µ must be kidding!

*Frictional Force

*Frictional Force

*The Force of Friction depends on:

*The surfaces in contact

*The amount of the normal force

*Whether the object is moving (kinetic) or stationary (static)

*Ff = µFn

*Coefficient of Friction

*µ = coefficient of friction which is a “unitless” number which depends on the surfaces in contact.

*µs = static coefficient of friction which is used when the objects are static (not moving relative to eachother.

*µk = kinetic coefficient of friction which is used when the objects are sliding relative to eachother.

*Coefficient of Friction

*µs > µk

Surfaces µ static µ kinetic

Wood on wood 0.4 0.2

Rubber on concrete 1.0 0.8

Banana peel on bugs bunny

0.1 0.0000006

*Static Frictional Force

*Fs µs Fn

*The static frictional Force is typically only as big as it needs to be.

*The static frictional force is a maximum when the above equation is an equality (like when an object on an inclined plane just starts slide as the angle of incline is increased)

*Kinetic Frictional Force

*Ff = µk F n

*When objects are sliding this is always an equality

*The kinetic coefficient of friction is less than the static coefficient of friction