Coefficient of Sliding Friction on an Inclined Plane
Name: __________________________
Partner: __________________________
Lab # _____
Date Performed: ___________
Date Due: ___________
Friction is so common in the world around that we often forget that it is a force that pushes on objects. Since
friction is present in almost every situation we look at in the world around us, it is important to understand
the factors that determine the amount of friction acting on an object and to be able to calculate the amount of
friction acting on an object. We often forget the benefits of friction but are quickly reminded when we try to
walk across a sidewalk covered with a sheet of ice, friction is reduced and it is difficult to walk. Here the
lack of friction is an inconvenience, however, machines are lubricated to reduce friction where it is not an
advantage.
In this activity, we will examine how the force of friction changes with the normal force exerted by the table
and determine the coefficient of friction between the material on the bottom of your object and the surface.
Figure 1
Figure 2
Procedure:
A. Nature of the Surface
1. Measure the mass of the block on a balance. Record the value. Calculate the amount of normal force that
the platform surface must exert on the block and record that value.
2. Clamp a pulley on the edge of the platform surface. Run the string level with the platform surface from
the hook on the block over the pulley and hang the hanger on the end of the string (see the diagram above).
3. With the block positioned with the sandpaper surface in contact with the platform, gradually add masses
onto the hanger and slightly push on the block to see if it will slide across the platform. Keep adding mass to
the hook until the block slides along the platform after you slightly push on it. Record the amount of mass
on the end of the hanger when it starts to move. Note remember to include the mass of the hanger in the
sum. Repeat until you have 3 measurements.
4. Wrap the block in paper. Tape the paper to the block so that the tape does not interfere with at least one of
the largest surfaces and with one of the narrowest surfaces.
5. With the block positioned with the largest surface in contact with the platform, gradually add masses onto
the hanger until the block just starts to move. Record the amount of mass on the end of the hanger when it
starts to move. Repeat until you have 3 measurements.
B. Area of Surface
1. Using the same paper wrapped block, turn the block onto one of its narrow edges and measure the force
as in the previous steps needed to just get the block to move. Record the value. Repeat until you have 3
measurements.
C. Mass of the Object
1. Using your paper wrapped block, place a 200 g or a 500 g mass onto the block. Measure the total mass of
the block with added mass and record the value. Calculate the amount of normal force that the platform
surface must exert on the block with the added mass and record that value.
2. Repeat the steps on attaching the block with the added mass to the hook with the string and pulley.
3. With the block positioned with the largest surface in contact with the platform, gradually add masses onto
the hanger until the block with the added mass just starts to move. Record the amount of mass on the end of
the hanger when it starts to move. Repeat until you have 3 measurements.
D. Inclined Surface
1. Using some old textbook elevate the end of the platform that has the pulley attached. Draw a schematic of
this arrangement (Figure 2). Measure the angle between the platform and the lab bench and record.
2. Record the mass of the paper covered block and set it on the platform with the largest surface side on the
platform. You may need to add a 200 g or a 500 g mass to your block if your block slides down the incline
without any mass added to the hanger or reduce the angle of inclination.
3. Repeat the steps to measure the force needed to just get the block to move. Record the value. Repeat
until you have 3 measurements.
4. Change the angle using a different number of books and repeat the above steps recording the values until
you have 3 measurements.
5. Change the angle again and repeat the above steps recording the values until you have 3 measurements.
Data:
Part A
Surface Material:
Block Mass
(kg)
Fg
FN
(N)
(N)
Fg
FN
(N)
(N)
Fg
FN
(N)
(N)
Fg
FN
(N)
(N)
Hanging
Mass (kg)
Ff
µ
(N)
Surface Material:
Block Mass
(kg)
Hanging
Mass (kg)
Ff
µ
(N)
Part B
Block Mass
(kg)
Hanging
Mass (kg)
Ff
µ
(N)
Part C
Block Mass
(kg)
Hanging
Mass (kg)
Ff
(N)
µ
Data: continued . . . . .
Part D
Angle:
Block Mass
(kg)
Fg
FN
(N)
(N)
Hanging
Mass (kg)
Ff
µ
(N)
Average Ff __________
Angle:
Block Mass
(kg)
Fg
FN
(N)
(N)
Hanging
Mass (kg)
Ff
µ
(N)
Average Ff __________
Angle:
Block Mass
(kg)
Fg
FN
(N)
(N)
Hanging
Mass (kg)
Ff
(N)
Average Ff __________
µ
Calculations:
Show one example calculation for each procedure.
Record the results of the calculations in the tables.
1) For each procedure, calculate the weight (Fg) of the block used, the normal force, the amount of friction
that the table exerted on the block with the masses to resist the force of gravity on the masses on the hanger.
For level surfaces remember to use the appropriate formulas to find the component forces.
Ff = - FHanger
Fg = mg
FN = - Fg
FHanger = weight force of the hanger + masses
For inclined surfaces remember to use the appropriate formulas including the angle you measured and
trigonometry to determine the component forces.
F║ = sin (θ) Fg
Fg = mg
FHanger = weight force of the hanger + masses
F┴ = cos (θ) Fg
FN = - F┴
Ff = FHanger + F║
(note F║ is negative)
Calculations: continued. . . .
2) Calculate the coefficient of friction for the different surface materials.
µ =
Ff
FN
3) On a seperate sheet of graph paper or using Excel, plot a graph using the Part D values of
FN as abscissas (x) and the average values of Ff as ordinates (y). Start your graph at (0,0)
Note we are interested in the magnitude of the values so plot as positive values.
Questions:
1. Does the coefficient of sliding friction depend on the area that is in contact? Verify your answer by
explaining using the results for your data calculations.
2. Why is the wooden block started with a sight push in the procedure for finding the coefficient of sliding
friction?
3. How do the values of µ for Part A compare to part B? Explain why they the same or any differences.
4. How do the values of µ for Part B compare to part C? Explain why they the same or any differences.
5. What is the relationship shown on your graph? Explain the meaning of the slope.
6. List two real life situations in which friction should be increased.
7. List two real life situations in which friction should be decreased.
Questions: continued . . . . .
8. A crate weighing 1800 N is pulled up a 15º incline at a constant speed by a force of 990 N. What is the
coefficient of sliding friction between the crate and the floor? Show all work.
Conclusion:
Summarize what you have observed/learned about friction and coefficients of friction from performing this
lab activity. Check reference tables and textbook for coefficient of frictions values – do you think your
values are reasonable?
How did the lab go? - What went well? What problems did you encounter? What would you do differently?
What is something you would like to investigate if you had to do another lab activity on friction?