Elizabethtown College
Department of Physics and Engineering
PHY200/103
Lab #5 Friction
Introduction
Friction, can’t live with it can’t live without it. In a world without coulomb friction, we
could build incredibly efficient vehicles and oil changes would be non-existent. On the
other hand getting around would be rather difficult, not to mention picking up a pencil. In
our textbooks we often solve problems “neglecting friction.” This allows us to simplify
problem to learn the concept at hand. Even in many real applications we can neglect
frictional effects and still expect a valid solution because the frictional forces are so much
smaller than the other forces present. However, friction is an important part of the
physical world we live in and we need to understand how it works.
We will explore the force of friction in this week’s lab.
The Physics Coulomb Friction
Friction is a non-conservative force. That is: friction converts mechanical energy into heat.
When friction is present, energy is still conserved but mechanical energy is not. Think about
moving a heavy sofa across your carpeted dorm room. The FBD for this situation is shown
in Figure 1. You have to keep pushing even just to keep the sofa moving at a constant
velocity. We have already demonstrated Newton’s second law to be true therefore some
force must be resisting the motion of the sofa. That force is called the friction force.
N
P
f
mg
Figure 1 – FBD of a sofa being slid across a carpeted surface
If you start pushing on the sofa with a very small force, the sofa doesn’t move. Again this is
because of the friction force. The friction force will match any force you apply to keep the
sofa from moving up to some maximum. If you exceed the maximum friction force the
friction will continue to resist your pushes but the couch will accelerate because your push is
now greater that the friction force. To move at a constant velocity you only need to push as
hard as the maximum friction force. Finally, you have probably noticed that in this type of
situation it is often harder to get the sofa moving than it is to keep it moving. The maximum
force of friction is greater when the couch is stationary than it is while the couch is moving.
The force of static (stationary) friction is only as large as it needs to be to keep the object
from moving:
f s  s N
(5.1)
where is the coefficient of friction for the two materials interacting and N is the normal
force between the two forces. When the object is moving we assume the force of friction to
be constant:
f k  k N
(5.2)
The static coefficient of friction is usually greater that the kinetic coefficient. Values tend to
be between 0.05 and 1.0.
Notice in the above equations that the friction force is not dependent on the surface area of
contact. The reason for this is actually still a topic of investigation in experimental and
theoretical physics (see the 11/20/2001 issue of Nature).
Experiment Coulomb Friction
Figure 1 – Experimental equipment
Using the force sensor along with the friction accessories (Figure 1 – activity Lab5.ds). Test
the following hypotheses:
The kinetic friction force is proportional to the normal force. Test for all three
surfaces on the aluminum track. Hint: try to keep accelerations low. A large
acceleration would confound your results.
The kinetic friction force is independent of the surface area of contact.
Experimentally determine the static and kinetic coefficient of friction of each of the three
surfaces (felt, cork and Teflon) on aluminum.
Hints: make sure to keep the force transducer and the string connecting the transducer to
the friction block horizontal at all times. Start and stop several times on each trial and try to
in the smallest force possible to make the block start sliding. If you pull too hard too fast you
will not see the static coefficient.
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