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What is frictional force? Problems with Solution

Apr 6, 2023

Frictional Force

Frictional force can be defined as a force that opposes motion when the surface of a body comes into contact with that of another body. Friction force decreases the mechanical advantage of a machine, or it decreases the ratio of output to input. Around one-quarter of its energy is used by an automobile in limiting friction force, but the friction in the tires allows the car to stay on the road, and that in the clutch makes it possible for us to drive. 

Ranging from matchsticks to machines to molecular structures of every simple and complex object, friction is one of the most prominent processes in the physical world. This article will discuss the frictional force and its types in detail. 


What is Frictional Force?

What is frictional force? The force generated by two surfaces of two objects when they come in contact and slide against each other is called the force of friction. The factors affecting frictional force include:

  • The frictional force is significantly affected by the texture of the surface and the quantity of force that propels them together.
  • The force of friction is affected by the position and angle of the object. 
  • If we place an object flat against another object, then the force of friction is equal to the object’s mass.
  • If we push an object against the surface of another object, then the amount of the frictional force is increased, which becomes higher than the object’s weight.

Calculating the Force of Friction

What is friction force calculation? The maximum frictional force applied on an object by a surface can be easily calculated by the following formula:


Ffrict = µ • Fnorm

Below are given the systematic steps that you can follow while dealing with calculations based on the frictional force:


Determine the Normal Force

The normal force exerted on a body in contact with another stable body is the support force, and in most cases, it can be simply described by the following friction force formula:

N = mg



M is the mass of the object


G is the acceleration due to gravity

If the surface is inclined, the strength of the force is decreased with the extent of inclination; therefore, the friction force formula becomes:


N = mg cos(θ)



The angle of inclination of the surface is θ

The force of a normal type of 2 kg wood block sitting on a surface is calculated in a simple calculation as,

N = 2 kg × 9.8 N/kg = 19.6 N

Calculating Frictional Force

The friction force formula is given by:

F = μN

Suppose we consider a wood block weighing 2 kg lying on a table to be moved from rest. We consider the static friction coefficient in this case, which is 00.5 in the case of wood.

The normal force can be calculated with the given details, 

N = 2 kg × 9.8 N/kg = 19.6 N 

As we now have the values of static friction coefficient and normal force, the frictional force can be calculated as follows:

F = 0.5 × 19.6 N = 9.8 N

fricitional force

Types of Frictional Forces 

Different types of frictional forces exist, such as kinetic frictional force, static frictional force, sliding frictional force, and rolling frictional force. The frictional force taking place between gases and fluids is called fluid friction. Following is the broad classification of the frictional force:

  • Static friction
  • Dry friction
  • Rolling friction
  • Kinetic friction
  • Fluid friction

Static Friction 

Static friction occurs when two surfaces are at rest with regard to one another. The value of static friction ranges from zero to the minimum force required to initiate motion. The force needed to initiate motion or to overcome static friction is always greater than the force needed to maintain motion or to overcome kinetic friction. 

Kinetic Friction 

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 that opposes motion. Although it also rests on the two objects in contact, the kinetic coefficient of friction is nearly always less than the static coefficient of friction.

The formula for the kinetic frictional force:

Fk= μk η 

Dry Friction

The reaction between two solid bodies is described by dry friction. When the two solid bodies are in motion, their friction is described as kinetic friction. When they are not in motion, it can be described as static friction. Both kinetic and static frictional forces are proportional to the normal force between two solid objects. 

Different coefficients of friction model the interaction of different substances. This means that certain substances are more resistant to movement as compared to others when the magnitude of normal force between them is the same. The values for each of these can be determined experimentally. 

Rolling Friction

Rolling friction comes into play when a ball, wheel, or cylinder rolls freely over a surface, as in the case of the ball and bearing rollings.

The distribution of energy involved in the twisting of the bodies involved is the main cause of friction in rolling. If we roll a hardball on a level surface, the surface is indented at the points of contact while the ball is somewhat packed down. 

The compression or elastic bend created at the main point of contact is interference to motion that is not entirely compensated as the substances revert back to their typical shape at the trailing section.

The internal losses in the two substances are parallel to those that prevent a ball from bouncing back to the level of its release. The sliding friction coefficients are usually 100 to 1,000 times greater than the rolling friction coefficients for corresponding materials. 

Examples of Rolling Friction

Rolling friction is a type of dynamic or kinetic friction that comes into play when one object rolls over another. For instance, when a circular disc, a ring, a sphere, or a wheel rolls over the surface, rolling friction is the force opposing its motion. Other examples of rolling friction are:

  • A basketball that is rolling on a court comes to a halt after a while.
  • A tire or skateboard wheels rolling on the road are also examples of rolling friction.
  • The motion of vehicles is affected by factors like size, shape, and weight. Suppose a heavily loaded twenty-wheeler truck will have greater rolling friction than a TATA Nano Car.
  • Use of paint rollers in the painting of the walls.

Fluid Friction

The force obstructing the flow of fluid is called fluid friction. In this situation, resistance is provided by the fluid between two surfaces. If the resistance is offered by both surfaces, it is called high dense and can be generally called greasy. Examples of fluid friction include:

  • Lubrication of the door hinges causes smooth functioning of the door and avoids creaking sounds while opening and closing them.
  • When a ball is dropped in a bucket full of water, the water splashes out of the bucket due to the buoyancy of the fluid.

Problems with Frictional Force

  1. A huge ice block is pulled across a frozen lake that has a mass of 300 kg. There is a small frictional force between the two ice surfaces equal to μk = 0.05. Calculate the frictional force acting on the ice block. 

Solution: The normal force acting on an object resting on a flat surface can be given by the formula N = mg. This can be used to calculate the frictional force as follows:

Ff =μN

Ff =μmg

In the above equation, if we substitute the data, we get:

Ff =0.05 × 300 kg × 9.8 m/s2

147 kg-m/s2 or 147 N.

The frictional force acting in the opposite direction as we pull the ice block across the lake is 147 N.

  1. A boat is pushed by a man on the shore to get water. The frictional coefficient between the mud and the boat is given by μ = 0.400. Calculate the magnitude of frictional force acting on the boat if the boat has a mass equal to 40 kg.

Solution: The normal force acting on an object on a flat surface is N = mg.

This information can be used to calculate frictional force as follows:

F = μN F = μ mg 

If we substitute the data in the formula, we get

F = (0.400)(40.0 kg)(9.80 m/s2)

F = 156.8 N

Therefore, the force of friction acting on the boat equals 156.8 N.


If you have gone through the whole discussion thoroughly, most of your doubts on the topic of frictional force will have been cleared and you can hopefully solve a lot of problems based on the topic. Keep practicing different types of problems related to the topic to gain a fundamental understanding of its concepts. 

Frequently Asked Questions

1. What is static friction?

The frictional force is experienced when we try to bring a stationary body to motion on a surface without stimulating a relative motion across the surface, and the body is called static frictional force.

2. Is frictional force a scalar or a vector quantity?

As frictional force involves both magnitude and direction, it is a vector quantity.

3. How to find the right coefficient?

The coefficient chosen depends on the object and the specific situation. The coefficient of static friction is used if the object is not moving across the surface, while the coefficient of sliding friction is used if the object under consideration is moving. The coefficient is also affected by the type of materials used. For instance, the coefficient is 0.6 if there is a block on a brick surface, while it ranges from 0.25 to 0.5 if it is on a block of clean wood. 

Frictional Force


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