2.7 Kinetic and Static Friction

Describe kinetic friction between two surfaces.

• Kinetic friction occurs when two surfaces in contact move relative to each other.
  • i. The kinetic friction force is exerted in a direction opposite the motion of each surface relative to the other surface.
  • ii. The force of friction between two surfaces does not depend on the size of the surface area of contact.
• The magnitude of the kinetic friction force exerted on an object is the product of the normal force the surface exerts on the object and the coefficient of kinetic friction. Relevant equation: FFfk kN, µ= 
  • i. The coefficient of kinetic friction depends on the material properties of the surfaces that are in contact.
  • ii. Normal force is the perpendicular component of the force exerted on an object by the surface with which it is in contact; it is directed away from the surface. TOPIC 2.7 Kinetic and Static Friction

Describe static friction between two surfaces.

• Static friction may occur between the contacting surfaces of two objects that are not moving relative to each other.
• Static friction adopts the value and direction required to prevent an object from slipping or sliding on a surface. Relevant equation:
  • i. Slipping and sliding refer to situations in which two surfaces are moving relative to each other.
  • ii. There exists a maximum value for which static friction will prevent an object from slipping on a given surface. Derived equation: FFfs s ,, maxN µ=
• The coefficient of static friction is typically greater than the coefficient of kinetic friction for a given pair of surfaces.