Types of Friction

Frictional forces are classified as two types:

1. Internal friction

This type of friction occurs when there is a relative motion between the layers of any liquid. Its is also said as viscosity.

2. External friction

This type of friction arises when there are two bodies either trying to move or actually moving w.r.t each other. It is also called contact friction. External friction are of three types

1. Static friction
2. Limiting friction
3. Kinetic friction

1. Static friction

The opposing force that comes into play when one body tends to move over the surface of another, but the actual motion has not yet started is called Static Friction.

Explanation

Let us consider a block of weight $mg$ lying on a horizontal surface as shown in figure. When a body presses against a surface, the surface deforms even if it appears to be rigid. The deformed surface pushes on the body with a normal force $R$ that is perpendicular to the surface. This is called normal reaction. It balances $mg$ i.e.

$ R=mg$

Suppose a small force $P$ is applied on the block to the right as shown. The force of friction $F$ opposes the motion. So long as the block does not move,

$F = P$

This means as we increase $P$, friction $F$ also increases, remaining equal to $P$ always.

As seen above, the magnitude of static friction is not constant. It always adjusts itself so as to be equal to the applied force.

2. Limiting friction

As we increase the applied force, a stage comes when the body is just at the verge of moving over the other. The static friction at this stage is obviously maximum. This maximum value of static friction is called Limiting Friction.

Hence, Limiting Friction is the maximum opposing force that comes into play when one body is just at the verge of moving over the surface of the other body.

3. Kinetic friction

When we increase the applied force slightly beyond limiting friction, the actual motion starts. This does not mean that friction has disappeared. It only means that the applied force is now greater than the force of limiting friction. The force of friction at this stage is called  Kinetic Friction or Dynamic Friction.

Therefore, Kinetic Friction or Dynamic Friction is the opposing force that comes into play when one body is actually moving over the surface of another body.

It is found that the force of kinetic friction depends on normal reaction and on quality of finish of the rubbing surfaces. It does not depend upon the area of contact. But the kinetic friction does depend (though to a small degree) on the velocity of relative motion of the bodies.

If we plot a graph between the applied force and the force of friction, we get a curve of the type shown in figure. The part OA of the curve represents static friction, \( F_l \), which goes on increasing with the applied force.

At A, the static friction is maximum. This represents the limiting friction \( F_l = OL \). Beyond A, the force of friction is seen to decrease slightly. The portion BC of the curve, therefore, represents the kinetic friction \( F_k = OK \).

We have ignored here the dependence of kinetic friction \( F_k \) on the velocity of relative motion of the bodies.

Also, we find that kinetic friction is always slightly less than the limiting friction.

This is because, once the motion starts, actually, inertia of rest has been overcome. Also, when motion has actually started, irregularities of one surface have little time to get locked again into the irregularities of the other surface.

Further, dynamic friction or kinetic friction may be of two types:

1. Sliding friction

The opposing force that comes into play when one body is actually sliding over the surface of the other body is called sliding friction. For example, when a flat block is moved over the flat surface of a table, the opposing force is sliding friction.

2. Rolling friction

The opposing force that comes into play when one body is actually rolling over the surface of the other body is called rolling friction. For example, when a wheel, a circular disc or a ring or a sphere or a cylinder rolls over a surface, the force that opposes it is the rolling friction.

Cause of Rolling Friction

When a body rolls on a level track, the area of contact is very small. Therefore, the pressure exerted, which is equal to weight/area, is very large. This causes a depression in the surface below and a mount or bump in front as shown in the figure.

In turn, the surface of the rolling body in contact gets slightly compressed. Thus, a rolling wheel (i) constantly pulls out of depression and goes uphill on the mount LM, (ii) simultaneously detaches itself from the road KL, which is opposed by the forces of adhesion between the surfaces in contact. This causes rolling friction.

When a tyre is properly inflated, it becomes hard and gets compressed by the road to a much smaller extent. Therefore, rolling friction reduces. Hence it is easier to drive a bicycle when its tyres are fully inflated.

Note that the velocity of the point of contact of the wheel with respect to the floor remains zero all the time, although the center of the wheel moves forward. Therefore, rolling friction is often quite small compared to the sliding friction. That is why heavy loads are transported by placing them on carts with wheels. Thus sliding friction is converted into rolling friction. For example, rolling friction of steel on steel is hardly 1% of sliding friction of steel on steel.

Laws of Limiting Friction

The limiting friction obeys the following laws, which are based on experimental observations only:

1. The value of the limiting friction depends upon the nature of the two surfaces in contact and their state of roughness.

2. The force of friction is tangential (parallel) to the two surfaces in contact and acts opposite to the direction in which the body would start moving.

3. The value of limiting friction between two given surfaces is directly proportional to the normal reaction between the two surfaces.