Friction

• Friction is the force that resists motion between two contacting surfaces.
• Friction is proportional to the normal force between the surfaces. (The normal force is the force vector that is perpendicular to the surface.)
• A box resting on the ground has F=mg (g is the gravity 9.81 for Earth).
• If the box is on an inclined ramp (theta), F=mg(cos(theta)).
  
  
• Friction also depends on the materials of the surfaces.
• Each material has a coefficition of friction, and this is multiplied by the normal force.
• F = umg. (or F_F = MuF_N)
• The direction of the force of friction an object experiences is always in the opposite direction to any net external force applied to the object.
• Coefficient of friction for ice-ice .1, steel-steel .7, and copper-steel .53
• There is a different component for at rest and moving friction. ice-ice moving = .03