Drag forces in classical fields.

Inclusions, or defects, moving at constant velocity through free classical fields are shown to be subject to a drag force which depends on the field dynamics and the coupling of the inclusion to the field. The results are used to predict the drag exerted on inclusions, such as proteins, in lipid membranes due to their interaction with height and composition fluctuations. The force, measured in Monte Carlo simulations, on a pointlike magnetic field moving through an Ising ferromagnet is also well explained by these results.