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Kinetic Friction




(3) Kinetic or dynamic friction



  • We already know that static frictional force exists between two surfaces in contact before there is relative motion between two surfaces
  • When applied force becomes greater then limiting friction force,then motion of the body starts and kinetic friction comes into existence
  • Thus when bodies in contact moves relative to each other then friction developed between them is called kinetic or dynamic friction
  • Consider the figure below in which a block of mass m is kept over the surface S
    Friction forces acting on the object due to its motion
  • Initially the block is at rest,now we push the block on the surface such that it begins to move or start sliding on the surface
  • When the block is sliding over the surface each body exerts a frictional force on the other side parallel to the surface in contact
  • The force of friction acting on the block B due to surface S is along the direction opposite to the motion of B with respect to S.Thus force of kinetic friction opposes the relative motion between the bodies in motion
  • The frictional force acting on the block is along the direction towards the left and an equal force acts on the surface S directed towards the rights
  • Thus we can say that sliding friction on a body B against surface S is opposite to the velocity of the body B with respect to S
  • Force of kinetic friction is denoted by fk and its magnitude is always less then the magnitude of limiting friction i.e maximum static friction
    i.e fk < fms
  • kinetic friction force is proportional to the normal contact force between the surfaces i.e
    fk is proportional to N
    => fkkN
    Where N =Normal contact force
    μk is the proportionality constant known as coefficient of kinetic friction and its value depends on the nature of the two surfaces in contact
  • fk < fms, this inequality shows that force required to start the motion is greater then the force required to maintain the further motion of the body


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