  We discuss the relative grain motions due to MHD turbulence in interstellar
medium. It has been known for decades that turbulent drag is an efficient way
to induce grain relative motions. However, earlier treatments disregarded
magnetic field and used Kolmogorov turbulence. Unlike hydro turbulence, MHD
turbulence is anisotropic on small scales. Moreover, compressible modes are
important for MHD and magnetic perturbations can directly interact with grains.
We provide calculations of grain relative motion for realistic interstellar
turbulence driving that is consistent with the velocity dispersions observed in
diffuse gas and for realistic grain charging. We account for the turbulence
cutoff arising from abmipolar drag. Our results on grain shattering are
consistent with the customary accepted cutoff size. We obtain grain velocities
for turbulence with parameters consistent with those in HI and dark clouds.
These velocities are smaller than those in earlier papers, where MHD effects
were disregarded. Finally, we consider grain velocities arising from
photoelectric emission, radiation pressure and the thrust due to molecular
hydrogen formation. These are lower than relative velocities induced by
turbulence. We conclude that turbulence should prevent these mechanisms from
segregating grains by size.
