The unsteady motion of a small sphere in a viscous liquid

The drag experienced by a sphere moving with velocity dependent on a single time scale t 0 in an unbounded viscous liquid is considered under the assumption that the Reynolds number is small. It is shown that, unless t 0 is sufficiently large, an asymptotic expansion in the Reynolds number becomes invalid for large times. Moreover, when the expansion is valid for large times, the drag can differ considerably from that predicted by the unsteady Stokes equations.