Types of Friction

TYPES OF FRICTION

Depending upon the conditions of contacting surfaces, friction can be categorised into three different types, as shown in Fig.6.1.

1. Dry (or Coulomb) Friction

• The friction that exists between two unlubricated surfaces is known as dry friction. There is much resistance to the relative motion between the surfaces.

• The dry friction is often called Coulomb friction, since C.A. Coulomb studied its characteristics in 1781.

• It is further divided into two types:

(a) Sliding (or solid) friction: The friction that exists when one surface slides over another surface is known as sliding or solid friction. Example: Friction between nut and bolt. 

(b) Rolling friction: The friction that exists when one surface rolls over another surface is known as rolling friction. Example: Friction in ball and roller bearings.

• The rolling friction is always less than the sliding friction. For example, a given load can be easily shifted from one place to another by rolling than dragging or sliding it on the ground.

2. Skin (or Greasy or Boundary) Friction

• When the two surfaces in contact have a minute thin layer of lubricant between them, then it is known as skin or greasy or boundary friction.

• In this type of friction, a thin layer of lubricant forms a bond between the two rubbing surfaces. The lubricant is absorbed on the surfaces and forms a thin film. This thin film of the lubricant results in less friction between them.

3. Film (or Fluid or Viscous) Friction

• The friction that exists when the contacting surfaces are separated by a film of fluidis known as fluid friction.

• It arises when a film of fluid separates two surfaces, and when layers of the fluid move over each other at different speeds.

• Unlubricated surfaces mean contacting surfaces in the absence of a lubricating fluid.

• Fluid refers to both gas and liquid.

• The fluid friction is important in the problems involving the flow of fluids through pipes and channels or dealing with bodies immersed in moving fluid.

• This type of friction is also termed film friction or viscous friction.

Note The friction may also be classified as:

1. Static friction: It is the friction, experienced by a body, when at rest.

2. Dynamic friction: It is the friction, experienced by a body, when in motion.

1. Laws of Dry (or Coulomb) Friction

Following are the laws of dry or Coulomb friction:

1. The frictional force (ie., force of friction) is directly proportional to the normal reaction between the surfaces.

2. The frictional force opposes the motion or its tendency to the motion.

3. The frictional force depends upon the nature of the surfaces in contact.

4. The frictional force is independent of the area and the shape of the contacting surfaces.

5. The frictional force is independent of the velocity of sliding of one body relative to the other body.

2. Limiting Force of Friction (F)

• The ultimate (maximum) value of frictional force, which comes into play, when a body just tends to move, is known as limiting force of friction or limiting friction or friction of impending sliding.

• In other words, the maximum value of frictional force acting on a body when the body is on the point of motion, is called limiting force of friction.

3. Coefficient of Friction (μ)

• The coefficient of friction is defined as the ratio of the limiting friction (F) to the normal reaction (RN) between the two bodies.

• Coefficient of friction,

4. Limiting Angle of Friction (ϕ)

• Definition: The limiting angle of friction (ϕ) is defined as the angle at which the resultant reaction R makes with the normal reaction RN.

Relationship between F and ϕ

Consider a body A of weight W is resting on a horizontal plane B, as shown in Fig.6.2.

If a horizontal force P is applied to the body, no relative motion will take place

until the magnitude of this applied force is equal to the force of friction (F), acting opposite to the direction of motion. The magnitude of this force of friction is given by

F = μW = μ • RN

where RN = Normal reaction.

In the limiting case, when the motion is just about to start, the body A will be in equilibrium under the action of the following three forces:

1. Weight of the body (W),

2. Applied horizontal force (P), and

3. Reaction (RN) between the body A and the plane B.

According to first law of friction, friction force is directly proportional to the normal reaction between the surfaces in contact.

where μ = Constant of proportionality and is called as coefficient of friction. 

The inclination of the resultant, from Fig.6.2, is given by

The angle ϕ is known as the limiting angle of friction or angle of friction.

Note

1. If tan ϕ = μ or ϕ = tan-1μ, then the body will move over the plane irrespective of the magnitude of the force F.

2. If tan ϕ < μ or ϕ < tan-1μ, then no motion of body on plane is possible irrespective of how large the magnitude of F may be.

3. The tan ϕ value cannot be greater than μ, or ϕ value cannot be greater than tan-1μ.

5. Angle of Repose (α)

• Definition: The angle of repose is defined as the limiting angle of inclination of a plane at which a body just starts sliding down the inclined plane.

• The angle of repose (α) is equal to the limiting angle of friction (μ).

Proof: Consider a body of weight W resting on an inclined plane B, as shown in Fig.6.3. The weight of the body W can be resolved into the following two components:

(i) W sin α, parallel to the plane, and 

(ii) W cos α, perpendicular to the plane. 

The body begins to move downwards on the plane, when W sin α ≥ F. From the geometry of figure,

Hence, the angle of repose a is equal to the limiting angle of friction ϕ.