What is Friction?

Friction is defined as the force that prevents any solid surface, any layer of fluid, or any kind of element or material from sliding.

Examples of Friction

When two surfaces come in direct contact with each other and move with respect to each other, the contact between the two surfaces converts kinetic energy into thermal energy due to friction. This can be seen with the example of pieces of wood, which when rubbed together, produce fire. This is one of the classic examples of energy conversion. Wear is also an example of the results of friction, which causes poor performance or damage to a material.

Friction is necessary in providing traction, which helps in easing motion on land. Friction is considered a non-conservative force, since the work done against friction is dependent on the path. A tribometer is the device used to measure the friction on a surface.

Types of Friction

Dry Friction

In this type of friction, the relative lateral motion between two solid surfaces is being resisted. Dry friction relates to the two types of friction:

• Static friction, which occurs between non-moving surfaces
• Kinetic friction, which occurs between moving surfaces

The force of dry friction is calculated by a model based on the coulomb friction. The coulomb model is governed by:

Ff ≤ µF_n

Where F_f is frictional force exerted by both the surfaces on each other, µ is the frictional co-efficient, and F_n is the normal force which is exerted by both the surfaces on each other, which is perpendicular to the surface.

Static Friction

This is the type of friction which takes place between two or more solid bodies that are not moving with respect to each other. This type of friction can be shown when an object is restricted from sliding when it is placed on an inclined surface. Its coefficient is denoted with µ_S. The coefficient of static friction is higher than that of kinetic friction. The maximum possible frictional force between the surfaces when the object is restricted from sliding is given by multiplying the normal force (F_n) with the coefficient of static friction.

F_max= µ_S x F_n

Kinetic Friction

This is the type of friction which takes place between two or more bodies that are moving with respect to each other. This type of friction can be shown when two surfaces rub together against each other, when they are in direct contact to each other. Its coefficient is denoted with µ_K. The value of kinetic friction is always less than that of static friction. The maximum possible frictional force between the surfaces when objects are sliding against each other is given by multiplying the normal force (F_n) with the coefficient of kinetic friction.

F_max= µ_KF_n

Rolling Friction

This is the type of friction which occurs on a surface when the motion of a rolling body is resisted by a force. The smoother the surface, the less the force of rolling friction. The factors affecting rolling friction include the:

• surface
• load
• diameter of the rolling body
• surface area of the body

The coefficient of rolling friction is denoted by µ_r. It is defined as the ratio of force of rolling friction to the total weight of the body. This can be expressed as:

F_r=µ_rW

Where F_r is the force of rolling resistance, µ_r is the coefficient of rolling resistance and W is the weight of body.

Fluid Friction

This is the type of friction which occurs in the layers of fluids that move with respect to each other. The flow of the fluid is resisted to some extent, and this resistance is called viscosity. In general, viscosity is defined as the thickness of fluid. Water is considered less viscous, while the honey is considered to have high viscosity. All fluids provide resistance, hence are considered viscous. Only the ideal fluid or superfluid is considered to have zero resistance to flowing and hence is non-viscous. Practically, this type of inviscid fluid does not exist.

Lubricated Friction

Lubricated friction is a type of fluid friction, but in this type of friction, solid surfaces are separated with the help of a fluid or lubricant. Lubrication is defined as the process or a technique of applying a lubricant, which is done between two surfaces that are in close contact to each other to reduce any kind of wear between the two surfaces.

When the surfaces of metals or some other components do not have lubricant between them, they can rub against each other, causing stress. Being in direct contact like this can cause generation of heat at the point of contact and can possibly lead to failure or damage.

Internal Friction

Internal friction is the type of force that resists the motion of the elements within any solid material while it is undergoing its deformation phase. This can be seen in the case of plastic deformation; when stress is applied to the system, it is resisted by internal friction to some extent. If the stress is not too high, the internal force will totally resist it, and when the force is removed from the body, it will again get back to its original shape.

Ways of Reducing Friction

Devices like wheels, ball bearings, roller bearings, air cushions, and other kinds of fluid bearings can help in changing sliding friction into rolling friction.

We can also use lubricants, like oil, grease, or water, between the surfaces of two materials. This decreases the coefficient of friction and will also help protect the surfaces from wear and damage due to heating.

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for:

• Bachelors in Engineering / Bachelors in Technology of Civil Engineering
• Masters in Engineering/ Masters in Technology of Civil Engineering