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# Kinetic Friction

## Kinetic Friction - Introduction

Last updated date: 19th Mar 2023
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The force of friction or frictional force is one of the basic types of force experienced in our everyday lives. The frictional force is the type of force that results due to the kinetic bodies that are in mutual interaction. There are various reasons that cause frictional force. The frictional forces are further subdivided into static friction and kinetic friction. Kinetic friction, which is also often referred to as sliding friction, and is a crucial factor for the objects under motion. Static friction influences the body before it starts moving (or when the body is at rest).

In this article, we will discuss what kinetic friction is, the kinetic friction formula, and a few solved examples for a better understanding of the concept.

## Charles-Augustin De Coulomb

Charles-Augustin de Coulomb was a French commander, engineer, and physicist who lived from 14 June 1736 to 23 August 1806. He is most recognised for discovering what is now known as Coulomb's law, which describes the electrostatic attraction and repulsion force. He also contributed significantly to the field of friction. In 1880, the coulomb, a SI unit of electric charge, was named after him.

Coulomb looked at the effects of four primary aspects on friction: the nature of the materials in contact and their surface coatings; the size of the surface area; normal pressure (or load); and the amount of time the surfaces were in contact (time of repose). Coulomb also evaluated the effects of sliding velocity, temperature, and humidity in order to choose between the various offered explanations for the nature of friction.

## Kinetic Friction

Most objects are not as smooth as they seem. On a microscopic level, even seemingly smooth surfaces are a landscape of small hills (rough surfaces) and valleys that are too small to observe naked eyes, but that makes a huge difference when it comes to determining the relative motion between two surfaces in contact. These small imperfections or the defects in the surfaces interlock, creating the frictional force that opposes any movement and must be calculated to determine the net force on the object.

There are a few different types of friction, but kinetic friction is also known as sliding friction, while static friction affects the object before it begins to move (i.e., the static friction is accounted for the object at rest), and rolling friction relates specifically to rolling objects such as bicycles.

So now, what is kinetic force?

## Kinetic Friction Types and Definition:

The kinetic friction force can be defined as the resistance experienced by the object which is under motion as a result of contact between the surface and the object moving against it. The kinetic friction force always acts in a direction opposite to the direction of motion of the object. This is the reason we experience that, when we try to push something forward, the kinetic friction will push that object backward.

In other words, retarding force between the two moving planes when they are in contact with each other is commonly known as Kinetic Friction.

The kinetic friction force only applies to an object which is subjected to motion and in general, the kinetic friction force is known as the sliding friction. Another important type of frictional force is the friction between the solids and it is known as static friction. The static friction force is the resistance applied to the motion as a result of the friction between an object which is at rest and some surface. The coefficient of static friction is regularly found larger than the coefficient of kinetic friction.

## Kinetic Friction Examples

• Friction also plays a very important role in everyday events, such as the rubbing of two objects. The resulting movement turns into heat and thus causes fire in some cases.

• It is also responsible for wear and therefore we need oil to lubricate the machine parts as it reduces friction.

• When two objects rub against each other, the frictional force converts into heat energy, in some cases giving rise to a fire.

• Kinetic friction is responsible for the wear of machine parts, so it is important to lubricate parts of the machine with oil.

## Kinetic Friction Formula

The physical quantities such as the frictional force are best defined using an equation. The frictional force depends on the coefficient of friction for determining the type of friction under consideration and the magnitude of the normal force that the surface exerts on the object. For kinetic friction force, the frictional force is given by:

$\Rightarrow F_k=\mu_kN$……(1)

Where,

μk-The coefficient of kinetic friction formula

N-The normal force acting on the given object

Also, if the problem involves a horizontal surface and no other vertical forces are acting then the normal force formula is given by:

$\Rightarrow N=mg$……(2)

Where,

• m - The mass of the object

• g - The acceleration due to gravity

Since friction is a force, the unit of the frictional force is the newton (N). Also, the coefficient of kinetic friction will be a unitless quantity.

The equation for static friction will be basically the same. But the exception is that the sliding friction coefficient will be replaced by the static friction coefficient μs. This is really best thought of as a maximum value because it increases up to a certain point, and then if you apply more force to the object, it will start moving.

## Interesting Facts

• Bumps on two adjacent surfaces are supposed to produce friction. Asperities are the name for these pimples. These two rough surfaces collide and become caught, preventing motion until the asperities are broken or slid over. These blemishes can be seen in even the smoothest materials.

• Electromagnetism is the cause of friction. When the atoms that make up the items get close enough to touch, they are attracted together and may unite.

### Examples:

1. A vehicle is moving at a uniform speed with the normal force of 950 N. If the kinetic friction force applied on the vehicle is 400 N. Then calculate the coefficient of the kinetic friction required here?

Sol:

Given that,

The normal force exerted on the vehicle= N = 950 N

The kinetic friction force acting on the vehicle Fk = 400 N

We are asked to calculate the coefficient of kinetic friction for this vehicle. We know that, the kinetic friction formula is given by:

$\Rightarrow F_k=\mu_kN$

Where,

μk-The coefficient of kinetic friction

N - The normal force acting on the given object

$\Rightarrow \mu_k=\dfrac{F}{N}$……(1)

Substitute, the given data in the above equation and simplify, we get:

$\Rightarrow \mu_k=\dfrac{F}{N}$

$\Rightarrow \mu_k=\dfrac{400}{950}=0.421$

Therefore, the coefficient of kinetic friction for this vehicle is 0.421.

2. A guy is playing football. Calculate the kinetic friction here, if the friction coefficient is 0.35 and the football is kicked with the force of 120 N?

Sol:

Given that,

The normal force acting on the football = N = 120 N

The Coefficient of kinetic friction = μk=0.35

We are asked to determine the kinetic friction acting on the football. We know that the kinetic friction formula is given by:

$\Rightarrow F_k=\mu_kN$

Where,

μk - The coefficient of kinetic friction

N - The normal force acting on the given object

Substitute, the given data in the above equation and simplify, we get:

$\Rightarrow F_k=\mu_kN$

$\Rightarrow F_k=0.35\times 120N$

$\Rightarrow F_k=42~N$

Therefore, the kinetic friction acting on the football is 42 N.

## Conclusion

Friction is a force that acts to slow down the movement of two objects that are in contact. Frictional energy is converted into sound and heat. There are two types of friction: static and kinetic. When the frictional force is insufficient to stop all motion, kinetic friction is applied.

Heat production is one of the effects of kinetic friction. When friction is powerful enough to inhibit movement between two objects, it is called static friction. Car brakes, bags, and shoes are just a few examples of daily systems that use friction. Friction is used by even a person sliding across the floor.

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## FAQs on Kinetic Friction

1. What are kinetic friction examples?

We encounter kinetic friction in our everyday life most frequently. One of the best and evident examples of kinetic friction is when we try to push an object placed on a rough surface we notice that it will be hard to push that, this resistance is due to the kinetic friction force acting on the object.

2. What is kinetic friction and its formula?

The slowing force (retarding force) between the two planes when they are in contact with each other is known as kinetic friction. The kinetic friction is directly proportional to the normal force acting on the particle or an object. The kinetic friction force is given by the following formula:

$\Rightarrow F_k=\mu_kN$

Where,

μk - The coefficient of kinetic friction

N - The normal force acting on the given object

3. What are static and kinetic frictions?

The static frictional force is the frictional force that opposes the applied force to an object, and the object continues to be at rest until the force of static friction is overcome. Whereas kinetic friction is the frictional force resisting the motion of an object.