# Types of Frictional Force

Oct 19, 2022

## Types of Frictional Force

Friction is a natural phenomenon which resists the motion of a solid material body upon another solid surface or liquid mass or gaseous atmosphere. Therefore, if there is a relative motion between two surfaces, a frictional force is exerted on the body in a direction opposite to the direction of movement of that body surface.

Frictional force comes to play in action when any solid body moves or tend to move –

1. upon another solid surface
2. in a liquid mass or
3. in a gaseous atmosphere.
4. Frictional force also occurs when, one liquid layer slides or tends to slide over another layer.

Therefore, frictional force is a natural phenomenon which resists the motion of a solid material body upon another solid surface or liquid mass or gaseous atmosphere. It always acts in a direction opposite the the direction of motion of surfaces.

## Types of Frictional Forces

Depending upon the surfaces in interaction, the frictional force can be classified in following manner –

### 1) Dry Friction or Solid Frictional Force

This is the friction experienced between two contacting solid surfaces.

This is of two types –

1. When a solid body is in rest it may have two types of friction –
1. Static friction – This the lowest friction experienced by a body in rest.
2. Limiting friction – This is the highest friction experienced by a body in rest.
1. Sliding friction – This is the friction when a body slides over another surface.
2. Rolling friction – This is the friction when a body rolls over another surface.

### 2) Fluid Frictional Force

This is the force of friction experienced by fluid layers. It is also called as viscous force of friction.

### 3) Skin Frictional Force

This is the force of friction felt by a solid body surface when it is dragged in a fluid environment.

### 4) Internal Frictional Force

When a solid body undergoes deformation, then friction develops between its molecules. This is called molecular friction.

In a diagram, different types of friction can be represented as given below –

021301 DIFFERENT TYPES OF FRICTIONS

## Dry Friction or Solid Friction

Dry friction or Solid friction acts between two solid surfaces which are in contact.

It is the most common type of friction experienced by us in our daily life. It is further classified depending upon the state of rest or motion of body as follows –

When a solid body is in rest, it may have two types of friction –

### (i) Static friction

021302 STATIC FRICTION

Consider about a body of weight ( W ) resting on a horizontal rough plane AB . Let, a pulling force ( P ) is applied in to move the body.

Consider that, initially the pull ( P ) is so small such that it doesn’t move. In this condition the body is said to be in static equilibrium in the state of rest . The effect of applied pulling force ( P ) is neutralizes by force of friction ( f ) developed between body surface and plane AB as shown in figure.

Since the body is in static equilibrium, hence the developed frictional force ( f ) is called Static friction.

Now consider that, the applied pull ( P ) is increased by small amount such that body doesn’t move. In this case also the effect of increased applied force of pull ( P ) is balanced by the increased force of friction ( f’ ) between body and plane.

In this way, if we go on increasing the pull ( P ) , the developed frictional force ( f ) also goes increasing and resisting the movement of body.

This means that the magnitude of static friction acting on a body at rest is equal to the applied force on that body.

Therefore, Magnitude of static friction = Magnitude of applied force to move the body from rest.

### (ii) Limiting friction

By increasing the pulling force, a situation will arise when the the force become ( P_ ) and body is just at the point of motion. In this situation the developed frictional force ( f_ ) , will be the maximum possible value of static friction. This is called Limiting friction.

Therefore, Limiting Friction = Maximum possible value of static friction required for a combination of surfaces in contact before body starts to move.

## Kinetic friction

When a solid body is in motion, it will have Dynamic or Kinetic friction.

If the pulling force is further increased just a little more from ( P_ ) , the body commences to move on the surface. The amount of friction now experienced by the body is called dynamic friction or kinetic friction.

Now, the body has started to move and continue to move under the action of pull ( P’ ) which is now just greater than the value of ( P_ ) .

If the pull ( P’ ) is just decreased a little bit to the value ( P_ ) or below it, then the body should come to rest. But it not happens. It is seen that body still continues to move on though value of pull is now considerably less than the value of pull ( P_ ) at limiting frictional force.

This implies that, friction acting on a moving body is lower than the friction acting on body when it is in the state of rest or just to move.

Therefore, kinetic friction is always less than limiting friction or maximum static friction for a given pair of surfaces in contact.

Kinetic friction is of further classified in two types –

### (i) Sliding Friction –

When a body slides or tends to slide on another surface, the developed friction is called Sliding friction.

### (ii) Rolling Friction –

When a body rolls or tend to roll on a surface, the developed friction is called Rolling friction.

In general sliding friction is always greater than rolling friction between same pair of contact surfaces. This is the reason that, to move a wooden log, it is always easier to roll it in comparison to drag it on the surface.

## Fluid friction

Friction developed in the flow of different layers of a fluid is called fluid friction.

As the fluid viscosity increases, fluid friction also increases.

## Skin friction

When a body is dragged in a fluid or gaseous environment, the felt friction is called Skin friction.

Movement of hydroplanes, boats, submarine, air planes etc. experience this type of friction.

## Internal friction

When a solid body undergoes deformation under the action of a load, molecular friction comes to play which resist the deformation. This is called internal friction.

## Comparison between different Frictional Forces

Suppose, a body is at rest on a surface. A force is applied on this body to move. Different types of frictional forces will develop at different stages of body from rest to motion.

Different types of frictional forces acting on the body is represented in an illustrative diagram as shown –

021303 COMPARING DIFFERENT FRICTIONS

## Frictional Force: Definition, Types, Calculation of Force of Friction

Friction is the force of resistance offered by surfaces that are in contact with each other. It is the force of resistance experienced by an object or body from another when they are either moving or rubbed against each other. Frictional force is a contact force i.e. for an object to experience friction, it must be in contact with another object. For e.g. walking on the road, writing on a notebook or blackboard or on any surface, sliding on a slide, or applying brakes on a vehicle.

Friction depends on three factors namely:

• Roughness of the surface
• Mass of the object
• Area of contact

Let us learn more in detail about frictional force, its types, and the steps to calculate the force of friction along with FAQs.

## Frictional Force

Work done by the frictional force is always negative. This is because frictional force is an opposing force. The force experienced by the object works in the direction opposite to that of the direction of applied force or projectile motion.

The formula for calculation of frictional force is

Here, F= force of friction

= coefficient of friction

The S.I. Unit of friction is Newton and is denoted by N.

## Types of Frictional Forces

Force of friction that takes place between two surfaces is classified as static and kinetic friction. Kinetic friction is further of two types: rolling and sliding friction. The friction that takes place between fluids (liquids and gases) is known as fluid friction. The broadly frictional force is of two types, let us study them:

• Dry Friction
• Fluid Friction

### Dry Friction

It is the force of friction between two solid bodies in contact that are in motion (kinetic friction) and not in motion i.e. static friction. We can say that dry friction is of two types- kinetic friction & static friction. It generally arises due to the interaction unevenness of the two surfaces. For e.g. rubbing hands, lighting matchsticks, and writing on paper or blackboard.

#### Static Friction

The opposing force experienced when we try to move a stationary object on a surface without actually triggering relative motion between the body and the surface is known as static friction. For e.g. a car parked on a hill, papers on a table, clothes hanging in a rack.

#### Kinetic Friction

It is the force of friction acting between two moving bodies or surfaces. A moving body experiences force in the opposite direction with respect to its motion. This opposing force in moving bodies is known as kinetic friction. For e.g. brakes applied in a moving vehicle, rubbing sandpaper on a surface, or movement of a snake. Kinetic friction is again of two types- rolling and sliding friction. Let us study more about them:

Rolling Friction:

It is the force that opposes the motion of a rolling body. Whenever a ball is rolling on the ground, its motion comes to halt after a few seconds, this is because the ball is experiencing rolling friction, which opposes its motion. For e.g- ball bearings experience rolling friction, car tires, bowling ball, etc.

Sliding Friction:

The resistive force existing between two bodies while sliding against each other is known as sliding friction. This force is required to keep the surface sliding over the other. For e.g. sliding down a slide, or sliding a block on the floor.

### Fluid Friction

The force that opposes the flow of fluids is known as fluid friction. The fluid is responsible for the friction between two surfaces. These are generally known as greasy or highly viscous in the case of highly resistive fluids in fluid dynamics. For e.g. air pushing our hand backwards when we stick it outside a car’s window, water pushing against our body while swimming, submarine in the water.

## Calculation of Force of Friction

Let us learn how to calculate the force of friction,

We know the formula for friction is given as

Here, F= force of friction

= coefficient of friction

In order to calculate this we have to follow certain steps-

Finding Normal force (N):

A normal force is a force acting on the object that is in contact with another stable or stationary object. It can be simply found by the formula-

m= the mass of the object & g= acceleration due to gravity.

The value for g = 9.8 (m/s^2)

Now when the surface is inclined, the strength of the normal surface is reduced the more the surface is inclined and the formula becomes:

Finding Coefficient ( ):

The coefficient required for use, depends on the situation and objects in the specific situation.

If the object is not moving, we use ( _) and if it is moving or sliding, we use ( _).

We put these values in the equation and solve them.

Let us take an example for calculating frictional force:

Example. Consider a wood block of 2-kg mass on a wooden table, being pushed from stationary. Calculate the force of friction.

Solution:

Here, we are given that,

Mass of wooden block (m) = 2kg

We know acceleration due to gravity (g)= 9.8 (m/s^2)

( _) = 0.25 to 0.5 for wood, taking it as 0.5

By using the formula,

Therefore, Force of friction = 9.8N

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 If you are checking Frictional Force article, also check the related maths articles in the table below: Types of friction Laws of friction Nodal analysis AC circuit Ampere Types of circuits

## Frictional Force FAQs

Ans.2 Friction is an opposing force i.e. it acts in the direction opposite to the direction of applied force or motion.

## Common Forces in Mechanics and Types of Forces

Common Forces in Mechanics are friction force, drag force, tension, spring force, pseudo force, etc. They are seen in most of our daily life problems. Hence, we call them Common Forces in Mechanics. We studied how the forces act and work. Let’s understand the concept of force and the different nature of these forces.

## Force and its Types

The interaction, which after acting on a body, changes or tries to change the state of rest or state of motion of the body is called force. There are six types of forces in nature.

### Conservative force

The force whose work is independent of the path followed is called conservative force. The work done by the conservative force only depends on the initial point and the final point. Hence, the work done by a conservative force in a closed loop is zero.

Example: Gravitational force, Electrostatic force, elastic force

To get the details on Kinetic Theory of Gases, candidates can visit the linked article.

### Non-conservative force

If the work done by a force depends on the path followed by the particle then it is called the non-conservative force.

Example: Friction force, air drag force, etc.

### Constant force

The force that does not change with respect to time and distance is called constant force. It always remains constant.

### Variable force

The force that changes with respect to time and distance is called variable force. It is a function of either time or distance or velocity etc.

### Contact Force

Requires direct contact between the body and the agency of force.

For details on Distance and Displacement, refer to the linked article.

### Non-Contact Force

Does not require direct contact between the body and the agency of force.

In mechanics, we deal with only one type of non-contact force, that is, Gravity. There are other non-contact type forces like magnetic force, electrostatic force, etc. However, they do not come under the branch of mechanics. We shall know about them in the upcoming articles. However, many questions asked in the exams are majorly based on the different types of Contact Forces. Hence, we shall study them.

## Types of Contact Forces

1. Tension force
2. Friction force
3. Spring force
4. Drag force
5. Pseudo force

Let’s discuss each one in detail.

### 1. Tension force

It is a tension in the string due to which it balances the mass tied to it.

### 2. Friction Force

“When two objects are in contact and one object moves or intends to move, then a force develops between the two surfaces called the frictional force”

In other words- “Friction is the force resisting the relative motion of solid surfaces”

For example, there is a friction force in the tyres of the car moving on the road. When we push a heavy stone and the stone is not moving, it is due to friction force that opposes the motion.

The friction force is usually denoted by “f”.

Friction force (f) = μ N

Where N is the normal force and μ is the coefficient of friction.

You can also check details about Vector.

#### Types of Friction

Depending on the state of rest and state of motion, friction is classified into the following:

#### Static Friction:

The friction acting between the surfaces of two bodies when they are not moving with respect to each other but tends to move is called static friction. It is the friction when the body is at rest.

For example: When a block is kept at rest on a table and we apply some force to move it but the block is not moving. Then the friction acting, in this case, is static friction. Here force applied is balanced by the static friction force.

The static friction is adjusting friction. When the external force applied is increased then the value of static friction also increases until it reaches a certain maximum value. This maximum value is called the limiting value of static friction.

The static friction only comes into the picture when there is an external force applied to the body. If a body is kept at rest and no force is applied to move the body then no static friction will act.

The limiting value of static friction is independent of the area of contact. It only depends on the nature of the surface and the normal force.

Where (μ_s) is the coefficient of static friction and N is the normal force acting on the block.

In general, the force of static friction is given by:

If the external force (F) exceeds this limiting friction ((f_s)_) then the block starts moving on the surface.

Q. Determine the maximum acceleration of the bus in which a block lying on its floor will remain stationary, given that the coefficient of static friction between the block and the bus floor is 0.25.

Ans. Here the acceleration of the block will be due to the static friction by the floor.

Coefficient of static friction((μ_s)) = 0.25

Let a is the maximum acceleration of the bus.

So the force on the block (F) = ma

Normal force (N) = mg

Thus the maximum acceleration of the bus = 2.5 (m/s^2)

#### Kinetic Friction

The frictional force that opposes the relative motion between two surfaces is called kinetic friction. It is denoted by (f_k).

This friction comes into the picture only when there is a relative motion between two surfaces.

It is also independent of contact area and depends only on the nature of the surface and the normal force.

Kinetic friction ((f_k) = μ_k N)

Where (μ_k) is the coefficient of kinetic friction and N is the normal force.

Check Power in AC Circuit for details here.

Q. What is the acceleration of the box and block system shown in the figure if the coefficient of kinetic friction between the box and the surface is 0.05? Also, find the tension in the string (Take g = 10 (m s^)). Neglect the mass of the string.

Ans.

Coefficient of kinetic friction ((f_k)) = 0.05

Let a be the acceleration of the block and the box.

For box; Normal (N) = 200 N

Friction ((f_k) = μ_k N) = 0.05 × 200 = 10 N

Acceleration (a) = 20/23 = 0.87 (m/s^2)

Tension (T) = 10 + 20 a = 10 + 20 × 0.87 = 27.4 N

#### Rolling Friction

When a body is rolling on a surface then the friction acting between the surface of the body and the surface is called rolling friction.

In the case of rolling motion, there is no relative motion of the point of contact of the surface with respect to the surface on which the body is moving.

During the rolling motion of a body, the surfaces in contact get momentarily deformed a little, and this results in a finite area (not a point) of the body that is in contact with the surface. The net effect is that the component of the contact force parallel to the surface opposes motion.

Rolling friction is denoted by ((f_R)).

For example: When a wheel of the car is rolling on the road then the friction by the road on the wheel is rolling friction. Friction on the wheel of cycle.

### Points to Remember

• The static coefficient of friction is greater than the kinetic coefficient of friction.
• The coefficient of friction depends on the nature of the surface and the temperature.
• Rolling friction is always less than static friction because in order to roll any object over a surface the force of friction between object and surface must be large enough so that it can’t slide on it and perform a pure rolling motion.
• The work done by the rolling friction is zero because there is no relative motion of the point of contact.

Also, check the Types of Thermodynamic Process in detail to boost your preparation.

### 3. Drag Force

Drag force is the resistance faced by bodies inside a fluid. Fluids include both liquids and gases.

C is the drag coefficient,

A is the area of the object facing the fluid,

and ρ is the density of the fluid.

Consider an object falling through the air under the effect of gravity. The two forces acting on it are the force of gravity and the drag force. The downward force of gravity remains constant regardless of the mass and velocity of the object. However, as the object velocity increases, the magnitude of the drag force also increases until the magnitude of the drag force is equal to the gravitational force. Thus it produces a net force of zero. Zero net force means that there is no acceleration. This can be shown by Newton’s second law. At this point, the object’s velocity becomes constant and we say that the object has reached its terminal velocity.

At the terminal velocity,

Using the equation for drag force, we have

Solving for the velocity, we obtain

### 4. Force in Spring

It is a restoring force that restores the spring into its original length.

F = – K x where k is spring constant, x is elongation or suppression of spring length

A negative sign shows that it works in opposition to the applied force.

To get the details on Nuclear Physics, candidates can visit the linked article.

### 5. Pseudo Forces:

When we observe any object with respect to an accelerated body then there is a force acting on the object which we are observing. This force is called a pseudo force.

Pseudo force is an imaginary force.

Pseudo force ((F_P)) = ma

Where m is the mass of the object which we observe and a is the acceleration of the body with respect to which we are observing the object.

For example: If we are sitting in an accelerating car and observing a body outside the car, then a pseudo force will act on that body.

## Types of Non-Contact Forces

These forces act at a distance.

### 1. Gravitational Force:

The force of gravity exerted on one object by another due to its mass is called gravitational force. It is directly proportional to the product of those objects’ masses and inversely proportional to the square.

### 2. Electrical Force:

The repulsive or attractive interaction between any two charged bodies is called an electric force.

### 3. Magnetic Force:

Magnetic force is the attraction or repulsion that arises between electrically charged particles because of their motion.

For details on Oscillations, refer to the linked article.

We hope this article helped you to understand the important concept of the Common Forces in Mechanics. For more such informative articles, updates, and notes, get the Testbook App now! Download the App to access free mock tests and practice sets and enhance your exam preparations today!

 Check out other Important topics which will help you crack the examination: Mechanical Properties of Fluids Fluid Dynamics Thermal Properties of Matter Thermal Conductivity Quantum Mechanics Optics Interference of Light Ray Optics Electromagnetic Devices Mechanical Properties of Solids

## Common Forces in Mechanics FAQs

Ans.1 When a body is rolling on a surface then the friction acting between the surface of the body and the surface is called rolling friction.

Ans.2 The friction acting between the surfaces of two bodies when they are not moving with respect to each other but tends to move is called static friction. It is the friction when the body is at rest.

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