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Friction Factors
We are constantly aware of the frictional force that opposes the motion of one surface in contact with another. When there is a sheet
of ice on a sidewalk, friction is reduced, and it is difficult to walk. The lack of friction is an inconvenience. However, machines are
lubricated to reduce friction where it is not an advantage.
If you pull an object horizontally at constant velocity, the applied force just balances the frictional force. You will perform a series of
short experiments in which you vary one property of the two surfaces in contact and measure the force necessary to move an object at
constant velocity.
Objective: Study the effect of various conditions on the force of friction.
Equipment:
string
several blocks
paper
wax paper
sandpaper
plastic wrap
spring scale
Al foil
Cloth
Halls carriage
masking tape
Procedure: Record your observations on your data sheet. Record several trials and average results.
A. Nature of the Surface
1. Tie a string around a block so that the string does not interfere with the sliding surface.
2. Practice until you can pull the block with constant velocity. Have a lab partner read the spring scale while the block is in
motion. Record the force in Table A1. Make sure you pull the scale in a horizontal position each time.
3. Pull horizontally from rest and measure the minimum force required to set the block in motion. It occurs just before the block
starts moving, and may be hard to catch. This force is equal in magnitude and opposite in direction to the force of static
friction. Record this force in Table A2.
4. Vary the surface interaction by pulling the block over a variety of surfaces. Be sure to hold the surfaces still and taut during the
pull.
B. Area of Surface
1. Position the block with its largest surface in contact with the table. Measure the force required to pull the block across the table
at constant velocity. Record the force in Table B1. Be sure to make your readings while the block is moving uniformly.
2. Turn the block on one of its narrow edges and measure the force as in step one. (You may have to relocate the string) Try to
pull the block with the same velocity as before. Record this force in Table B1
3. Turn the block once more to measure the force necessary to overcome friction on the third side.
C. Speed of motion
1. Measure and record in Table C1 the force necessary to move a block uniformly at different velocities.
D. Rolling and Sliding Motion
1. Place a piece of masking tape around a Halls Carriage so that it will slide rather than roll when pulled. Place a 500g mass in
the carriage. Measure the force required to make it move at a constant velocity. Record your measurements in Table D1.
2. Remove the tape and measure the force necessary to roll the carriage at approximately the same constant velocity. Record
your observations.
E. Force Pressing the Surfaces Together
1. Measure and record in Table E1 the force needed to pull a single block uniformly.
2. Place another block on top of the first and repeat the procedure, recording your observations.
3. Repeat with a third block.
Calculating the Coefficient of Friction:
1. Using the data from Table A1, determine µk (kinetic) for each of the surfaces tested. Make any additional measurements
needed for your calculations. (Ff = µFN)
2. Using the data from Table A2, determine µs (static) for each of the surfaces tested.
Name:
Period:
Friction Factors Data Sheet
Prior to recording data, please predict the outcome based on your prior knowledge of friction.
Table A1: Nature of the Surface - Kinetic Friction
Prediction: Which surface will exhibit the greatest kinetic friction? _________________________________________
Results: Which surface exhibited the greatest kinetic friction? ___________________________________________
Surface
Trial 1
Trial 2
Trial 3
Average
Wood-wood
Wood-paper
Wood-wax paper
Wood-plastic
Wood-foil
Wood-cloth
Wood-sandpaper fine
Wood-sandpaper rough
Table A2: Nature of the Surface – Static Friction
Prediction: Which surface will exhibit the greatest static friction? _________________________________________
Results: Which surface exhibited the greatest static friction? ________________________________________
Surface
Trial 1
Trial 2
Trial 3
Average
Wood-wood
Wood-paper
Wood-wax paper
Wood-plastic
Wood-foil
Wood-cloth
Wood-sandpaper fine
Wood-sandpaper rough
Table B1: Area of Surface
Prediction: Will more or less surface exhibit the greatest friction? ________________________________________
Results:
Surface area (cm²)
Trial 1
Trial 2
Trial 3
Average
Table C1: Speed of Motion
Prediction: Which speed will result in the most friction? _________________________________________________
Results:
Relative speed
Trial 1
Trial 2
Trial 3
Average
Slow
Moderate
Fast
Table D1: Rolling and Sliding Motion
Prediction: Will rolling or sliding motion exhibit more friction? ____________________________________________
Results:
Motion
Trial 1
Trial 2
Trial 3
Average
Sliding
Rolling
Table E1: Force Pressing the Surfaces Together
Prediction: Will increased weight cause greater friction? __________________________________________
Results:
Number of blocks
Trial 1
Trial 2
Trial 3
Average
Calculating the Coefficient of Friction:
Surfaces
Ff = µk FN
Wood-wood
Wood-paper
Wood-wax paper
Wood-plastic
Wood-foil
Wood-cloth
Wood-sand paper
µ kinetic
Ff = µs FN
µ static
Analysis:
1. Which factors influence the force of friction?
2. Which material had the greatest static friction?
The least?
3. Which material had the greatest kinetic friction?
The least?
4. Leather, rather than rubber, is generally recommended for the soles of toddler’s shoes. Why?
5. Sand is often scattered on roads during snowstorms and icing. Why?
6. If there were no friction would it be possible to tie a knot in a piece of string or rope? Would the knot hold? Explain.
7. Knowing that the static friction for a rubber tire on a concrete road is 1.0 while the kinetic (sliding) friction is only
0.7, explain the benefit of anti-lock braking systems.
Write a brief paragraph summarizing your findings and relating several interesting or surprising findings.