Friction is a necessary evil

Posted by Phisyks on 09 Oct 2015


“Friction is a necessary evil” – A very famous quote

Friction is the force that opposes the motion of an object that is in contact with another body. The contact is important, since friction is a contact force.

The cause of friction is a combination of several surface phenomena including irregularities in surfaces, Electrostatic force of attraction etc.

Frictional force are of 2 types:

  • Static friction
  • Kinetic friction


Static friction:

The friction that keeps the body in rest even if enough force is applied. This force has a limit. This limit is called the Maximum static friction or the limiting friction.

The formula for limiting static friction is:


Where N is the normal reaction on the body and µs is the coefficient of static friction.



Where α is the angle of the inclined plane. Now the value of friction for different situations is as follows

  • When the body is in rest
      fs = mg x sin (α)  
  • The maximum static friction or the limiting friction is
      fmax = µs x N


Kinetic friction:

When the body starts to slide down the inclined plane the kinetic friction acting on the body is given by


These are the general properties of friction.