> Pfffft. The only correct answer is to wake up or stop haullcinating, because there's no such thing as a
> perfectly smooth (frictionless) surface.
You must not have met many physicists... frictionless surfaces, two dimensional
potentials, spherical horses... I could go on and on. And this is during real
work, not just exam questions.
Knowing _which_ parameters you can idealize is one of the things which
separates a good physicist from a bad one!