Download Coefficient of Sliding Friction

Coefficient of Sliding Friction
Name __________________
Date ___________
Introduction
When a block is at rest on an inclined plane its weight, FW , is concentrated at the
center of gravity of the block and acts vertically downward. Since the block can not
move in that direction, the weight of the block must be considered in terms of its two
components of force. The first component is the force that acts parallel to the plane of
the incline, FP , and tends to slide the block down the plane. The other component, FN , is
the normal force that acts at a right angle to the plane and tends to break it or to make the
block “stick” to the plane. If the slope of the plane is great enough to cause the block to
slide at a constant velocity, no acceleration, the ratio of the parallel force to the
perpendicular force is the coefficient of kinetic friction between the block and the plane.
The coefficient of sliding friction may also be defined as the ratio of the force
required to slide an object at uniform speed over a surface to the weight of the object
itself. This can be found by weighing the object and then using a spring balance to
measure the force needed to slide the object at slow, uniform speed. For example, if a
force of 8Nis required to slide a 20-N block over a horizontal surface at constant rate, the
coefficient of friction is 8N/20N or 0.4. The coefficient of friction depends on the
materials and the nature of their surfaces.
Objective
The completion of this experiment should allow you to measure the coefficient of
sliding friction of an object that is located on an inclined plane.
Apparatus
Inclined plane
Pulley attachment
Wooden block, with hook attachment
Meterstick
Set of masses
String
Protractor
Platform balance
Procedure
Find the angle at which the wooden block slides down the inclined plane at a
constant velocity when it is given a slight push to start it. This can be done by trying
different angles until the correct angle is found. Record this angle in a data table. Also
record the mass of the block as measured with a platform balance or triple-beam balance.
Repeat the above procedure, but place the block on its narrow side instead of its
broad side. Measure and record the angle as before.
For the remaining trials, attach a piece of string to the hook on the wooden block
and pass the string over the pulley at the upper end of the inclined plane. Set the angle of
the plane at 5 degrees and attach enough masses to the free end of the string so that the
block will slide with a constant velocity when it is given a slight starting push. Record
your data in a data table.
Increase the angle to 15 degrees and repeat the above procedure. Repeat this
process at 10 degree intervals until an angle of 45 degrees has been reached. Record all
data in data tables.
Calculations
1. Calculate the weight of the wooden block from its mass. Record this weight as FW .
2. Using the data in the data tables and the vector methods discussed previously in class,
calculate FP, FN, and Ff for the trials without the attached masses.
3. Use your data to calculate the coefficient of sliding friction for the trials without the
attached masses.
4. For the trials with the attached masses, calculate the weight of the mass attached to
the string and record as Fh . Then calculate the coefficient of sliding friction for these
trails.
5. Plot a graph for the trials with the attached masses using the values of the frictional
forces on the “y” axis and the values of the normal forces on the “x” axis.
Questions
1. Does the coefficient of sliding friction depend on the area that is in contact? Verify
your answer from your data and calculations.
2. Why is the wooden block started with a slight push?
3. What is the relationship between the shape of the graph and the coefficient of sliding
friction?
4. How do the values of the coefficients of sliding friction compare to “theoretical”
values as can be found in your text?