# PHYS 2001 Lecture Notes - Lecture 9: Friction, Glossary Of Video Game Terms, Fibronectin

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16 Jun 2018
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4.9 Frictional Forces
We’ve been discussing forces on objects who have surfaces in contact. The
Normal Force
is perpendicular to the surface.
Normal
Force
is
perpendicular
to
the
surface.
There is another force due to surfaces in contact in which the force is parallel to
the surface. This is call the Frictional Force.
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Fi ti
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e w
ill
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ft
en
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t
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er
t
o
thi
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orce as
F
r
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on.
Friction comes in two types: Static Friction
Kinetic Friction
Consider the situation of a block sitting on a table:
The Frictional Force tends to oppose the direction of motion, i.e. it tries to slow
the object down. But not always!!!
Consider
the
situation
of
a
block
sitting
on
a
table:
If I want to slide the block across the table, then I have
to overcome a frictional force just to get it to move
(
static friction
)
(
static
friction
)
.
And then I have to overcome another frictional force to
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eep
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r
i
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ti
on, or
f
r
i
c
ti
on o
f
motion.
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fsF
fk
Let’s say I want to try and pull the block to the right.
As I start to pull with some force F, the block doesn’t move because of the
static frictional force (fs).
Notice that
f
s
is
p
arallel to the surface of contact
,
and it o
pp
oses the direction
f
s
p,pp
that I’m trying to pull the block.
So I keep making my pulling force Fbigger and bigger. fskeeps getting bigger
and bigger too, canceling out my pulling force until fsreaches its maximum value.
Once I go past this point so that Fis now greater than fs, the block breaks free
and starts to slide.
Now that the block is sliding I no longer have to overcome the static frictional
Now
that
the
block
is
sliding
,
I
no
longer
have
to
overcome
the
static
frictional
force, fs, but I have to overcome the kinetic frictional force, fk, to keep the block
moving.
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