FRICTION AND ITS TYPES
BY
BHORANIYA AHEMAD ABBAS A.
(130460106008)
What is Friction
• Friction is a force
• A frictional force can exist when two
substances contact each other.
• The molecules of each surface interact
according to Newton’s Laws of Motion.
• Friction always opposes motion, i.e., it is
opposite to the direction of velocity.
• If there is no motion, then friction opposes
the sum of all the other forces which are
parallel to the surfaces in contact.
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Friction Force
When an object is in contact with a surface there is a force acting on that
object. The component of this force that is parallel to the surface is called
the friction force.
This resistive force is exerted on a moving object due to
viscosity or other types of frictional property of the medium in or surface on
which the object moves.
Always opposite to the movement!!
Contact Force
• Force that occurs between objects
that are in contact with each other.
• Contact forces can be resolved into
components that are perpendicular
and parallel to the surfaces in contact.
• The perpendicular component is called
the normal force.
• The parallel component is called
friction.
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Contact Force in Running
Friction Force
Normal Force
Resultant
runner’s
force on
push
runner
During the push off phase in running, the normal force acts
upward on the runner, while the friction force acts forward on
the runner. The friction force is the only force capable of
moving the runner horizontally down the track. The normal
force can only accelerate the runner upwards.
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Friction and the Normal Force
• The maximum frictional force is
proportional to the normal contact
force.
• An increase in the normal force results
in an increase in the maximum friction.
• This is because the molecules on the
two surfaces are pushed together more,
thus increasing their interactions.
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“Normal” Forces and Frictional
“Normal” means
Forces
perpendicular
Reaction Force
From Ramp
Normal
Force
Friction
Force
Decompose Vector
Weight of block
Weight of block
Friction Force = Normal Force  (coefficient of friction)
Ffriction = Fnormal
Friction and Surface Area
• Friction is not affected by the size of
the surface area in contact.
• If the normal force remains constant,
but the contacting surface area is
increased, then the normal force is
spread out over more molecules, thus
the force on each molecule is reduced.
– Amontons (1699)
• What about race car tires?
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Calculating Friction
• Ff_max = FN
• Ff_max is the maximum force of friction
•  (Mu) is the coefficient of friction
• FN is the normal force
• Friction can range in value from -Ff_max to +Ff_max
•  depends on the types of surfaces that are interacting.
It would be low for rubber on ice, but high for rubber
on asphalt. It also depends on whether the surfaces are
moving relative to each other ( static or dynamic )
3/24/2016
Dr. Sasho MacKenzie - HK 376
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Types of Friction
Dry Friction
Fluid Friction
Occurs between the non-lubricated
surfaces of solid objects
Occurs with fluids,or
lubricated surfaces
Static Friction
When dry friction acts between
two surfaces that are not
moving relative to each other
3/24/2016
Dynamic Friction
>
When dry friction acts
between two surfaces that are
moving relative to each other
Dr. Sasho MacKenzie - HK 376
10
Static Friction
 the force exerted on a stationary object
by a surface that prevents the object
from starting to move (Fs)
 the object remains at rest because the
static friction is equal in magnitude and
opposite in direction to the applied force
 for the object to move, the applied force
must overcome the maximum amount of
static friction (Fsmax)
Static Friction
The Force of Static
Friction keeps a
stationary object at
rest!
f s  FN   s
FN
F
fs
Fg
 s  coefficient of static friction
Types of Kinetic Friction
 sliding friction : an object is scraping or
sliding across a surface
 rolling friction : an object rolls across a
surface
 fluid friction or air resistance : an object
travels through water or air and
experiences drag
Kinetic Friction
Once the Force of Static
Friction is overcome, the
Force of Kinetic Friction
is what slows down a
moving object!
FN
Motion
F
fk
Fg
f k  FN   k
 k  coefficient of kinetic friction
Kinetic or Dynamic Friction
 the force exerted on a moving object by a
surface (Fk)
 acts in the opposite direction of the
motion
 if applied force is equal in magnitude to
the force of friction, the object will move
with a constant velocity
 if the applied force increases, the object
will accelerate
“Fluid” Friction
This type of friction is what happens with
liquids and gases (In Physics, liquids and
gases are both called "fluids". They
behave in similar ways.)
Fluid friction is also known as "drag". On
aircraft it's also called "air resistance".
It depends on:how thick the fluid is
(its "viscosity")
the shape of the object
the speed of the object
Friction
Friction force (N)
Ff =  Fn
Normal force (N)
Coefficient of friction