Friction and its types | Laws | Coefficient of friction

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Definition of friction: 


A force that is acting in the opposite direction to the motion of the body is called friction. Friction is resistance to the motion of one object moving relative to another object. 

Frictional force f = µ × N

Where, f = frictional force
µ = coefficient of friction 
N = Normal force

Types of friction: 


Friction mainly classified into two types. 
  1. Static friction 
  2. Dynamic friction 
The friction experienced by a body, when an object is at rest is known as static friction. 

The friction experienced by a body, when the body is in motion is known as dynamic friction. It is also called kinetic friction. Kinetic friction also divided into two types which are the following below. 
  1. Sliding friction
  2. Rolling friction
The friction experienced by a body, when the one body slides over another body is known as sliding friction. 

The friction experienced by a body, when balls or rollers are placed between the two surfaces is known as rolling friction.  

There are some other types of friction also depending on surfaces which are following below.
  1. Dry friction
  2. Fluid friction
  3. Lubricated friction 
  4. Skin friction 
  5. Internal friction 

Limiting friction: 


The maximum value of frictional force, when comes into play, when a body just begins to slide over the surface of the other body is known as limiting friction. 

Laws of static friction: 

  • The frictional force always acts in one direction, opposite to that in which the body tends to move.
  • The magnitude of frictional force is exactly the same as the force which tends to move the body.
  • A constant ratio between the two surfaces and the natural reaction bears the magnitude of the limiting friction.
  • The frictional force is independent of the contact area between the two surfaces.
  • The force of friction depends on surface roughness.

Laws of dynamic or kinetic friction: 

  • The frictional force always acts in one direction, opposite to that in which the body tends to move.
  • The magnitude of the dynamic friction bears a constant ratio to the normal reaction between the two surfaces. 
  • The frictional force remains constant for moderate speeds. But with speed increase, it slightly decreases.

Coefficient of friction: 


The coefficient of friction is the ratio of limiting the friction F between the two bodies to the normal reaction. It is denoted by µ. 

Coefficient of friction µ = F / RN 

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