I have begun studying Ashcroft + Mermin on my own, and am trying to follow the math in the text. They suggest that an electron in a metal with some momentum p(t) and exposed to a force f(t) will at some time later (t+dt) have a contribution to the momentum on the order of f(t)dt plus another term on the order of dt*dt. My question is where does this dt^2 term enter? My instinct is to say that F=dp/dt, and that the change in momentum can then be given (very simplistically?) by f(t)dt. Obviously, a form of f(t) and an integral is in order, but I cannot see the logic of what is stated in the text. Any and all help or guidance on the matter would be greatly appreciated!