The nonlinear second harmonic and ponderomotive forces acting on electrons in an inhomogeneous rf field have been derived from the electron equation of motion accounting for the energy dependence of electron–atom collisions. It is shown that in a collisionless plasma (ω≫ν) the second harmonic force F2ω and the ponderomotive Fdc force are similar in magnitude and frequency dependence (ω−2), where ω is the driving frequency and ν is the electron collision frequency. In a collisionally dominated plasma (ω<ν) both forces are diminished and have a different frequency dependence: F2ω∝(ων)−1 and Fdc∝ν−2. Therefore, in a collisional plasma, the second harmonic force is always greater than the ponderomotive force. © 2000 American Institute of Physics.