Let us now consider small departures from the circular motion of the orbiting mass element caused by some perturbing force. We transform variables into a rotating frame moving with the orbiting mass element at angular velocity formula_14 with origin located at the unperturbed, orbiting location of the mass element. As usual when working in a rotating frame, we need to add to the equations of motion a Coriolis force formula_15 plus a centrifugal force formula_16 The velocity formula_17 is the velocity as measured in the rotating frame. Furthermore, we restrict our attention to a small neighborhood near formula_18 say formula_19 with formula_20 much smaller than formula_21 Then the sum of the centrifugal and centripetal forces is