In this Letter we show that recent observations using dynamic force spectroscopy can be described by a generalized Tomlinson model which includes the contribution of an external noise. We show that the measured friction forces depend on the microscopic potential and dissipation inherent to the system as well as on the mechanical properties of the setup (i.e. spring constant) and the external noise. Tuning the noise and spring constant offers ways to extract information about the microscopic properties. (C) 2002 Published by Elsevier Science B.V.