This work contains a short overview on two phenomenological models aiming to evaluate the loss of energy when a nanotip oscillates at the proximity of a surface. In one case, the surface is described as a continuous medium with a local stiffness k(s) and a local damping term gamma(s). The viscoelastic properties allows an easy evaluation of the energy loss as a function of the surface relaxation times tau(s) = gamma(s)/k(s). The main purpose of the second approach is to evaluate the effect of large fluctuations on the dynamic properties of the oscillator. The surface is built of atoms, each of them lying in a symmetric double-well potential and the oscillating nanotip produces a stochastic resonance process. The linear response theory is applied to evaluate fluctuations of the particle corresponding to interwell motions. Then from the amplitude, fluctuation is calculated the loss of energy. The result shows that a stochastic resonance process can increase the amount of dissipate energy of two orders of magnitude at a forcing frequency much smaller than the usual ones corresponding to atom motions. (C) 2002 Elsevier Science B.V. All rights reserved.