# How do I know that $\mu_k$ independent of velocity and the area of contact?

Doubts:- Although he is stating that till now there is no effective model to explain kinetic friction. (1)Why Kinetic friction is independent of the contact area of the body with the surface?

(2)Why kinetic friction is independent of the velocity of the moving object?

$$|\vec{F_k}|=\mu_k |\vec N|$$. What if $$\mu_k$$ depends on the area of contact and velocty of the object? How do I know it is not?