Class 11 Physics Laws of Motion Kinetic friction

Kinetic friction

  • Force that resists motion of one body over another with which it is in contact.
  • Denoted by fk
  • As motion starts, fs vanishes and fk appears


  • Kinetic friction is Independent of the area of contact and velocity of the body
  • It varies with Normal reaction, N

fk ∝ N

fk = constant * N

fk = μk N; μk is the coefficient of kinetic friction

Three scenarios can arise in a body’s motion

  1. When applied force > fk   

 Fa > fk

(Fa – fk) = ma

a = (Fa – fk)/m

  1. When applied force = fk

Fa = fk

Therefore, a = 0 i.e. the body moves with uniform velocity

  1. When applied force = 0

Fa = 0

A = -fk/m

No motion occurs, the body stops


Relation between Coefficient of Static & Kinetic friction

fk = μk N

fs =< μs N

fs > fk (to keep the body moving)

μs N > μk N

Or,   μk  <  μs

Coefficient of kinetic friction is smaller than the coefficient of static friction.


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