Static Friction Vs. Kinetic Friction

In your basic physics courses, friction is a very important physical concept to study. Friction has been a subject of study for years and we have an empirical theoretical formulation today, that models this phenomenon and quantifies it for study.

What is Friction?

Friction is the opposing, resistive force between two fluid, solid or any material surfaces that acts to hinder relative motion between them. Friction is always directed in the direction opposite to the direction of motion. Friction is not a fundamental force of nature. It is a macroscopic force that emerges out of electromagnetic interactions between two surfaces in contact with each other.

Walking is made possible due to the force of friction that arises between the underlying surface of shoes or sandals, with that of roads. As opposed to this, walking on highly polished surfaces like waxed floors is difficult because there is lesser friction generated between the shoe and the floor. As you look around, you will realize, that friction is an all pervading phenomenon at work, which has a large influence on the world around us.

There are many types of friction, depending upon the nature of materials, that it is created between. The static friction vs. dynamic friction comparison made here is a comparison between two types of 'Dry Friction'. Dry friction is the friction force created between solid surfaces in contact with each other. According to the empirical theory of friction, the force of friction (F) is given as follows:

F ≤ μN

where 'μ' is the coefficient of friction and 'N' is the normal force between the two surfaces.

Differences Between Static Friction and Kinetic Friction

Making a clear distinction between two things helps in understanding them better. Static and kinetic are two opposite ideas. Static refers to a system that is not changing with time, while kinetic refers to a system in motion. Let us see the main point of difference between static and kinetic friction.

Definition

A definition of both static and dynamic friction can make the differences between these two different types of dry friction very clear. Static friction is the force of friction that exists between two solid surfaces that are non-moving. The lateral relative motion possible between these two surfaces is stalled by the force of static friction. It is the opposing force between two static surfaces that is so strong, that it eliminates any possibility of motion. Due to static friction, there is no slipping and therefore no motion between two solid surfaces.

Kinetic friction is the force of friction between two moving solid surfaces in contact with each other. Even though objects are in motion, the force of friction cannot be eliminated completely and it continues to be a resistive force to motion.

Static Friction Vs. Kinetic Friction: Coefficient

The static friction coefficient and dynamic friction coefficient are two numbers that are calculated to get an estimate of dry friction felt between two surfaces. These both are ratios between the force of friction and the normal force felt between two solid surfaces. The coefficient of static friction is given by:

F = μ_static N

where μ_static is the coefficient of static friction. The strength of friction between two static surfaces can be estimated by calculating the coefficient of static friction.

Coefficient of kinetic friction is given by the equation:

F = μ_kinetic N

where μ_kinetic is the coefficient of kinetic friction. The static friction coefficient is generally higher than the kinetic friction coefficient for two solid surfaces in contact with each other.

Understanding dry friction and measuring it is very important, if one wants to overcome or minimize its effects in technology, leading to an increase in overall efficiency of functioning.