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Friction
Boy, this stuff rubs me the wrong way…
Friction
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Friction is a contact
force when two
surfaces touch
Friction is always
in the opposite
direction of the
motion
How do we get friction?
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Friction force depends on two things:

The surfaces in contact with each other


For example, sandpaper on wood,
pavement on rubber, ice on metal
The mass of the object/surface that is
trying to move

We will work with keeping one surface still
and one surface moving
The interaction between two surfaces
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The part of friction that depends on the
two surfaces interacting is called the
“coefficient of friction”
Abbreviated “m”
The larger the m, the more friction force
there is
 For example, ice and skin has a small
m, sandpaper and skin has a large m
Just to confuse things…

There are two different types of friction:
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Static Friction
 The friction force that occurs between two
objects as one object is trying to start
moving
Kinetic Friction
 The friction force that occurs between two
objects once one object is moving
Typically, static friction is higher than
kinetic friction (easier to keep moving than
to start moving)
The Equation
Mathematically, friction
forces look like this:
Ff = (m)(FN)
Example #1

A person exerts a 36 N horizontal force as he
pulls a 52 N sled across a cement sidewalk
with constant speed. What is m between the
cement and the metal sled runners?
FN
36 N
Ff
Ff = 36 N (because motion
is constant speed = no
acceleration)
Ff = (m)(FN)
36 N = (m) (52 N)
52 N
.692 = m
Example #2

Suppose the same sled is on packed snow.
The m now equals .12. If a person weighing
650 N sits on the sled, what force is needed
to pull the sled at constant velocity?
FN = 702 N (the weights of
the two objects added)
FN
?
Ff
Ff = (m)(FN)
Ff = (.12) (702 N)
52 N
650 N
Ff = 84.2 N
Friction on slanted surface
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Is friction higher or lower on a slanted
surface?
Well, let’s look at the free-body diagram…
Notice what happens to the
normal force…
The Fg is acting
straight down
Ff
FN
Only a portion of the
weight is reflected on the
FN
The Ff is smaller!
Fg
Friction Homework
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Chapter 6
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#100,000,001 – 101,000,000
Every 3rd problem, except the palindromes (like
101,000,101)
Just Kidding
#33, 34, 37