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 few hundred grams on the block. Attach the force sensor to the block
with a string and gradually pull until the block starts moving and then continue pulling to
keep the block moving at a constant speed. Use Data Studio to look at the force as a
function of time. Do this carefully enough to see how the force changes just before to
just after the block starts moving.
Now place the wooden block on the table with the felt side that has the largest area
against the table. With no added mass, measure the average force required to keep the
block moving at a constant speed. Add a mass to the block and repeat. Do this for about
4-5 different masses up to 1 kg or so. Now repeat using the small (edge) area of the
block that has the felt.
Using Excel, on a single graph plot the frictional force (force sensor measurement) as a
function of the normal force (weight of block and added mass) for the large surface and
for the small surface.
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 coefficient of kinetic friction for the large
surface and for the small surface. Was your response to Question 3 correct?