We know that two forces act on a stationary object on a horizontal table, one is the weight of the object which acts downwards and the other is the normal reaction force that acts upwards. These two forces - The force of gravity or weight and the normal reaction force always cancelled each other and make the object at rest.

In such a situation, if a small external force is applied to the object, there are two things that can happen with the object - the object may remain stationary or move away from its initial position. Because we know from experience that a small applied force may or may not be enough to move the objects.

This clearly shows that some other force comes into play in the horizontal direction and opposes the applied force. This is known as the frictional force, which is generated parallel to the surface of the body in contact.

## What is Frictional Force?

When two surfaces are moved or try to move relative to each other in opposite directions, the opposing force that is created between the two surfaces is called frictional force or simply friction.

The source of this frictional force is the roughness of the surfaces in contact. The higher the roughness of the surface, the more friction the surface produces.

## Types of Frictional Force

There are different types of frictional forces. The frictional force that takes place between solid surfaces is classified as static friction, kinetic or sliding friction, and rolling friction.

Static Friction: The frictional force that opposes impending motion between surfaces in contact is called static friction.

Suppose you are pushing an object on a table with an external force F but the object is not moving from its initial position. In this situation, an opposite force is generated against the force you applied to the object.

Here this opposite force that is generated against the applied force is static friction. But it is remembered that static friction does not exist by itself, when there is no applied force, there is no static friction.

Experimentally it is found that the static friction is independent of the area of contact, and the limiting value of static friction (fs) is directly proportional to the normal reaction force (N). So

Where  𝜇s  is a proportionality constant known as the coefficient of static friction. It depends only on the nature of the surfaces in contact.

Sometimes the formula can be written as follows

Because for the maximum static friction (fs)max = 𝜇sN and the other value of (fs) is always smaller than 𝜇sN, never exceeds the value of 𝜇sN. Now if the applied force F exceeds (fs)max then it enters in the new types of friction called kinetic or sliding friction.

Kinetic or Sliding Friction: The frictional force that opposes relative motion between surfaces in contact is called kinetic or sliding friction.

In this type of friction, the body just begins to move.  Here the applied force F exceeds (fs)max and the body begins to slide on the surface.

Like static friction, experimentally it also found that the kinetic or sliding friction is independent of the area of contact, and the kinetic or sliding friction (fk) is directly proportional to the normal reaction force (N). So

Where  𝜇k  is a proportionality constant known as the coefficient of kinetic or sliding friction. It also depends only on the nature of the surfaces in contact.

It must be remembered that kinetic or sliding friction is always smaller than static friction. ie

Motion in a rough surface:

If we applied a force F on an object of mass m and the object get an acceleration a, then according to Newton's second law of motion

Now for the surface to be rough, the kinetic or sliding friction resists the motion of the object. Then the equation goes like this

If the object moves with constant velocity, then F = fk i.e a=o. If the applied force on the object is removed, its acceleration is – fk /m. This means the object starts retarding and finally comes to stop.

Rolling friction: The frictional force that opposes the motion of a body that is rolling over the surface of another is called rolling friction.

This rolling frictional force is caused by the distortion of the rotating object and the distortion of the surface in contact with the object. The formula for rolling friction goes like this

Where  𝜇r  is a proportionality constant known as the coefficient of rolling friction. It also depends only on the nature of the surfaces in contact.