Molecular processes are both dynamic and energetic in nature and thus exhibit both conservative (elastic) and dissipative (viscous) forces. We present a method suitable for application to a conventional atomic force microscope (AFM) that is capable of measuring this complex quantity and use it to study the force-extension response of a single molecule. The dynamic force sensitivity of the AFM cantilever was increased with a positive feedback system producing a stable effective quality factor of over 300 in a liquid environment compared to the nominal value of similar to 1. This facilitated tracking of the resonant frequency and the separation of conservative (elastic) and dissipative (viscous) forces. With access to this extra information, not available by conventional static force spectroscopy, it is possible to estimate the effective viscosity of a single dextran molecule to-be 6000 Pa s.