# What is Frictional Force?

Last Updated : 05 Oct, 2023

Frictional Force or Force of Friction is the opposing force when two surfaces come in contact with each other to move either in the same direction or in opposite directions. As friction is a force that opposes or resists the motion of an object that comes in contact and slides against each other. Hence, the frictional force is a Contact Force. Friction is a force that always opposes motion. The frictional forces are majorly affected by the texture of the surface of the objects that came in contact with each other and the amount of force exerted on them.

## What is Frictional Force?

The frictional force is the force created when two surfaces come into touch with each other and slide against each other. These forces are mostly influenced by the surface roughness and the amount of force required when they are combined. The volume of frictional force is affected by the angle and position of the object. The forces of attraction are the primary cause of friction between objects.

The frictional force between two surfaces always acts in opposition to the applied force, as shown in the figure above. The surface’s texture, the amount of force pushing them together, the position of the object, the angle of the force applied, etc. are some factors that have a big impact on the frictional force. The force of friction is equal to the mass of the thing when it is flat against another object. When we push an object up against the surface of another object, the frictional force increases and exceeds the weight of the pushed object.

## Frictional Force Formula

The maximum frictional force applied on an object by a surface can be easily calculated by the following formula:

Ffriction = Âµ Ã— N

where,

### Unit of Frictional Force

As Frictional Force is a force acting on an object, the unit and dimension of frictional force are the same as force.

• The unit of frictional force is kg.m/s2 or Newton (N).
• The dimensional formula of frictional force is [M L T-2].

## Calculation of Frictional Force

There are three steps involved in the process of calculating the force of friction i.e.,

Step 1: Finding Normal Force.

The normal force i.e., represented by N, is the force exerted by a surface to support the weight of an object. The direction is this force is always perpendicularly outward from the surface on which the object is resting. For any object resting on the horizontal surface, the normal force balances the weight of the object, thus it can be calculated the same as the weight of the object i.e., mg [where m is the mass of the object and g = 9.8m/s2 is the acceleration due to gravity]. However, if the object is on an inclined plane or if other forces are present, the normal force may be different. So, determine the normal force based on the specific situation.

Step 2: Finding the right Coefficient of Friction.

The coefficient of friction (Âµ) is a relative coefficient that represents the interaction between any surfaces in contact. It shows the resistance between surfaces is there to start any motion. The coefficient of friction depends on both surfaces and their nature and the coefficient of friction for some common surfaces is given in the following table:

Surfaces

Static Friction

Kinetic Friction

Aluminium on Aluminium (Dry)

1.05-1.35

1.4

Steel on Steel (Dry)

0.6

0.4

Steel on Steel (Greasy)

0.1

0.05

Teflon on Steel

0.41

0.04

0.3

Silk on Silk (Clean)

0.25

Rubber Tires on Dry Pavement

0.9

0.8

Metal on Ice

0.022

0.02

Rubber Tip of Crutch on Rough Wood

0.7

Step 3: Calculating Frictional Force using Formula Ffriction = Âµ Ã— N.

After calculating the normal force and coefficient of friction, you can calculate the frictional force using the formula:

Ffriction = Âµ Ã— N

The resulting force acts opposite to the direction of the attempted motion or motion of the object.

## Factors Affecting Frictional Force

The magnitude of the frictional force depends on the following factors listed below:

• Surface textures and the amount of force pushing them together have a major impact on these forces.
• The amount of frictional force is affected by the position and the angle made by the object.
• The frictional force will be equal to the weight of the object if it is put flat against another object.
• The frictional force will increase and exceed the weight of the thing if an object is pushed up against the surface.

However, the frictional force between two surfaces is independent of the actual area of contact.

## Types of Frictional Forces

The following are the two major types of frictional forces, based on the type of material in contact, that are:

• Dry Friction
• Static Friction
• Kinetic Friction
• Rolling Friction
• Sliding Friction
• Fluid Friction

## Dry Friction

When two solid bodies come into contact, both when they are moving (kinetic friction) and when they are not, there is a reaction known as Dry friction (static friction).

The normal force applied between the solid bodies determines how much friction there is, both in static and kinetic terms. Different coefficients of friction are used to represent the interaction of various materials. Hence, this implies that for a given normal force between two substances, certain substances have a stronger resistance to movement than others.

Based on the type of motion between the two objects, the dry frictional force may be classified further as:

### Static Frictional Force

Static friction is the amount of resistance that exists between an object and the surface it is resting on. The basic condition for moving an object that is resting on a surface involves applying a force that is greater than the frictional force generated by the surface. Another name for it is limiting frictional force. Static friction includes things like walking, rock climbing, and other activities.

### Kinetic Frictional Force

When the box is sliding, kinetic friction develops beyond the point of coming motion. When there is kinetic friction, the normal force between the box and the surface multiplied by the kinetic coefficient of friction will equal the amount of friction force opposing motion. Although it also rests on the two objects in contact, the kinetic coefficient of friction is nearly always less than the static coefficient.

### Rolling Frictional Force

Rolling friction is defined as the resistance created when one body is forced to roll across the surface of another. In comparison to kinetic friction, it is considerably less. Some of the applications where rolling friction can be easily seen include roller skates, ball bearings, etc.

### Sliding Frictional Force

When one object is dragged across the surface of another, sliding friction results. Compared to static friction, it is weaker. Examples of sliding friction include dragging a block that is kept on the table, writing, playing on a slide, etc.

## Fluid Friction

A substance that can flow and can adopt the shape of the container in which it is kept is called a fluid. Fluid friction is the term for the resistance that a liquid or gas offers to a moving object. Hence, in simple words, the frictional force exerted by fluids is called the fluid friction.

The air resistance force or drag force is the term used to describe the frictional force produced by the air. Fluid friction makes it challenging to walk through a pool of water in the middle of a storm.

### Fluid Friction Examples

There are many examples of fluid friction around the daily life of a common being. Some of these examples are as follows:

• Lubrication of hinges in doors, machines, or anything else is the most common example of Fluid friction, where we use lubricants (fluids with less frictional coefficient) to ensure the smooth operation of the hinges in doors or machines.
• A flying paper plane is also an example of fluid friction, as without the frictional force acting on it by air it can’t sustain its flight in the air. If we try to fly a paper plane in the vacuum chamber, it won’t be able to fly at all.
• The working of parachutes is another example of fluid friction, where we leverage the concept to make the world a better place.

## Frictional Force Examples

Frictional Forces are an essential part of our daily life without these we can’t even able to walk properly. Some of these different real-life examples of different types of frictional force  experiences are as follows:

### Sliding a Sledge

Sliding a sledge over an icy surface is a good example of Sliding friction. The amount of friction between an object and the surface is reduced by the ice that has been deposited on the land or ground. The lack of friction on the surface makes objects on it easily slide and slid, that’s why it slides efficiently on snow or sand but not on concrete.

### Walking

Walking is only possible when there is a sufficient amount of force of friction between the ground and the feet. As frictional force is a contact force, it can only exist between the points of the intersection. If there is any substance with less coefficient of friction, then walking becomes hard as the frictional force between the ground and the feet decreases.

### Climbing

A force that tends to resist the climber’s ability to move on the rock can be felt when they do. The frictional force is the term given to this force. Rock climbing depends on friction to create a secure hold between the climber’s feet or hands and the rock’s surface. It helps climb the rock and keeps the climber’s hands and feet from slipping.

### Rubbing Hands

Sliding friction is illustrated by rubbing hands together. A force that opposes this motion of the hands is activated when they brush against one another. The frictional force is the force resisting motion that is exerted in a direction that is counter to the direction that the hands are moving, and it is what creates heat energy.

## Solved Problems on Frictional Force

Problem 1: A box of mass 5 kg is being pushed along a horizontal surface with a force of 20 N. The coefficient of friction between the box and the surface is 0.2. Calculate the frictional force acting on the box.

Solution:

The frictional force can be calculated using the formula

i.e., Ffriction = Âµ Ã— N

Since the box is on a horizontal surface, the normal force acting on it is equal to its weight,

N = 5 x 9.8 = 49 N.

Thus, frictional force = 0.2 x 49 N = 9.8 N

Problem 2: What is the minimum force required to move a 10Kg box which is placed on a flat surface with a coefficient of static friction of 0.5?

The minimum amount of force required to move a box is equal to the static friction.

i.e., Fs = Î¼sN

where Î¼s is the coefficient of static friction and N is the normal force.

N = mg = 10 Ã— 9.8 = 98 N

Therefore, Fs  = 0.5 Ã— 98 N = 49 N.

The minimum force required to start moving the box is slightly larger than F_s, so we’ll apply a force of 50 N.

Problem 3: A car of mass 1000 kg is moving on a flat road. The coefficient of kinetic friction between the tires of the car and the road is 0.3. Calculate the force required to maintain a constant speed of 20 m/s.

Solution:

The force required to maintain a constant speed is equal and opposite to the frictional force acting on the car.

The frictional force can be calculated using the formula,

i.e., Ffriction = Âµ Ã— N

Since the car is on a flat road, the normal force acting on it is equal to its weight,

N = 1000 x 9.8 = 9800 N.

Thus, frictional force = 0.3 x 9800 N = 2940 N.

Hence, the force required to maintain a constant speed of 20 m/s is 2940 N and direction of this force is opposite direction to the frictional force.

Problem 4: A block of mass 10 kg is placed on a horizontal surface with a coefficient of friction of 0.2. If a force of 50 N is applied to the block, what is the acceleration of the block? (Use g = 10 m/s2)

Solution:

Given m = 10 Kg, Âµ = 0.2 and Applied force = 50 N

The normal force is equal to the weight of the block,
i.e., N = m x g = 10  x 10 = 100 N

Thus, Frictional force = Âµ Ã— N = 0.2 x 100 N = 20 N

Since there is a force of 50 N acting on the block, the net force is:
Net force = 50 N – 20 N = 30 N

The acceleration of the block can be calculated using the formula:
Net force = mass x acceleration
â‡’ Acceleration = Net force / mass = 30 N / 10 kg = 3 m/s2

Therefore, the acceleration of the block is 3 m/s2.

## Practice Problems on Frictional Force

1. A 1000 kg car is parked on a flat road. If the coefficient of static friction between the tires and the road is 0.75, what is the maximum force that can be applied horizontally to the car before it starts to move?

2. A 50 kg box is sliding on a horizontal surface with a coefficient of kinetic friction of 0.2. If a horizontal force of 100 N is applied to the box, what will be its acceleration?

3. A 200 g book is initially resting on a wooden table with a coefficient of static friction of 0.4. If the table is gradually tilted, at what angle (in degrees) will the book start to slide?

4. A 500 N block is placed on an inclined plane with an angle of 30 degrees. The coefficient of kinetic friction between the block and the plane is 0.3. Calculate the net force acting on the block parallel to the inclined plane if it is moving at a constant velocity.

5. A 10 kg block is hanging from a rope over a pulley. The coefficient of kinetic friction between the block and the surface is 0.2. If the block is released from rest, what will be its acceleration?

## FAQs on Frictional Force

### 1. What is Frictional Force?

The frictional force is the force created when two surfaces come into touch with each other and slide against each other.

### Q2: Which Factors influence Frictional Force?

The two important factors that influence the friction force the most are:

• Nature of the two surfaces in contact, and
• Nature of the force that are acting on these surfaces.

### 3. How to Calculate Frictional Force?

The frictional force can be calculated by using the following formula:

Ffrict = Âµ Ã— Fnorm

where,

• Âµ is the coefficient of friction, and
• Fnorm refers to the normal force acting upon the said object, given as F = mg (where m is the mass and g is the acceleration due to gravity).

### 4. What is Unit of Frictional Force?

The unit of frictional force in SI unit is kg.m/s2 or Newton (N).

### 5. Explain how Lubricant Decreases Frictional force between the objects.

The lubricants cover the surface imperfections where the contact is made. Thus, friction is decreased when moving objects make touch with a lubricant.

### 6. Give two Examples of Frictional Force.

Following are the two examples of frictional force:

• Only when there is a force of friction between the ground and the feet can one walk. Since friction is a contact force, it exists between the points of the intersection. It helps in creating a solid connection between the foot and the ground.
• A force that tends to resist the climber’s ability to move on the rock can be felt when they do. Frictional force is the term given to this force.

### 7. How the Maximum of Static Friction is Calculated?

The maximum force of static friction is equal to the coefficient of static friction times the normal force on an object.

### 8. What is the Parity of the Work Done by Frictional Force?

The work done by frictional force is always negative in parity.

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