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Friction
ALSO KNOWN AS MU - Μ
Normal Force
 Remember that normal force is perpendicular
to the plane that an object is in contact with.
 Normal force of a book sitting on a table
In this case, the normal force is equal and opposite of the Force of the
weight of the object.
Static Friction
 When an object is at rest, it tends to stay at
rest. When you do push on an object, there is
a resistive force that keeps it from moving.
This force is called static friction.
 Ex: pushing a car
Static Friction
 When the applied force finally exceeds the
static frictional force, then the object will
move.
Kinetic Friction
 Once your car actually begins to move
(overcome the static frictional force), it still
experiences a retarding frictional force called
kinetic friction.
 The Net Force acting on an object is the
difference between the applied force and the
frictional force.
Properties of Friction
 Friction is proportional to the
Normal Force
 For Friction:
 Ff = µFn
 So how do you get µ by itself?
Friction depends on the
surfaces in contact.
Coefficients of Friction







Materials
Steel on Steel
Aluminum on Steel
Copper on Steel
Rubber on Concrete
Wood on Wood
Glass on Glass
Teflon on Teflon

Synovial joints in humans
µ Static
0.74
0.61
0.53
1.0
0.25-0.5
0.94
0.04
0.01
µ Kinetic
0.57
.47
0.36
0.8
0.2
0.4
0.04
0.003
1) Chris pushes a pencil on the floor with a force of .02N.
The pencil experiences a frictional force of .01N. What is
the net force of the pencil?
2) Unbeknownst to the students, when the school floors
are newly waxed, Mr. Engel likes to slide down the hallways
in his socks. If Mr. Engel weighs 600N and the coefficient
of sliding friction between his socks and the floor is .120,
what is the force of friction that opposes his motion?
3) Ti’Ara decides to move her TV stand so she can lay in bed
and watch TV. If her 100kg bed resists the motion with a
resistive force of 320N, what is the coefficient of kinetic
friction between Ti’Ara’s bed and the floor?
4) Jordan (80kg) steps on spilled milk. The coefficient of friction
between Jordan’s feet and the floor is reduced to µ =.040.
What is the force of the sliding friction between Jordan and the
floor?
 4) A 15kg chair initially at rest on a horizontal floor
requires a 36N horizontal force to set it in motion.
Once the chair is in motion, a 32N horizontal force
keeps it moving at a constant velocity. Find the
coefficients of static and kinetic friction.
6) Chasity moves a box of books down the hall by pulling on a
rope attached to the box. Chasity pulls with a force of 185N at
an angle of 250. If the coefficient of friction is .02 and the mass
of the box is 40kg, What is the force of friction?
7) Brayan pushes a 10kg desk across the room (so he can sit on
it while he does his online homework). He pushes down at an
angle of 60o with the horizontal with a force of 5N. If µ between
the desk and the floor is .08
>What is
is the
the normal
Ff experienced
by the desk?
>What
force experienced
by the desk?