Catastrophes are often caused by friction weakening, as shown by the empirical volume effect of landslides. We use velocity-dependent, normal stress-dependent, power density-dependent and constant friction coefficient friction laws to determine the dominant friction weakening factor among velocity, normal stress and power density. We observe that the power density-dependent friction law not only adequately reflects the volume effect, the apparent friction coefficient first rapidly decreases with average thickness and then gradually stabilizes, but also mirrors the topographical effect. The apparent friction coefficient typically increases with drop height and slope angle, except when the volume is too small (thickness < 5 m). Predictions based on the power density friction law show that if the volume is less than a certain threshold (thickness < 5 m), the factor affecting the topographical effect would be biased toward velocity, whereas if it was greater than the threshold (thickness > 5 m), the factor affecting the topographical effect would be biased toward normal stress. Although the influence of velocity is generally smaller than that of normal stress, its influence cannot be ignored on a small scale. Overall, we obtain the power density, product of velocity and shear stress, dominant friction weakening instead of velocity alone, and normal stress alone. Notably, the power density more appropriately reflects the energy dissipation of friction weakening. Further this paper will provide a theoretical basis for predicting the motion of large long-runout landslide based on the power density-dependent friction law.