Static and Kinetic friction

Question

How are the values of the coefficients of static and kinetic friction affected by the area of contact of a body?

Answer 1

The coefficients are independent of contact areas, but can determined experimentally for a block and incline as described below:

 

The coefficient of Static Friction between the bottom of a block and the top of an incline
- is determined by the tangent of the angle at which a block begins to slide down and incline.

 

The Coefficient of Kinetic Friction between the bottom of a block and the top of an incline
– is determined by the tangent of the angle at which a block slides down on an incline at constant velocity.

 

This method can be use for various materials.

 

Answer 2

The greater the area of contact, the greater the coefficients of friction.  This is true for both kinds of friction.