  Graphene nanoribbons (GNRs) physisorbed on a Au(111) surface can be picked
up, lifted at one end, and made slide by means of the tip of an atomic-force
microscope. The dynamical transition from smooth sliding to multiple stick-slip
regimes, the pushing/pulling force asymmetry, the presence of pinning, and its
origin are real frictional processes in a nutshell, in need of a theoretical
description. To this purpose, we conduct classical simulations of frictional
manipulations for GNRs up to 30 nm in length, one end of which is pushed or
pulled horizontally while held at different heights above the Au surface. These
simulations allow us to clarify theoretically the emergence of stick-slip
originating from the short 1D edges rather than the 2D "bulk", the role of
adhesion, of lifting, and of graphene bending elasticity in determining the GNR
sliding friction. The understanding obtained in this simple context is of
additional value for more general cases.
