Fig. 2. Sliding friction dynamics in steady sliding (A) and periodic stick–slip motion (B). During the static phase (orange), frictional forces increase linearly until they reach the critical frictional force value (F_static) when the slip phase (blue) starts. (A) In steady sliding systems, the kinetic friction force (F_kinetic) is measured once the system reaches a constant sliding velocity. (B) In stick–slip motion, however, the slip phase is immediately followed by another stick phase. Thus, stick–slip systems often never reach a constant velocity during the slip phase, thereby making calculations of F_kinetic difficult. In the present study, we calculated the maximum change in the coefficient of friction (µ) per stick–slip cycle by using the time points at which maximum and minimum frictional forces occurred during each stick–slip cycle, which correspond to F_static and the minimum frictional force (F_min), respectively. (Adapted from Persson, 2000.)