**Coefficient of Friction**

Friction is a force which holds back the movement of a sliding object. We can find friction everywhere in our surroundings. This force acts in the opposite direction to the way an object wants to slide. Besides, the coefficient of friction is a very important term here.

In addition, while running on the sidewalk you stop quickly, we can stop because of the friction between our shoes and the cement. In this topic will elaborate coefficient of friction properly.

**Concept of Friction**

Friction is the resistance which an object encounters in moving over another. Friction only happens with solid objects, but we do get resistance to motion in both liquids and gases. This doesn’t involve sliding surfaces as friction does. It is like a colliding situation, not a sliding one. Also, it is air resistance, if the gas is air.

Imagine that someone is trying to push a book along with a table with his finger. If he applies a very small force, the book will not move. This means that the frictional force is equal to the force with which he is pushing the book.

If the frictional force was less than the force produced by his finger, the book would slide forward. Furthermore, if it was greater, the book would slide backwards.

If he pushes the book a bit harder, it would still remain stationary. Thus the frictional force must, therefore, have increased, or the book would have moved. If he continues to push harder, at last, a point is reached when the frictional force increases no more. Also, if he slightly pushes it harder, then the book will start to move. If a body is moving, then friction will be taking its limiting value.

**Coefficient of Friction**

The measure of friction is based on the type of materials that are in contact. Concrete material has a very high coefficient of friction. Actually, this coefficient of friction is a measure of how easily one object moves in relation to another.

So, a high coefficient of friction represents that there is a lot of friction between the materials. Concrete on concrete has a very high coefficient, and Teflon with mostly all things has a very low coefficient. So, Teflon is used on surfaces where we don’t want things to stick, like pots and pans.

The coefficient of friction is a unit-less value which represents the friction between two surfaces. It is obvious that between two equal surfaces, the coefficient of friction will be the same.

**Computation of Coefficient of Friction**

Friction generally occurs in two ways, which are kinetic and static. Kinetic friction acts on an object that slides on a surface, on the other hand, static friction occurs when friction prevents the object from moving.

A simple, as well as an effective model for friction, is that the force of friction, f, is equal to the product of the normal force, N, and a number called the coefficient of friction, μ. It means

F= N × μ

This normal force is the force perpendicular to the interface between two sliding surfaces. In addition, the maximum frictional force (when a body is sliding or is in limiting equilibrium) is equal to the product of the coefficient of friction and the normal reaction force.

After this, the frictional force will act parallel to the surfaces in contact and in a direction to oppose the motion that is taking/ trying to take place.

**Example:** A particle having a mass of 5 kg is at limiting equilibrium on a rough plane which is inclined at an angle of 30 degrees to the horizontal. Now, find the value of the coefficient of friction between the particle and the plane.

Solution: Frictional force, F = 5×g× sin30 (g is the acceleration due to gravity)

Resolving perpendicular to the plane:

Normal reaction Force, N = 5×g× cos30

In limiting equilibrium, F = N× μ

5gsin30 = 5gcos30 × μ

μ = sin30/cos30 = 0.577

So, Coefficient of friction is = 0.577

**Solved Question for You**

**Q: What will happen when frictional force is at its maximum?**

**Ans:** The frictional force between two objects is not constant, but increases until it reaches a maximum value. When the value of frictional force is at its maximum, this body will either be moving or will be on the verge of moving.