1. A method for determining when to stop chemical mechanical polishing (CMP) of a workpiece, where the workpiece has a first-to-be-polished layer composed of a first material and an underlying layer structured to include a plurality of sacrificial pads composed of a second material, where planarized areas of the second material exhibit less friction against a utilized CMP slurry than do like-sized and planarized areas of the first material, the stop time determining method comprising: (a) while the first-to-be-polished layer is being polished by said utilized CMP slurry and before the sacrificial pads are substantially exposed, testing a friction-indicating signal that is indicative of magnitude of friction between the workpiece and a slurry holder to detect a first change in slope versus time, where the slope of the friction-indicating signal after the first change has a negative value that is more negative than a predefined, threshold negative slope and where the slope of the friction-indicating signal after the first change indicates that exposure of the sacrificial pads has substantially begun; (b) after the first change in slope is detected by said first-recited testing and while said utilized CMP slurry continues to be used for polishing of both the first and second materials, further testing the friction-indicating signal to detect a signature point in the friction-indicating signal, where the signature point is indicative of a more progressed, but not yet complete exposure of the sacrificial pads, and where the more progressed exposure constitutes a substantially greater exposure of the sacrificial pads to the utilized CMP slurry than the exposure of the pads when the first change in slope of the friction-indicating signal was detected.