your
answer in both m/s and mph.
13. If the posted speed limit on the road was 45 mph, could the driver be charged with
exceeding the posted speed limit?
14. Based upon the results of this experiment, evaluate your answer to interpretation
question #1. How would you answer this question knowing what you know now?
ANALYSIS OF ERROR:
1. Suppose you had accelerated the friction block or drag sled (in the direction of
motion) instead of pulling it at constant velocity. Would this have made your
measured frictional forces too large or too small? Use Newton’s second law to
explain your answer.
2. How would the error in #1 have affected your computed coefficients of friction?
3. If you were to do Part I of this experiment over again, would you get the exact
same results for your computed coefficients of friction for the surfaces? What
could account for any differences?
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