# What is friction-Definition,Types/2021

What is friction? – Its Types and Explanations

## Introduction

It is the force that exists as one of the most significant phenomena in the physical world. Every object exerts friction on another object.

Each object resists motion on another object when the surfaces of two objects come in contact with each other. If we rub two sticks together then it causes fire due to friction.

While you walk on the road, you exert a contact force on the road and the road exerts the same contact force on you which are equal in magnitude but the direction is opposite to each other.

This contact force follows Newton‘s third law of motion. This phenomenon is due to friction.

Now the question arises in your mind what would be the direction of that contact force? This contact force can be resolved into two components.

One is perpendicular to the contact surface called normal contact force and the other parallels the contact surface is called friction.

## Definition

We can say that friction is nothing but a force that is generated due to the contact between two surfaces of objects and which resists the motion of an object due to sliding, rolling, or moving relative to another.

## Types

### 1. Static friction (fs)

This is the force due to which the surface of two objects that contact with each other and one object does not slide with respect to another object. It is experienced when heavy objects are kept stationary on another surface.

Explanation

Consider a heavy almirah kept on the ground. We want to shift it to another place. Suppose the almirah has its weight(Mg)which is kept on the ground and the ground exerts with normal force say ‘N’ which is opposite to it in a stationary condition. We push it with force F in the right direction to shift it. But due to heavyweight almirah does not move in a single position.

Why does it not move? It is due to the frictional force exerted by the ground on the almirah which acts in opposite directions i.e. towards the left with the same force exerted by us. In this case, frictional force exists between the surface of the almirah and the ground and this is called static friction (fs).

If we increase force F to push the almirah then this static fs also increases. It means static friction is self-adjustable.

However, this static frictional force cannot go beyond the maximum frictional force(f max).

Again when we applied a force greater the than maximum frictional force, this static frictional force fails to increase its value and object starts slipping.

The maximum static frictional force exerted on the body(i.e. almirah) is called limiting friction (f max)which is proportional to the normal contact force between two bodies.

i.e, f maxS N, where N is normal force =Mg ( M= mass of almirah and g= acceleration due to gravity=9.8 m/s2)

So for a body at rest relative to other fs≤ f maxSN, Where μS coefficient of static friction force respectively.

The maximum external force required to move an object is:-

F>fs>f max

In the above example, if we push the almirah to left then the ground pushes the almirah in the right direction. The almirah does not move until the applied force by persons is greater than the limiting frictional force that is exerted by the ground.

Laws of fs

• The fs does not depend on the area of contact as long as the normal force N is the same.
• fs is self-regulating friction.
• The limiting friction f max(the maximum external force that an object can exert on another object in contact) is proportional to the normal contact force between two objects.
• μS depends on the material and roughness of the two surfaces in contact.

Examples of fs:

• Vehicles parked on the hill
• Heavy objects placed on the table

### 2. Kinetic friction (fk)

It exists when the surfaces of two objects are in contact and they move with respect to each other. They are in contact with each other and rub their surface in contact. In this condition, frictional forces generated between them are called kinetic friction (fk)

Explanation:-

Suppose object A moves with velocity V which is placed in contact with another object’s surface B. Let body A move towards the right, then the friction acting on A is due to body B which opposes the velocity of body A and the direction of this frictional force is towards the left.

The direction of frictional force acting by A on B is towards the right which is opposite to the velocity of B with respect to A.

In this case, f k= μk N, where μk is the coefficient of kinetic friction force. N is the normal force and equal toMg

Laws of f k

• Its magnitude is proportional to the normal force acting between two objects and does not depend on the area of contact as long as the normal force N is the same.
• μk depends on the nature of surfaces in contact and the Weight of two objects in contact.
• If the surface is smooth μk will be small and if the surface is rough μk will be large.
• μk- does not depend on the speed of the sliding or slipping objects.

Examples

A box is placed on the moving car

When we rub two sticks it generates heat which causes fire

Vehicles moving on the road cause wear and tear of tyres. There is wear and tear in moving machines, so lubrication is required.

### 3. Sliding friction (Fs)

When any two object slides against each other on the inclined surface, it creates opposing friction. This resistive is also known as kinetic friction.

In this way, we can be defined sliding friction as a resistive force that opposes the movement between two contact surfaces that slide against each other on an inclined surface. It means sliding friction is needed to keep a surface sliding along another surface.

Explanation

Suppose a block B of mass m is placed on an inclined surface and the angle of inclination is θ, here the angle of inclination is adjusted so that it slides on an inclined surface. The weight of block mg has two components

1. Perpendicular to the inclined surface which is mgcosθ and equal to normal contact force N.

2. Parallel to the inclined surface which is mgsinθ which is in the direction of sliding but opposite to fk.

Hence, the component parallel to the incline,

At the equilibrium state,

Fs=fk=mgsinθ=μ kN

Component perpendicular to the incline

N=mgcosθ,

Equation of sliding friction force perpendicular to the surface, Fs=μs N= μs mgcosθ

μs=Fs/N=mgsinθ / mgcosθ

Also, μ k=F k /N

Note:- in most of the materials FS is less than fs.

### 4. Rolling friction (fr)

The force that resists the motion of a rolling body. The Rolling of the ball bearing is an example. Rolling friction is very small in comparison to static and kinetic friction and there is much less energy loss. Laws offr

• With the increase of smoothness, frdecreases.
• The rolling frictional force is directly proportional to the weight of the body and inversely proportional to the weight of the rolling body i.e. F=μ W/r

Where W = weight of the body, r =radius of the rolling body.

μr = FR/W

Where fr and μr =resistive force and coefficient of rolling friction.

W= weight of the rolling body

### 5. Fluid friction

The force exerted by fluids is called fluid friction. It occurs due to resistance between two layers of fluid. Also, it is called viscous force or Viscosity. It prevents the flow of fluid.

An example of honey is a fluid that has more fluid friction than water.

## Laws of friction

• It depends on the contact surface.
• If the object slips over each other then, kinetic friction force fk= μk N, Where, N= normal contact force, μk= coefficient of kinetic friction between the surfaces.
• The direction of kinetic friction on a body is opposite to the relative velocity between bodies.
• In static friction, the force of friction is given by fs ≤ f max=μS N where μS =coefficient of static friction between two bodies.
• fs and f k do not depend on the area of contact as long as the normal force N is the same.
• In sliding, the angle and position of the object affect the amount of friction force
• If the flatted object is placed on other objects then friction force is equal to the weight of the object i.e. mg, where m= mass of an object and g=acceleration due to gravity=9.8 m/s².
• Generally, μs is slightly greater than μk

Q 1. Why friction is important?

Ans. Friction is important in our daily life. It helps us to walk on the road because it prevents our legs to slip while walking or running. It stops car tyres from skidding on the roads. It helps vehicles to park on the hill and other many things stay in their position due to friction.

It acts to grip the ground and prevent sliding, rolling, and slipping.

Q 2. Why static friction(fs) is greater than kinetic friction(fk)?

Ans. The aim of the answer to this question is whyfs≥fk, wherefsandfkarestaticandkinetic friction.

Ist of all we should understand static and kinetic friction

(fs):-

It is defined as the frictional force which tends to act between two bodies that are in contact with each other but do not slide or slip with respect to each other. The friction that generates which tends to oppose sliding or slipping is called static friction.

This friction tends to oppose sliding or slipping unless the maximum friction is greater than static friction.

This maximum friction(f max )is called limiting friction which is

f max=μS N

So we can write,

For a body at rest relative to others,fs≤ f max=μS N

μS=coefficient of static friction between the bodies and depends on the material and roughness between two surfaces in contact.

N is the normal force between them.

(fk):-

It is defined as thefrictional force which acts between two moving objects and the surfaces of moving objects that are in contact with each other.

In this case, f k= μk N, whereμkis the coefficient ofkinetic friction.N is the normal force and equal to mg

m=mass of an object, g= acceleration due to gravity=9.8 m/s2.

• μk depends on the nature of surfaces in contact and the Weight of two objects in contact.
• If the surface is smooth μk will be small and if the surface is rough μk will be large.
• μk- does not depend on the speed of the sliding or slipping objects.

The static condition creates when the object is in static condition and the kinetic condition acts when two objects move with each other.

In general, μs is found slightly greater than μk.

So the value μS N is greater than μk N, where N is the same.

Hence,fs≥fk.