Friction
The force between two surfaces in contact
that causes a resistance of the motion or
pending motion between the surfaces.
Friction always opposes the direction of motion
or pending motion between the two surfaces in
contact.
•Friction is always parallel to the two surfaces in contact.
Motion direction
friction
Applied force
Friction always opposes the direction
of motion or pending motion.
Motion Direction
Applied force
friction
What Causes Friction?
• Irregularities between two surfaces in
contact.
A microscopic view
of the two surfaces
in contact.
Friction
• Measured in Newtons (N) because it is a type of
force.
• Frictionless objects are fictitious.
• Static friction – the type of friction present when
two surfaces are at rest with respect to each
other
• Kinetic (Sliding) friction – the type of friction
present when one surface slides across another
surface.
• Static friction is usually greater than kinetic
friction
Friction is only affected by:
• The texture or surface properties between the
two surfaces in contact. (Coefficient of friction)
• The amount of contact between the two surfaces.
(Normal force)
Friction equation:
f  μη
f = frictional force
μ = coefficient of friction
η = normal force
f  μη
Friction is fun
Coefficient of friction
• Surface properties between the two surfaces in contact that
affects pending motion or motion
• The larger the value the more resistance between the two
surfaces.
• The coefficient of static friction only applies when an object is
not sliding over another surface
• The coefficient of sliding (kinetic friction) only applies when
one object slides over the surface of another surface.
• The coefficient of static friction is usually greater than the
coefficient of sliding friction.
• Dimensionless quantity
• μ = The Greek Letter “mu”
• The ratio to the frictional force to the normal force
• μ = 0 for a frictionless surface.
• Value range typically between 0.03 (smooth ice on smooth
ice)  1.20 (rubber on concrete)
• The value for coefficient can be interpreted as a percent. The
force needed to move ice on ice is only 3 % of the normal
force.
• Table
Frictional force
Static Friction increases to a
maximum level
Kinetic
friction
Applied force
• Static friction
matches the applied
force to a certain
level. After the
maximum static
friction value is
obtained the object
moves and kinetic
friction is active.
Normal force
• Represents the amount of contact
between two surfaces
• Measured in Newtons.
• Always directed away from the surface
and perpendicular to the two surfaces in
contact.
η
η
η
Two types of friction
• Since there are two types of coefficient of
frictions (static and kinetic), there are two types
of friction
• fs=static friction (the friction present when one
object does not slide over the surface of another
surface
• fk = kinetic friction (the friction present when an
object slides over the surface of another
surface.)
• fs=μsη
• fk=μkη
If the applied force and frictional force
are equal, then the object will move
at constant velocity
Motion direction
Friction =f
FA=fs
FA=fk
Applied force = FA
for an object not in motion
for an object in motion that slides over the surface
of anther surface