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Friction is an interested phenomena which makes an attention to new products and technologies over a decade of years. When we talk about the friction the contact between two objects is also have to be considered. Both contact and friction always go together and are merged in many ways in real systems. For example, consider sledding with out snow. It is not very easy. We have to push hard to make the sled move. Here friction plays a role. Fro this example, we can say that friction is a force that slows the speed of an object when they rub together. Rough surface like grass produce more friction than smooth surface like snow. So, generally friction is resistive force which opposes the motion of a particles. There are different types of friction; dry friction, fluid friction, lubricated friction, skin friction, internal friction etc. When two surfaces comes in contact with each other, the kinetic energy is converted into heat. It is not a fundamental force but it arises from the electromagnetic force which is a fundamental force.

What is Friction?

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A force on substance which is in contact with other surface and it opposes the motion of the body. This resistive force is the frictional force. There are two different kinds of friction, one is the static friction and other one is the kinetic friction. Static friction is nothing but a frictional force arises when the two objects are not in motion. It increases up to a limit and then it starts to move. So we can say that it is the force which can sustained by an object before motion. The other type is kinetic friction. It arises due to the motion of an object along a surface. In the case of two hard surfaces, the kinetic friction is less than that of the static friction. That is more force is needed to move the objects is at rest than to keep them in motion. The further details about the friction is comes under the following sections.

Definition of Friction

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Friction is defined as the force that resists the motion of two objects which is in contact with each other. Smooth surface causes less amount of friction whereas rough surface exhibits larger amount of friction. The main types of friction are static friction and kinetic friction.

Friction Equation

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The general equation for calculating the frictional force between the two objects is the product of coefficient of friction and the normal force which is pushing the two objects. The mathematical representation of the above statement is given below:
F = μN
F is the resistive force
μ is the coefficient of friction
N is the normal force acting on the object

Friction Units

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The SI unit of friction is Newton (N). The coefficient of friction is unit less. Because it is given as force divided by force. The dimensional formula is given as MLT-2 .

Friction Force

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There are mainly two types of friction related how the objects are moving with respect to the other surface. The equation for this friction force is given by,

For static friction force:

F = μsN

Where F is the static friction force
μs is the coefficient of static friction
N is the normal force acting on the body

For kinetic friction force:

F = μkN

Where F is the kinetic friction force
μk is the coefficient of kinetic friction
N is the normal force acting on the body

Friction Velocity

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Friction velocity is nothing but is a form by which shear stress may be written in terms of velocity. So it is also known as shear velocity. It is used in fluid mechanics to compare the true velocities of the fluids. The equation of the friction velocity is given by,

$u_{*}$= $\sqrt{\frac{\tau}{\rho}}$
Where $u_{*}$ is the friction velocity
$\tau$ is the shear stress
$\rho$ is the density of the fluid

Friction Factor

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Friction factor can be referred as Darcy friction factor, Fanning friction factor and Atkinson friction factor. Out of these three two are related to fluid dynamics. Darcy friction factor relates the pressure loss due to the friction to the average velocity of a fluid. The second friction factor is Fanning factor. It relates the shear stress at the wall to fluid velocity and density of the fluid. Atkinson factor is a measure of resistance to the airflow of duct. Mine ventilation industry used this friction factor.

What Causes Friction?

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Friction is a resistive force between two objects.Mainly three parameters are responsible for the frictional force. They are,
  • Surface roughness
  • Adhesion or molecular attraction between the molecules
  • Deformations in the material

Friction Irregular Surfaces

Friction Graph

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The graphical representation of the frictional force is given below.
Friction Graph

Some of the pictures related to the friction is also enclosed here.

Friction Sledding
Friction Car
Friction Man

Examples of Friction

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Some of the examples of the friction is given below:
  • Striking a match
  • Writing with pen
  • Walking
  • Space shuttle
  • Skating
  • Chewing food
  • Breaks of a vehicle
  • Standing on the floor
  • Rubbing of two materials

Friction coefficient: The friction coefficient or coefficient of friction is a numerical number which represents the friction between the surfaces. So it is defined as the ratio of the force which maintain the connection between the body and surface to the force which opposes the motion of the body. Since it is the ratio of two forces, it is a unit less and dimension less quantity. Generally it is denoted by μ.

Types of friction: The important types of friction are given below:

Friction Problems

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The problems related to the friction force are given below.

Solved Examples

Question 1: Calculate the coefficient of friction if the mass of the wood piece is given as 12 Kg which is placed on another piece of wood. The frictional force is given by 35 N.

From the question it is given that,
Ff = 35 N , m = 12 Kg
First we have to find out the normal force. Hence,
$F_{n}$ = mg
$F_{n}$ = 12 $\times$ 9.8
$F_{n}$ = 117.6 N
We know the equation for friction force
$F_{f}$ = $\mu F_{n}$
$\mu $= $\frac{F_{f}}{F_{n}}$
$\mu$ = $\frac{35}{117.6}$ = 0.30


Question 2: A boy is sliding on the ice whose weight is 35 Kg. The value of coefficient of friction is given as 0.4. Determine the frictional force acting between the ice layer and the boy?

Given parameters are
m = 35 Kg, μ = 0.4
Normal force is given by
$F_{n}$= mg
$F_{n}$ = 35$\times$ 9.8 = 343 N
The equation for the friction is
$F_{f}$ = $\mu F_{n}$
$F_{f}$= 0.4$\times$ 343 = 137.2 N