obtained independently using a tribometer for tuo sheets of the same aper. In this case UN) is simply the erternally-applied load, and the error bars correspond to the standard deviation of the measurements that have been repeated at least five times. The inset shows a fit to Eg. (8) (dashed line) of a traction erperi- ment(plain line for M 50. nown to be dependent on the normal load. Specifically,