Mechanical Transmission System: Friction, Definition, Causes and Laws. (Basic Technology Note)

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When one object rubs and grates another, it causes friction.

Friction can be defined as a force that resists the relative motion of solid surfaces, fluid layers and other forms of objects sliding against each other. The four known types of friction are Sliding, Static, Rolling and Fluid friction.

When a solid material slides over another solid surface, there is always resistance to motion. This resistance is what the scientists refer to as Friction. There is friction between the wheels of a car and the road. Friction exists between our shoes/legs and the ground. To sit on a chair there must be friction between our buttocks and the chair so that we don’t slip off the chair.


  •      The tougher the surfaces rubbing against each other, the more    the friction.
  •      The smoother the surfaces rubbing against each other, the less the friction.

Causes of Friction

The causes of the friction are molecular adhesion, surface roughness, and the plowing effect. Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Trying to slide objects against each other requires breaking these adhesive bonds.

Fig. 1

Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Trying to slide objects against each (as shown in Fig. 1) other requires breaking these adhesive bonds. For years, scientists thought that friction was caused by surface roughness, but recent studies have shown that it is actually a result of adhesive forces between the materials.

When two objects are brought into contact, many atoms or molecules from one object are in such close proximity to those in the other object that molecular or electromagnetic forces attract the molecules of the two materials together. This force is called adhesion. Trying to slide one object across the other requires breaking these adhesive bonds.  

Adhesion is the essence of friction.

Surface roughness is a factor when the materials are rough enough to cause serious abrasion. This is called the sandpaper effect.

All solid materials have some degree of surface roughness. If you looked at what seems to be a smooth surface under a high-powered microscope, you would see bumps, hills and valleys that could interfere with sliding motion.

Fig. 2

Plowing effect (or deformation of the object) occurs when one or more of the materials are relatively soft, hence the tougher one deforms the smoother one; as shown in Fig. 2.


     1.    Friction plays a vital role in our daily life. Without friction we are handicap.

     2.    It is difficult to walk on a slippery road due to low friction. When we move on ice, it becomes difficult to walk due to low friction of ice.

     3.    We cannot fix nail in the wood or wall if there is no friction. It is friction which holds the nail.

     4.    A horse cannot pull a cart unless friction furnishes him a secure Foothold.

Despite the fact that the friction is very important in our daily life, it also has some disadvantages such as: 

     1.    The main disadvantage of friction is that it produces heat in various parts of machines. In this way some useful energy is wasted as heat energy.

     2.    Due to friction we have to exert more power in machines.

     3.    It opposes motion.

     4.    Due to friction, noise is also produced in machines.

     5.    Due to friction, engines of automobiles consume more fuel which is a money loss. 

There are a number of methods to reduce friction in which some are discussed here.

Use of lubrication: 
The parts of machines which are moving over one another must be properly lubricated by using oils and lubricants of suitable viscosity.

Use of grease: 
Proper greasing between the sliding parts of machine reduces the friction.

Use of ball bearing: 
In machines where possible, sliding friction can be replaced by rolling friction by using ball bearings.

Design modification: 
Friction can be reduced by changing the design of fast moving objects. The front of vehicles and airplanes made oblong to minimize friction.

Laws of friction

There are five laws of friction. These are:

     1.    When an object is moving, the friction is proportional and perpendicular to the normal force (N).

     2.    Friction is independent of the area of contact so long as there is an area of contact.

     3.    The coefficient of static friction is slightly greater than the coefficient of kinetic friction.

     4.    Within rather large limits, kinetic friction is independent of velocity.

     5.    Friction depends upon the nature of the surfaces in contact.

Class activity:

    1.    How can friction be created?

    2.    How can fiction be reduced?

    3.    What are the advantages of friction?

    4.    What are the disadvantages of friction?


A lubricant is an oily substance that makes surfaces smooth and slippery, thereby reducing heat, wear, friction and vibration caused when two or more surfaces rub together.

Lubrication reduces friction. To lubricate means to apply oil in order to reduce friction, heat, and wear and tear.


    1.    What is friction?

    2.    What occurs when two objects rub against each other?

    3.    What is the frictional difference between tougher surfaces and smoother surfaces?

    4.    What are the three main causes of friction?

    5.    What is surface roughness?

    6.    What are the five laws of friction?

    7.    What is a lubricant?

    8.    What does it mean to lubricate?

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