# Classroom Presentation Toolkit Friction ```Section 2: Friction
Friction is a type of force between two touching surfaces.
K
What I Know
W
What I Want to Find Out
L
What I Learned
Essential Questions
• What is the friction force?
• How do static and kinetic friction differ?
Friction
Vocabulary
Review
New
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force
kinetic friction
static friction
coefficient of kinetic friction
coefficient of static friction
Friction
Kinetic and Static Friction
• Friction is a force that opposes the motion of two surfaces that are in
contact with each other.
• There are two types of friction—kinetic friction and static friction.
• Static friction is the force exerted on one surface by another when there
is no motion between the two surfaces.
• For example, the force applied to the couch below is balanced by the static
friction force.
Friction
Kinetic and Static Friction
• If you push hard enough, the couch will begin to move and kinetic friction
will act on it.
• Kinetic friction is exerted on one surface by another when the two
surfaces rub against each other because one or both of them are moving.
Friction
Kinetic and Static Friction
• Frictional force depends on the materials that the surfaces are made of.
• The different lines correspond to dragging a block along different surfaces.
Friction
Kinetic and Static Friction
• The slope of this line is the coefficient of kinetic friction (μk) between the
two surfaces and relates the frictional force to the normal force, as shown
below.
Kinetic Friction Force
Ff, kinetic  μk FN
• Notice that the normal force (FN) also plays a role in the size of the friction
force.
• Similarly, μs is the coefficient of static friction between the two surfaces.
Static Friction Force
Ff, static  μsFN
• μsFN is the maximum static friction force that must be overcome before
motion can begin.
Friction
Kinetic and Static Friction
Friction
FN
Kinetic and Static Friction
vg
Ff
Fchild
Use with Example Problem 3.
Fg
Problem
A child drags a heavy, rubber-soled
shoe by its laces across a sidewalk at
a constant speed of 0.35 m/s. The
shoe has a mass of 1.56 kg and the
coefficient of kinetic friction is 0.65.
What is the force exerted by the child?
(Assume the sole is in contact with the
sidewalk, not bouncing around and
that the child pulls horizontally.)
Response
SKETCH AND ANALYZE THE PROBLEM
• Draw a vector diagram.
• List the knowns and unknowns.
KNOWN
v = 0.35 m/s
m = 1.56 kg
μk = 0.65
UNKNOWN
Fchild = ?
SOLVE FOR THE UNKNOWN
• Because the shoe is not accelerating in the
y-direction, Fnet, y = 0.
• Thus, the normal force and the shoe’s
weight are equal in magnitude.
FN  Fg  mg
• Because the shoe is not accelerating in the
x-direction, Fnet, x = 0.
•
Thus, the force the child exerts and the
frictional force are equal in magnitude.
Fchild  Ff  μk FN  μk mg
 0.651.56 kg9.8 N/kg  9.9 N
• Force is in newtons, so the units are correct.
Friction
Kinetic and Static Friction
Ff
FN
Fchild
Fg
a
Use with Example Problem 4.
Problem
If the child in the previous Additional In-Class SOLVE FOR THE UNKNOWN
Example pulls with an extra 2.0 N in the
• Find the net force in each direction.
horizontal direction, what will be the
• Fnet, y = 0 and FN = mg, as it did in the
acceleration of the shoe?
previous example.
• Ff is same as it was in the previous
Response
example, so Fnet, x = Fchild − Ff = 2 N.
SKETCH AND ANALYZE THE PROBLEM
• Use Newton’s second law.
• Draw a force diagram.
•
List the knowns and unknowns.
KNOWN
UNKNOWN
Fchild = 11.9 N
a=?
m = 1.56 kg
μk = 0.65
Fnet  ma
Fnet, x
2.0 N
ax 

 1.3 m/s2
m
1.56 kg
• 1.3 m/s2 is about (1/8)g, so it is a reasonable
acceleration.
Friction
Review
Essential Questions
•
What is the friction force?
•
How do static and kinetic friction differ?
Vocabulary
• kinetic friction
• static friction