A wooden crate is pushed along a rough horizontal warehouse floor, then brought back to its starting point along the same straight-line path. During both legs of the trip, a constant kinetic-friction force of magnitude Ff acts opposite the direction of motion.
(a) Describe two essential characteristics that distinguish conservative forces from nonconservative forces, using precise physics terminology.
(b) For the closed round-trip described, state whether the total work done by the kinetic-friction force is positive, negative, or zero. Explain your reasoning without making a numerical calculation.
(c) Using your answers to parts (a) and (b), justify why it is impossible to define a position-dependent potential-energy function Ufriction(x) for kinetic friction that is analogous to gravitational or elastic potential energy.