Grain velocities as a function of the grain radius for a homogeneous grain of specific density 3 g cm ⁻³. The model of grain acceleration by turbulence Ormel & Cuzzi (2007) is presented at different densities. Thick lines represent the relative rms velocity between the grain and the gas, while thin lines represent the relative rms velocity between grains of the same size. The contribution of Brownian motion to the velocity of grains is almost always negligible. The velocity limit for coagulation is presented in two versions: the theory from Chokshi et al. (1993) and the experiments from Poppe & Blum (1997). The sound velocity (T = 10 K) in this isothermal phase is added for reference.