Laws of Friction

  • The frictional force between two stationary surfaces is called the static friction,  fs.
  • The frictional force between two sliding surfaces is called the kinetic friction, fk.
  • It can be shown empirically that between two particular surfaces
    • static friction fs ranges from zero to a maximum value.

0 < fs < μsN

    • kinetic friction fk is constant and independent of speed.

fk = μkN 

    • maximum static friction is larger than kinetic friction.

This video shows clearly how the static friction increases until reaching its maximum value, then drops to the constant kinetic friction.

Unlike drag force, friction is unaffected by the sliding speed. It is also unaffected by the area of the surfaces in contact. All that matters is the material of the two surfaces and how hard they are pressed against each other.

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