The physical nature of the forces acting between a laterally vibrating probe in air near a solid surface has been investigated using a calibrated method for the experimental determination of dynamic force. The frequency response of the probe vibrating near its fundamental bending resonance was measured as a function of distance as the probe approached the surface. The experimental results were compared with a continuumhyphen;mechanical model of bending waves in a rod; the nature of the forces was inferred by determining the effect of various boundary conditions on the standinghyphen;wave motion of the probe. The results conclusively show that while in the initial stage of approach the force is dominated by fluidhyphen;dynamic effects, in the final stage of approach an elastic force appears. The appearance of the elastic deformation is characterized by a shift in the resonant frequency of the vibrating probe. copy;1996 American Institute of Physics.
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