I am reading a book which mentions that frictional force is not exactly proportional to the normal force at high speeds. Quoting:
Once the bodies slip on each other frictional force is, $kN$. This is approximately true for relative speeds not too large (say for speeds <10 m/s).
$$k = Frictional constant$$ $$N = Normal force$$
If so is the case,then what is the general way to find out the frictional force?