The Atomic Force Microscope (AFM) can be used to measure the minute forces between a sharp tip and a surface resulting from the chemical, topographical (shape), and mechanical properties of the surface at the nanometer scale. We have developed a method to rapidly and quantitatively measure the normal force vs. tip-surface separation, by probing the nonlinear dynamics of an oscillating cantilever as it interacts with the surface. The method involves the frequency mixing of two pure excitation tones and measurement of the intermodulation, or frequency mixing of these tones. The method allows one to examine in detail the tip-surface interaction at each pixel of an image acquired at typical AFM scan speeds. Being a dynamic method, it allows for investigation of not only conservative (elastic) tip-surface iterations, but also dissipative (viscous) interaction.