It's pretty simple. There is static friction and kinetic friction because an object can be static relative to the surface it sits on or it can be in motion.
The static frictional force can be anything between 0 and ?N depending on the external force applied. Once the applied force that opposes friction surpasses ??, the object is in motion and now experience kinetic friction. Notice that we can never have the coefficient of kinetic friction be greater than coef. of stat. fric. because then getting an object to move would be impossible. There is no theoretical reason that the two coefficients couldn't be equal, however, though we typically observe it that ?_k<?_s.
>okay but whyNot science. There isn't more to it. Static and kinetic friction are just macroscopic approximations that account for a shitload of microscopic interactions at the interface between the materials as shown in the pic here.
>>11367525What more could you be confused about?
