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Dynamic energy losses in thin Ni-Fe films during magnetization reversal under sinusoidal magnetic drive fields of 15-Oe peak at frequencies up to 50 kHz have been studied by direct observation of the dynamic M-H loop. Empirically it is found that the energy losses depend neither upon frequency alone nor peak drive fieId alone, but rather upon the slope in Oe/s of the drive field at the instant of magnetization reversal. Energy losses are proportional to for small and to for large up to Oe/s, the apparatus limit. Simultaneous observation of easy-axis and hard-axis pick-up loop voltage waveforms indicates that magnetization reversal occurs by coherent rotation for large and by domain-wall motion for small . Observed behavior of the energy losses is shown to be in conformity with the classical descriptions of domain-wall motion and coherent rotation.