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What is Friction?

Kundan Pandey
The concept of friction is an essential part of physics. This post discusses its basic concept, as well as important related information. Have a look...
Friction is a resistive force. If this seems like a small riddle for you, let us try and understand the basic concept of friction.

Definition

Friction is the tendency to oppose relative motion. It is an inherent force that resists the motion of an object which is in contact with another object or a surface. If you try to move a heavy book kept on a table, you will have to apply some force. This force is required to overcome the frictional force between the contact area of book and table.
Have you ever tried ice skating? You may have thought that there is no frictional force present when you're ice skating, and hence, it's easier to slide on the ice.
It is not that there is zero friction between the ice and the skates. Zero friction is an ideal concept, one that we use while solving theoretical problems.
In the practical world, some friction always exists between two objects coming in contact with each other. In case of ice skating, it is present, but is very less, making it easier for skaters to perform. If friction becomes zero, then it would be impossible for skaters to stop, or for you to sit on a chair, or run on the road. You will be in motion forever.

Is it a Force?

Friction is a force that is resistive in nature. It obstructs the motion of another object by applying some force. But, from where are these frictional forces generated?
First, let's understand friction from a molecular level. Friction that we observe in daily life can seem to be caused by the roughness of a surface. This is what scientists believed for a long time to be the main cause of friction. However, studies today have suggested molecular adhesion and plowing effect to be the major causes.
Molecular adhesion is defined as the force that exists between two surfaces coming in contact, by the virtue of molecular or electromagnetic attractions between their molecules.
To overcome resistance, it becomes essential to break the molecular attraction bond or the adhesive bond. Plowing effect is the frictional force that occurs due to deformation of the object, near the point of contact.
For example, if a hard object slides on a soft platform, it is deformation in the surface of the soft object that generates friction.

Measurement

Being a force, the SI unit for its measurement is Newton. The value of the frictional force that acts between objects in relative motion, depends on two major quantities, coefficient of friction (µ) and net normal reaction (N). The frictional force equation is, F = μN, where, F = frictional force, μ = coefficient friction, and N = net normal reaction.
Understanding the coefficient of friction (μ) is very important to comprehend this concept completely. The coefficient of friction is a scalar quantity, which represents the resistance that a surface exerts on the objects moving over it.
Mathematically, it is the ratio of frictional force between the surfaces, and the external force applied to the bodies. The coefficient of static friction is a term that is used for static objects, while the term coefficient of kinetic friction is used for objects in motion.
Solving related mathematical problems will help you develop a more holistic understanding of friction.