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# Laws of : Static Friction, Kinetic Friction And Limiting Friction for JEE

## Introduction to the Friction

Last updated date: 30th Mar 2023
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We all have heard the word ‘friction’ but what is friction? Let us know what it is? When we throw a ball on the ground, it begins to move with some velocity. However, no force should be acting in the direction of motion, and the ball should continue to roll according to Newton's first law, but this does not occur. Instead, the ball stops after a certain distance, implying that a force is acting on it. This force is referred to as "friction.". Let us know more about this concept in this article.

## What is Friction?

The force that resists the relative motion of two bodies in contact is known as Friction. It can be either static, like the friction between a book and a table, or kinetic, like the friction between your car tires and the road.

In general, friction depends on three things

• The nature of the surfaces in contact.

• The force pressing them together.

• And how fast they're moving.

For example, you can easily slide a book across a table top because there's very little friction between paper and wood. But if you try to push that same book along the floor, it's much harder because there's more friction between carpet and wood.

## Static Friction Definition

Static friction is the force that resists the motion of an object when it is not already in motion. This force arises from the interaction between the surfaces of the object and the surface it is resting on. The magnitude of static friction depends on a number of factors, including the roughness of the surfaces involved, the amount of contact between them, and the forces acting on them.

The formula of static friction is:

${{F}_{s}}={{\mu }_{s}}\times N$

where

${{\mu }_{s}}$ = coefficient of static friction

N = normal force

## Kinetic Friction Definition

Kinetic friction is the force that resists motion when two surfaces are in contact and sliding against each other. It is also known as sliding friction or dynamic friction. The amount of kinetic friction depends on the roughness of the surfaces, the force pressing them together, and the type of material.

The formula of kinetic friction is:

${{F}_{k}}={{\mu }_{k}}\times N$

where

${{F}_{k}}$ is the force of kinetic friction (in Newtons)

${{\mu }_{k}}$ is the coefficient of kinetic friction

N is the normal force (in Newtons)

## Limiting Friction

Limiting Friction is the highest value of static friction which comes into play when an object is just about to slide over the surface of a different object. For an exerted external force greater than the limiting friction, the body begins to move. We have to remember that once the motion has started, static friction cannot be considered. A new type of friction termed as kinetic friction comes into play.

## Laws of Static Friction

• The maximum force of static friction is not dependent on the area of contact.

• The maximum force of static friction is comparable to the normal force, i.e., if the normal force increases, the maximum external force that the object can endure without moving, also increases.

## Laws of Kinetic Friction

• The force of kinetic friction (${{F}_{k}}$) is directly proportional to the normal reaction (N) between two surfaces in contact, where ${{\mu }_{k}}$ is constant called the coefficient of kinetic friction.

• Force of kinetic friction is independent of shape and apparent area of the surfaces in contact.

• It depends upon the nature and material of the surface in contact.

• It is independent of the velocity of the object in contact provided the relative velocity between the object and the surface is not too large.

## Laws of Limiting Friction

• The direction of limiting frictional force is always contrary to the direction of motion.

• The Limiting friction acts tangentially to the two surfaces interacting.

• The magnitude of limiting friction is proportional to the normal reaction between the two surfaces.

• The limiting friction turns upon the material, the nature of the surfaces interacting and their evenness.

• So long as the normal reaction is the same, the magnitude of limiting friction is free of the shape or the area of the surfaces in contact, for any two given surfaces.

## Applications of Kinetic Friction

• Kinetic friction has a number of important uses, both practical and theoretical. In many cases, it is desirable to have a frictional force opposing motion, such as in brakes and clutches. This allows for control over speed and prevents unwanted slipping or sliding.

• Kinetic friction also plays an important role in tribology, which is the study of wear, lubrication, and bearings. By understanding how different materials interact with each other under various conditions, engineers can design more efficient machines with less wear and tear.

• Finally, kinetic friction provides a dissipative force that helps maintain thermal equilibrium in systems where there are temperature gradients (e.g., between a hot object and its surroundings).

## Applications of Static Friction

Static friction is the force that resists the motion of an object when it is not in contact with another object. This type of friction is caused by the interaction of molecules on the surface of the objects. Static friction is important because it helps to keep objects from moving when they are not supposed to.

One common use of static friction is to keep a car from rolling down a hill. The static friction between the tires and the ground prevents the car from moving, even if there is no other object in contact with it. Another use of static friction is to keep a door closed. The static friction between the door and the frame prevents the door from opening, even if there is no wind or other force pushing against it.

Static friction can also be used to help people move heavy objects. For example, someone might use their feet to push against a wall in order to move a heavy piece of furniture across a room. The static friction between their feet and the floor prevents them from slipping, and allows them to apply enough force to move the object.

## Summary

At last we can summarise the whole article with a few important points. In this article, we discussed that friction is the force that resists the relative motion of two bodies in contact. It can be either static, like the friction between a book and a table, or kinetic, like the friction between your car tires and the road.

Static friction is the force that resists the motion of an object when it is not already in motion. This force arises from the interaction between the surfaces of the object and the surface it is resting on. Kinetic friction is the force that resists motion when two surfaces are in contact and sliding against each other. Along with these points, we discussed the applications of both static and kinetic friction where students get more idea about the concept.

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## FAQs on Laws of : Static Friction, Kinetic Friction And Limiting Friction for JEE

1. How can we say static friction is a self-adjustable force?

When an object is at rest on a surface, the static friction force between the object and the surface balances the force of gravity acting on the object. The static friction force is therefore self-adjusting, since it adjusts itself to ensure that the object remains at rest. It depends on the normal force applied by the body.

2. Does static friction apply on the moving objects?

Yes, static friction is also applied on moving objects. Let’s take an example of a moving car. The wheel of the car is moving along with it. But the bottom point of the wheel is at rest with the ground. At this point static friction is applied that helps the wheel to roll without slipping. If static friction is not present there, then it will slip only. Rolling in any moving object is possible due to static friction only.

3. How can we say that kinetic friction is independent of velocity?

The kinetic friction force always acts in opposition to the direction of motion, and its magnitude is independent of velocity. This means that as an object moves faster, the kinetic friction force increases in order to balance the increased forward force and keep the object moving at a constant speed. Kinetic friction only depends on the normal force and friction constant.