Coefficient of Friction
In this lab you will measure the coefficient of kinetic friction between a wooden block
and the table and examine some of the factors that may affect its value. The coefficient
of kinetic friction is defined as the ratio of the frictional force to the normal force while
the block is sliding,
k 
Ff
Fn
.
(1)
Similarly, the coefficient of static friction is defined as the ratio of the maximum
frictional force that resists motion to the normal force,
s 
F f ,max
Fn
(2)
We can easily measure k by measuring the horizontal force required to keep an object of
a known weight moving at constant velocity on a flat surface. For a horizontal pull, Fn is
just the weight of the object.
Question 1:
According to your textbook, how do k and s usually compare in value? (Which is
typically larger?)
Question 2:
How do you expect the coefficient of friction to depend on the normal force? For
example, do you expect k to increase, decrease, or remain constant as the normal force is
increased?
Question 3:
How do you expect k to depend on the contact area between the two sliding surfaces?
Experiment:
Place a mass of a 0.2 kg on the block. .
First, place the wooden block on the table with the largest area felt side facing down.
Attach the force sensor to the block with a string. Use Data Studio to look at the force as
a function of time. Gradually pull until the block starts moving and then continue pulling
to keep the block moving at a constant speed. Do this carefully enough to see how the
force changes just before to just after the block starts moving.
Add a mass to the block and repeat. Do this for 4 different masses up to 1 kg.
Now repeat using the small (edge) area of the block that has the felt.
Using Excel, on a single graph plot the frictional force, both the maximum static
frictional force and the dynamic frictional force (force sensor measurement).
Plot these as a function of the normal force (weight of block and added mass) for the
large surface and for the small surface, respectively. Thus four data sets in one graph.
Question 4:
From your first measurement, what do you conclude about the relative values of k and
s. Is this in agreement with your answer to Question 1?
Question 5:
Look at your graphs of Ff versus Fn. Are they in agreement with your answer to
Question 2? Explain.
Question 6:
From the slopes of your curves, determine the static and dynamic coefficients of friction
for the large surface and for the small surface. Was your response to Question 3 correct?
Turn in:
 One print-out of original DataSudio data (force as a function of time)
 The Excel graph with four data sets.
 Answers to all questions.