Physics

Sliding friction- Definition|Examples

Friction is an opposing resistive force developed when two bodies are in contact with each other. Based on the mode contact friction can be divided into two types, rolling friction, and sliding friction. So there are two possibilities, an object can roll or slide over a surface. In this article, we look at the definition and a few examples of sliding friction.

Definition of sliding friction

Sliding friction definition — Skiing invloves sliding friction the snow and the skiboard.
Image by Pexels from Pixabay

Sliding force can be defined as the force of friction between two bodies when they are sliding over each other. The point to be noted here is, the objects don’t roll over but just slide across the surfaces. For example, when we push a wooden box across a surface, it slides and does not roll. Newton’s first law states that an external force is required for an object to stop. This external stopping force is provided by the sliding friction when an object is sliding over a surface. For example, if you push a book over a table, it slides but ultimately comes to a halt due to sliding friction.

Factors affecting sliding friction

Sliding friction is dependant on many different factors which can be summed up into normal force and coefficient of sliding friction.

Roughness: Sliding friction mainly has its origin in the interlocking of irregularities between the object and the surface. So, the higher the roughness of the surfaces, the greater will the value of sliding friction. The coefficient of sliding friction gives us an estimate of the roughness.
Mass of the object: Friction is directly proportional to the mass of the sliding object.
Pressure: Friction is directly proportional to the force applied in the normal direction. In the case of an incline then the component of force in the normal direction is calculated.
Other factors: Factor like electromagnetic attraction/repulsion, chemical bonds, etc play a significant role in certain specific conditions.

Sliding friction formula

The formula for sliding friction is the same as the general formula for friction which is:

F_s = μ_sN

Where F_s is the static friction force, μ_s is the coefficient of static friction, N is the normal force at the point of contact.

Coefficient of sliding friction

The coefficient for sliding friction is denoted by the symbol μ_s. It is a very important factor in determining the frictional force acting on an object. The coefficient of sliding friction is unique for each pair of materials. For example, it is difficult to walk on sandy surfaces compared to hard surfaces. You can read more about the coefficient of friction in this article.

The direction of sliding friction

Sliding friction always acts opposite to the direction of force applied and parallel to the plane of contact.

In the above image, we can clearly see the direction of sliding friction acting opposite to the force applied.

Sliding friction in an inclined surface

In the case of inclined surfaces, we compute the component of forces in the normal direction. Rest, all the other factors remain the same.

Sliding friction and kinetic friction

Sliding friction and kinetic friction are different concepts and cannot be compared directly. Based on the state of motion friction can be classified into sliding and kinetic friction. So, based on the definition of sliding and kinetic friction, we can say that when an object is in motion and it slides then sliding friction is the same as kinetic friction.

How to increase sliding friction

We can play around with the factors affecting friction like roughness, adhesion, pressure, etc to increase friction.

Roughening of the surface of the object and the sliding surface. This can be done by patterning or changing the shape of the object.
Increase pressure on the object. This can be done by magnetic or electric attraction or even by chemical adhesion.

How to decrease sliding friction

We can decrease friction by again varying the factors affecting friction.

Smoothening of the surfaces of the object and the sliding surface. This can be done by grinding/polishing surfaces.
By magnetic levitation, we can reduce the contact between surfaces.
Lubricants like grease, graphite can reduce the sliding friction in small applications like door hinges.

Difference between sliding friction and rolling friction

Rooling friction and sliding friction — Sliding friction(left) is always greater than rolling friction(right)

The coefficient of static friction is always greater than the coefficient of rolling friction. That is why it is easier to roll an object rather than to push it by sliding. The above image explains this concept clearly. One recent innovation based on this concept would be the trolley bags, which have made traveling easy.

Examples of Sliding Friction

Example of sliding friction — Curling: A game in which a stone is made to slide across a ice-floor towards a target.
Image by Photo Mix from Pixabay

A wooden box pushes across the floor
Ironing an object involves sliding friction between the hot metal and the clothes
A long jump athlete sliding across the sandpit
A book being slide across the desk involves sliding friction
Kids playing on a playground slide
Strick a matchstick across a rough surface to generate fire

What are the most common examples of sliding friction in our daily lives?

Rubbing of hands in winter seasons to generate heat.
Washing/cleaning of surfaces by sliding a cloth across the surface.
Walking: When we walk, we push the ground and the sliding friction enables us to move forward.

What is static sliding friction?

The definition of static sliding friction is the same as sliding friction, except that the force applied is unable to make the object move. So, this cannot be truly considered as sliding friction. For example, suppose we want to push a washing machine across the floor, it takes some time to push it, till that time static sliding friction is opposing the movement.