FRICTION AND ITS TYPES

BY BHORANIYA AHEMAD ABBAS A.

(130460106008)

2

What is FrictionWhat is Friction• Friction is a forceFriction is a force• A frictional force can exist when two A frictional force can exist when two

substances contact each other.substances contact each other.• The molecules of each surface interact The molecules of each surface interact

according to Newton’s Laws of Motion.according to Newton’s Laws of Motion.• Friction always opposes motion, i.e., it is Friction always opposes motion, i.e., it is

opposite to the direction of velocity.opposite to the direction of velocity.• If there is no motion, then friction opposes If there is no motion, then friction opposes

the sum of all the other forces which are the sum of all the other forces which are parallel to the surfaces in contact.parallel to the surfaces in contact.

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.

Friction Force

This resistive force is exerted on a moving object due to

Always opposite to the movement!!

viscosity or other types of frictional property of the medium in or surface on which the object

moves.

4

Contact ForceContact Force

• Force that occurs between objects Force that occurs between objects that are in contact with each other.that are in contact with each other.

• Contact forces can be resolved into Contact forces can be resolved into components that are perpendicular components that are perpendicular and parallel to the surfaces in and parallel to the surfaces in contact.contact.

• The perpendicular component is The perpendicular component is called the called the normal forcenormal force..

• The parallel component is called The parallel component is called frictionfriction. .

5

Contact Force in RunningContact Force in Running

Normal ForceFriction Force

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.

runner’s pushResultant force on runner

6

Friction and the Normal Friction and the Normal ForceForce

• The maximum frictional force is The maximum frictional force is proportional to the normal contact force.proportional to the normal contact force.

• An increase in the normal force results in An increase in the normal force results in an increase in the maximum friction.an increase in the maximum friction.

• This is because the molecules on the two This is because the molecules on the two surfaces are pushed together more, thus surfaces are pushed together more, thus increasing their interactions.increasing their interactions.

““Normal” Forces and Normal” Forces and Frictional ForcesFrictional Forces

Weight of block

Decompose Vector

NormalForce

FrictionForce

Weight of block

Reaction ForceFrom Ramp

“Normal” meansperpendicular

Friction Force = Normal Force (coefficient of friction)Ffriction = Fnormal

8

Friction and Surface AreaFriction and Surface Area

• Friction is not affected by the size of Friction is not affected by the size of the surface area in contact.the surface area in contact.

• If the normal force remains constant, If the normal force remains constant, but the contacting surface area is but the contacting surface area is increased, then the normal force is increased, then the normal force is spread out over more molecules, spread out over more molecules, thus the force on each molecule is thus the force on each molecule is reduced.reduced.– Amontons (1699)Amontons (1699)

• What about race car tires?What about race car tires?

04/18/23 Dr. Sasho MacKenzie - HK 376 9

Calculating FrictionCalculating Friction

• FFf_maxf_max = = FFNN

• 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 )

04/18/23 Dr. Sasho MacKenzie - HK 376 10

Types of FrictionTypes of Friction

Dry FrictionOccurs between the non-lubricated surfaces of solid objects

Fluid FrictionOccurs with fluids,or lubricated surfaces

Dynamic FrictionWhen dry friction acts between two surfaces that are moving relative to each other

>Static FrictionWhen dry friction acts between two surfaces that are not moving relative to each other

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 FrictionStatic Friction

frictionstaticoftcoefficien

Ff

s

sNs

FN

fs

F

Fg

The Force of Static Friction keeps a

stationary object at rest!

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 FrictionKinetic Friction

frictionkineticoftcoefficien

Ff

k

kNk

FN

fk

F

Fg

Once the Force of Static Friction is overcome, the Force of Kinetic Friction

is what slows down a moving object!

Motion

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” FrictionFluid” FrictionThis type of friction is what happens with This type of friction is what happens with

liquids and gases liquids and gases (In Physics, liquids and (In Physics, liquids and gases are both called "fluids". They gases are both called "fluids". They behave in similar ways.)behave in similar ways.)

Fluid friction is also known as "Fluid friction is also known as "dragdrag". On ". On aircraft it's also called "aircraft it's also called "air resistanceair resistance". ". It depends on:-It depends on:-how thick the fluid is how thick the fluid is

     (its "viscosity")      (its "viscosity") the shape of the object the shape of the object the speed of the object the speed of the object

FrictionFriction

Ff = Fn

Normal force (N)

Coefficient of friction

Friction force (N)