As our frame of reference, we begin with Cartesian coordinates in 3space R. Assume the frame is non-accelerating (inertial). Suppose there is a mass m at a fixed point in space r′ = (x′, y′, z′). This is a point source. It produces a gravity field around it, which can be sensed by a test mass at another point in space. How strong and in what direction is the tug of the field as it acts on the t...