I've been studying the Basset-Boussinesq-Oseen (BBO) Equations and I don't really understand the unsteady forces terms which are: - Added Mass Effect - Basset Force It says the unsteady forces arise from an acceleration to the relative velocity vector. Can someone explain with an example? I'm, having difficulties really picturing what relative velocities are in this context. Also, the added mass effect comes from the fact that the particle does work on the surrounding fluid. I don't understand this because isn't this how lift/ drag is created? Why is this any different from that? Finally, regarding the Basset force, if there was a relative velocity (i.e. u-v was non-zero), wouldn't there be a lag in the boundary layer anyway without the need for an acceleration? As a sidenote, can someone elaborate why form drag and Buoyancy are different? I've read from another thread here that form drag is to do with the dynamic pressure and Buoyancy the static pressure, yet all the formulae to work these out appear to be exactly the same which is the integral of a pressure gradient around the surface :s.