Fig. 6. Synchronous measurements of plectrum displacement (A) frictional forces (B, F_f; C, F_n) and coefficient of friction (D, µ). The oscillating force and coefficient of friction waveforms are a product of the stick–slip friction dynamics of the plectrum moving over the file. Each force trace (F_f and F_n) is correlated with the movement of the plectrum. In B, the raw data are depicted in blue and the filtered data (low pass filter at 280 Hz) in red. Grey regions label the slip phases of the stick–slip cycling. In D (coefficient of friction), the linear fits of the static friction regions of the waveform are depicted in red.