We were discussing the importance of friction i.e. positive and negative effects of friction with the help of our previous post.

Now, we will be interested further to understand the classifications of friction with the help of this post i.e. explain the different types of friction in detail. We will find out here the classifications of friction and will also brief here each type of friction in this post.

Let us start here with the classifications of friction.

### Classifications of friction

We will classify the friction as mentioned below.

Fig: 1- Classification of friction

### External friction

External friction is basically defined as the friction due to the interaction between surfaces of two solid bodies in contact.

External friction will be further classified in two types of friction as mentioned below.
1. Static friction
2. Dynamic friction

### Static friction

Static friction will come in to picture when surfaces of two solid bodies in contact are at rest but there is tendency for a relative motion.

### Dynamic friction

Dynamic friction will come in to picture when surfaces of two solid bodies in contact are in relative motion.

### Internal friction

Internal friction will be further classified in two types of friction as mentioned below.
1. Fluid friction
2. Solid friction

### Fluid friction

Fluid friction is also termed as viscous friction.

Fluid friction is basically developed between fluid elements when adjacent fluid layers are moving with different velocities.

Frictional force developed here will be proportional to the relative velocity between the fluid layers and the fluid viscosity.

Fluid friction concept will be widely used in the problems based on flow through pipes and orifices, bodies immersed in fluids and lubricated surfaces.

### Solid friction

Solid friction will be found in all type of solid materials subjected to cyclic loading. Energy will be dissipated internally within the material.

Frictional force due to solid friction will be proportional to the displacement.

Let us see here one more type of friction i.e. Dry friction

### Dry friction

Let us assume that a block, as mentioned in following figure, is subjected with a force F1.
Fig: 2- Block stationary at the surface

Now we need to ask to our self that whether there will be frictional force over here or not. We can easily say that there will not be any friction and hence frictional force in above situation.

Because, no frictional force will be developed in the absence of an external force to cause a tendency for relative motion. That means if there will not be any force acting on a body that causes the tendency for relative motion, there will be no frictional force.

If we consider that only force F1 is acting on the body which is displayed above, there will be no frictional force as there will be no tendency for a relative motion between body and surface over which body is placed.

If we consider the pushing force F is acting on the body, frictional force will be started to develop at the interface of the objects.

Force F will be balanced by the frictional force and hence the object will remain in equilibrium and at the rest condition.

When we will increase the pushing force F, frictional force will also be increasing to maintain the equilibrium condition of the body. When pushing force F will reach to the maximum frictional force, the object will be on the verge of sliding i.e. if force F will be increased even slightly more than the maximum value of frictional force, object will start to move or slide.

We can see it in following figure left part, we can see the curve drawn between the applied force and the frictional force developed while body is in rest condition here. Frictional force fs (static frictional force) will be increasing linearly with the pushing force F. The body will remain in stationary condition up to a point where pushing force reaches to the maximum value of frictional force.

Fig 3- Frictional force Vs Applied force

The first part of above figure shows the curve between the friction force and applied pushing force while body is in rest condition.

The right part of the above figure shows the curve between the friction force and the applied force while body is in rest condition. When pushing force F will reach to the maximum frictional force, the object will be on the verge of sliding i.e. if force F will be increased even slightly more than the maximum value of frictional force, object will start to move or slide.

We must have to note it here that the frictional force will be dropped when movement will be started and further it will be independent of relative velocity up to a limit and beyond that frictional force will be dropped again as displayed here in the right part of the above figure.

Therefore, we have studied here the various types of friction and frictional force with the help of this post. We have also introduced here the term dry friction and we have also understood here the importance of static friction and dynamic friction with the curve drawn between the applied force and friction force.

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We will find out now the coulomb's law of dry friction in our next post.

### Reference:

Engineering Mechanics, By Prof K. Ramesh