Static Friction Coefficient

Friction is a force which is ever present everywhere around us. It comes into play when any two surfaces come in contact with each other. It is the major factor that causes wear and tear of machines with moving parts in contact with each other. Friction is created by the interaction of two surfaces in contact with each other and its magnitude is dependent on the nature of those surfaces and the forces applied between them.

What is Friction?
It is defined as a force that opposes or resists the relative lateral motion between two solid surfaces, material elements or even fluid layers. The two materials in contact with each other may be a liquid and a solid, a gas and a solid or even a gas and a liquid in contact with each other. The force of friction is not a fundamental force. It arises on a macroscopic level, from the electromagnetic force between the constituent atoms and molecules of the surfaces in contact.

Our understanding of friction today is the result of experimental research conducted by many scientists and engineers over the years, which include great minds like Leonardo da Vinci, Leonard Euler, Guillaume Amontons and Charles-Augustin de Coulomb. The empirical theory of friction that emerged in the field of physics, from their analysis of the subject, is still used today. The concept of static friction is a by-product of this empirical theory.

What are the Types of Friction?
According to the types of materials that come in contact, there are various types of friction. This includes dry friction (friction between solid surfaces), fluid friction (friction between fluid layers), skin friction (friction experienced by a solid traveling through a fluid) and internal friction (friction experienced by layers of a deforming solid internally). The theory discussed here is entirely restricted to dry friction between solids. It is the simplest of all the types of friction to analyze.

Dry friction is further classified into two types: 'Static Friction' & 'Kinetic Friction'. Static friction is the force of friction between non-moving or static surfaces. As opposed to that, kinetic friction is the force of friction experienced by two moving surfaces.

Some of the properties of friction that were empirically derived through experiments are the following. Friction between two surfaces is directly proportional to the applied load and is independent of the area of contact between the two surfaces. The second empirical law regarding area of contact does have a lot many exceptions, but is satisfied in most cases. Coulomb discovered that the force of kinetic friction between two surfaces in motion is independent of sliding velocity.

What is Static Friction Coefficient?
Static friction is the opposing or resisting force between two solid surfaces, that prevents any relative motion between them. The normal forces and the frictional forces are exactly balanced to stall motion between the two surfaces. For example, a rock may be stationed on a slope, due to the static friction between the slope surface and the rock surface.

To understand the theory of dry friction, one needs to understand the concept of normal force between two surfaces. It is the compressing force between two solids in contact with each other, which is perpendicular to both surfaces. For a block of wood placed on a flat surface, gravity is the normal force.

The coulomb theory describing dry friction of the static type, is governed by the following equation:

F_f = μ_s F_n

where F_f is the maximum force of friction, F_n is the normal force between the surfaces in contact and μ_s is the coefficient of static friction.

Thus, static friction coefficient can be defined as 'The ratio between the maximum force of friction felt by two solid surfaces in contact and the normal force exerted between two surfaces'. The direction of frictional force is opposite to the potential movement direction between these static surfaces. Being a ratio, the coefficient is a dimensionless scalar with no measuring unit. It is purely a number.

Calculation
Higher the value of static friction coefficient, higher is the frictional force exerted between two surfaces. The value of this coefficient cannot be determined theoretically. It can only be determined empirically through experiments. For most materials, the coefficient ranges between 0.3 and 0.6. No material object has a value of zero for the coefficient of static friction.

Materials like teflon do have a coefficient close to 0.04. Some objects like silicone rubber can have a coefficient value to be greater than 1. The coefficient varies according to the nature of the surfaces that are in contact with each other. Static friction between two surfaces depends on their physical and geometric properties.

This friction coefficient is an important parameter that needs to be taken into consideration in machine designing or in designing of any system where surfaces can come in contact with each other. Analysis of the macroscopic forces of friction, which lead to dissipative heat losses in systems is essential, if we plan to move towards a smoother and energy efficient future!