Class 11 Physics Laws of Motion Friction


  • Friction is a contact force that opposes relative motion.
  • No friction exists till an external force is applied.

There are 3 types of Friction:

  • Static friction
    • Force that resists initiation of motion of one body over another with which it is in contact
    • Opposes Impending motion
    • Denoted by fs  

Let the ball be at rest initially

Applied force, Fa = 0; Static friction, fs = 0

Later, applied force, Fa = F, then fs also increases but only up to a certain limit. As soon as Fa becomes greater than fs, the ball starts to move.


fs acts when a body is at rest. Hence called Static friction

  • Limiting value of fs depends on Normal reaction, and is independent of the area of contact

fs max ∝ N

fs max = constant * N

fs max = μs N

where μs is the coefficient of static friction

This coefficient depends on the nature of surfaces in contact.

  • According to the Law of Static friction, Static friction is always less than or equal to the limiting value of fs

  fs  =< fs max 

  fs max = μs

  fs  =< μs

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