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When the atomic force microscope (AFM) is used for force measurements, the driving speed typically does not exceed a few microns per second. However, it is possible to perform the AFM force experiment at much higher speed. In this article, theoretical calculations and experimental measurements are used to show that in such a dynamic regime the AFM cantilever can be significantly deflected due to viscous drag force. This suggests that in general the force balance used in a surface force apparatus does not apply to the dynamic force measurements with an AFM. We develop a number of models that can be used to estimate the deflection caused by viscous drag on a cantilever in various experimental situations. As a result, the conditions when this effect can be minimized or even suppressed are specified. This opens up a number of new possibilities to apply the standard AFM technique for studying dynamic …