Main Question or Discussion Point
when a body is slided or move in a horizontal floor, the Frictional force is directly proportional to the Normal force acting on the body. Is there is any proof (emperically or Theoretically)
In the case of tires, there's a load sensitivity factor, reducing the coefficient of friction as the load increases. Wiki artcile, although this is for static friction:In fact, like most simple physics laws, it is a "first order" approximation.