From a mathematical standpoint, however, it sometimes is handy to put only the second-order derivatives on the right side of this equation; that is we write the above equation by rearrangement of terms as: F + m r θ θ ˙ ˙ 2 r ^ ^ − − m 2 r ˙ ˙ θ θ ˙ ˙ θ θ ^ ^ = m a ~ ~ = m r ¨ ¨ r ^ ^ + m r θ θ ¨ ¨ θ θ ^ ^ , where a "coordinate" version of the "acceleration" is introduced: a ~ ~ = r ¨ ¨ r ^ ^ + r θ θ ¨ ¨ θ θ ^ ^ , consisting of only second-order time derivatives of the coordinates r and θ. The terms moved to the force-side of the equation are now treated as extra "fictitious forces" and, confusingly, the resulting forces also are called the "centrifugal" and "Coriolis" force.