Download Phy221 Lab 2

Name ______________________
Lab Partners __________________________
Friction
Introduction: Have you ever noticed how it is harder to start an object sliding across a surface than it is
to keep the object sliding? When a block rests on a table there is, literally, some kind of bonding going on
between the block and the table. To start the object moving you must apply a horizontal force parallel to
the table. If the force is too small it won’t overcome the “bonding” and the object still will not move. We
say that a static frictional force exists that balances (is equal and opposite to) the applied force. As the
applied force increases, the static frictional force increases, but the object still does not move until some
maximum static frictional force is reached. At this point the applied force is sufficient to break the
bonding between the block and the table, and the object moves.
Once the object is sliding there is still “bonding” between the block and table, but it is not as strong as
when the block is not sliding. Therefore, it requires less force to break these bonds and keep the object
sliding. The frictional force created by the making and breaking of these bonds during sliding is called the
kinetic frictional force.
In this lab you will investigate properties of the static and kinetic frictional forces. You will be given a
variety of supplies with which to make these measurements including wooden blocks, string, weights, a
weight hanger, an inclined/horizontal plane with pulley, etc. You will need to work with your lab partners
to determine a technique to measure the frictional forces and determine their dependence on various
adjustable parameters.
Part 1: Measuring the Static Frictional Force
Clearly, if you apply a horizontal force to a block on a horizontal surface and the block does not move
then the static frictional force has the same magnitude as the applied force. But, how would you actually
measure the maximum static frictional force, fsmax? Discuss this with your lab partners and your instructor
then describe your technique in question 1 of the data section. Carry out the experiment with a block of
wood with its larger surface in contact with the horizontal surface. You might want to record the weight
of the block. Hint: Do not press the block down upon the horizontal surface, set it down gently. Also,
perform several trials. Finally, be sure that your applied force is perfectly horizontal (Why?).
Now consider the effect of the weight of the block. Since the block is not accelerating vertically the
weight force must be equal in magnitude to the normal force. Thus, as we change the block’s mass, we
can study the dependence of friction on the normal force. Add an extra 100 g, 200 g, 300 g, 400 g, and
500 g to the block while each time recording the measured maximum static frictional force. Be sure to
actually measure the mass of the weights that you are adding. Do not rely on the values written on them.
Enter your data in the table of question 2 in the data section.
Part 2: Measuring the Kinetic Frictional Force
Now you need to come up with a technique to measure the kinetic frictional force. You will need to figure
out a way to apply a known force so the block slides with zero acceleration (constant velocity) so the
kinetic force balances the applied force. Describe your technique in question 5 of the data section.
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Add mass to the block, as you did previously, to increase the normal force and measure the kinetic
frictional force. Enter your data in question 6 of the data section.
Data and answers to questions:
1.
Describe your technique to measure the maximum static frictional force. (Show free-body
diagram.)
2.
Data for dependence of maximum frictional force on normal force with large surface of block
facing down.
Total mass
Normal Force
fsmax
3.
Plot a graph between maximum static frictional force and the normal force? What kind of function
would best fit your data? What is the equation for this function? Hint: clearly if there is zero normal force
(i.e. the block is not touching the horizontal surface) there would be zero kinetic frictional force. Thus,
include the point (0,0) in your data when you make a graph.
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4.
From your measurements and analysis, and your understanding of maximum static frictional force,
A) can you come up with an equation that describes how fsmax depends on the normal force?
B) Is fsmax linearly proportional to the normal force?
C) What is the proportionality constant?
D) What are its units?
E) Does the proportionality constant depend on the materials of the block and horizontal surface?
5.
Describe your technique to measure the kinetic frictional force.
6.
Data for dependence of kinetic frictional force on normal force with large surface of block facing
down.
Total mass
Normal Force
fk
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7.
Plot a graph between kinetic frictional force and the normal force? What kind of function would
best fit your data? What is the equation for this function? Hint: clearly if there is zero normal force (i.e.
the block is not touching the horizontal surface) there would be zero kinetic frictional force. Thus, include
the point (0,0) in your data when you make a graph.
8.
From your measurements and analysis, and your understanding of the kinetic frictional force,
A) can you come up with an equation that describes how fk depends on the normal force?
B) Is fk proportional to the normal force?
C) What is the proportionality constant?
D) What are its units?
E) Does the proportionality constant depend on the materials of the block and horizontal surface?
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