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Friction may be defined as the resistive force acting in opposite direction in which the body tends to move or it moves. Frictional force always acts tangentially at points of contact.

Friction may be classified into two catagories.

• Static Friction and
• Kinetic Friction

Static Friction is the friction experienced by a body when it is at rest under the action of external friction.

Kinetic Friction is the friction experienced by a body when it moves. Kinetic friction may be classified as Sliding Friction and Rolling Friction.

As a body can move in two ways, one is sliding and the other is rolling, there are two types of kinetic friction

• Sliding Friction and
• Rolling Friction

Sliding Friction: When a body slides over a surface without having any rotational tendency about a horizontal axis, it experiences a sliding friction. Sliding frictional force always try to dampen the movement as quickly as possible.

Rolling Friction: It is the friction experienced by a body when it rolls over the surface with an angular velocity as well as linear velocity. Rolling is a combination of translation as well as rotation about a horizontal axis passing through the centre of gravity.

Suppose we have a block of weight (W) lying on the ground as shown in the figure. The block will be at equilibrium as the weight will be neutralised due to the normal reaction provided by the ground.

Now let a horizontal (P) force is gradually applied to the block. Initially, when the force P is small, the block will not move, but if we increase the magnitude of P, then one moment will come when the block will start to move along the direction of applied force.

But from Newton's 2nd law, we know that whether the magnitude of force P is small or large, in all the cases the block should start to move with an acceleration F/m. But, practically the block starts to move when the magnitude of the applied forces reaches a definite value. Why does the block behave so? What does it mean?

Well, it means that there must exist an opposite resistance force that acts opposite to the applied force, having the same magnitude as that of applied force. But, this resistance force has a limit. When the resistance force reaches a maximum value, then the further increase in applied force can not be neutralized and as a result the body starts to move. When the value of the resistance force becomes maximum, any further increase of applied force set the movement in the body. The condition just before a body starts to move is called as "Limiting Conditions." The resistance force is called Frictional force and it becomes maximum, when the body attains the limiting condition. At this position the maximum frictional force is called "Limiting Friction."

Now just look at the experiment again, if we applied an infinitesimal force, the block doesn't move, which means the frictional force thus generated must be equal to the infinitesimal applied, else the body will experience a net force and the body will start to move. Now, if we increase force by some amount, the body will still be static, until the applied force reached the value of limiting friction. So, what does this indicate?

It indicates that frictional forces are variable. As we increase applied load, the frictional force also increases from zero to the highest value of limiting friction.

When the horizontal applied force is zero, the frictional force must be zero else the body will experience a net force. It means that when there is no applied force, frictional force just vanishes. Hence, it is known as pseudo force as its existence is dependent upon the applied force. (Remember there is another force centrifugal force which depends upon the centripetal force in rotational motion, hence, it is also a pseudo force.)

So, as the applied force starts to increase, the frictional force also increases thus maintaining the equilibrium conditions. But, the frictional force can not neutralize the applied force P, when its magnitude crosses the maximum value frictional force that can be generated due to friction on the contact surfaces.

Therefore when P > (fs)max, where (fs)max is the maximum magnitude of frictional force, the body will be in motion. When P = (fs)max, we call it as the case of impending motion or limiting condition.

So, on what factor does the maximum frictional force on a surface depend upon? Certainly, it doesn't depend upon the applied force, although frictional force depends upon the applied force, but how much will be the maximum value of friction entirely depends upon the surface and the geometry of that surface and the Normal reaction the surface produces to counter balance the weight of the body. Here, one should remember normal reaction depends upon the mass of the body and the inclination of the surface with horizontal.


So, Coulomb's Law of dry friction states that, when there is a body resting on a surface is subjected to an applied force P, the maximum frictional force that would be generated directly depends upon the value of Normal reaction experienced by the body.

(fs)max ∝ N

N = mg cos θ, where mg is the weight of the body and θ is the inclination angle of the plane with horizonal.

(fs)max = µ x N

where µ is the proportionality constant and N is the normal reaction the body experiences from the surface.


Here the constant (µ) plays a vital role. On what factor does the constant mu depend upon? It has been observed that the value of (µ) is greatly affected by the roughness of the surface upon which the body rests. It's value is a combined property of the contact surface as well as the surface roughness of the body itself. If we replace the body with another body of same mass but different material the value of (µ) changes. Also, if we place the body upon a different surface then also the value of mu changes. So, the value of mu is such a property that defines the characteristics of friction between the body and the contact surface. Hence, it is aptly named as the coefficient of friction.

There are basically two types of Co-efficient of Friction.

Co-efficient of Static Friction
Co-efficient of Kinetic Friction


When a block of mass is at rest on a surface and a horizontal force P is applied on the body to move it, a frictional force will be there to oppose any movement of the body. This force will act on the contact surface. Normal reaction is also acting upward on the contact surface. So total force on the contact surface will be resultant of normal reaction and frictional force. The angle made by this resultant force with normal reaction is called the angle of friction.


The direction of a frictional force depends upon the tendency of movement.

Suppose we get two identical block of weight W in identical planes at an angle ß with horizontal as shown in the figure.

Due to the force component W.sin ß acting downwards along the plane, the body will have a tendency to move downwards along the plane.

As the body would try to move downwards, a frictional force will be generated at the contact surface which would try to oppose the tendency to move downwards of the body, i.e., it would try to resist the downwards movement of the body. So, it will act upwards along the plane.

Normal reaction produced by the inclined surface at the contact point or area. The normal reaction will be equal and opposite the force component of the weight of the body at a perpendicular direction to the inclined plane hence, N = W cos ß, where N is the normal reaction.

Now suppose we plane adjust the inclination of the plane, it means we can either increase or decrease the inclination of the plane. When the inclination is very small, the downward force component W sin ß will be small and an equal magnitude frictional force will be produced and neutralize the downward force. Hence, the body will be at rest.

Now if we increase the inclination of the plane, the downward force component W sin ß will increases too, and frictional force will also be increased. Gradually, a condition will arrive when the downward weight component becomes equal to the maximum frictional force generated on the contact surface. This is limiting condition and also known as impending motion. If we increase the inclination angle ß by a small amount, the body will start to move downwards. The angle of the plane when the body is at limiting condition is known as angle of repose.


We can define angle of repose as the angle of the inclination of a plane when a body on the plane is at limiting condition of impending motion due to its self weight component along the inclined plane.

It is numerically equal to the angle of friction. It is denoted by (α).


It is an imaginary cone generated by revolving resultant reaction R about the normal reaction N. R is the resultant of the frictional force and normal reaction.

Properties of Cone of Friction:

The radius of this cone represents the frictional force (fs)max.
The semi apex angle of the cone represents the angle of friction.
For co-planar forces, in order for motion not to occur the reaction R must be within the cone of friction.

Friction is widely used in belt drive, where power is transformed from one place to another using a pulley-belt mechanism.

We use friction while we walk. We press the ground beneath us backward while friction present on the contact surface.

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