  We present a study of hydrodynamic drag forces in smoothed particle
simulations. In particular, the deceleration of a resolution-limited cold clump
of gas moving through a hot medium is examined. It is found that the drag for
subsonic velocities exceeds that predicted by simple physical approximations.
This is shown to be a result of the hydrodynamical method which encourages the
accretion of particles from the hot medium onto a shell around the cold clump,
effectively increasing the radius of the clump. For sonic and supersonic
velocities, the drag is shown to be dependent on the effective cross section of
the clump. The consequences for cosmological simulations are discussed.
