On the Motion of Free Material Test Particles in Arbitrary Spatial Flows
  We show how the motion of free material test particles in arbitrary spatial flows is easily determined within the context of ordinary vector calculus. This may be useful for everyone, including engineers and other non-specialists, when thinking about gravitational problems. It already has valid application to simple problems such as the problems of motion in rotating and accelerating frames and to the gravitational problem of the single spherically symmetric attractor. When applied to the two body gravitational problem, it may help us determine the actual direction of the flow. 
Tom Martin