The final equation derives from Newton's third law ; the force of the second body on the first body (F 21) is equal and opposite to the force of the first body on the second (F 12). Thus, the equation of motion for r can be written in the form μ μ r ¨ ¨ = F where μ μ is the reduced mass μ μ = 1 1 m 1 + 1 m 2 = m 1 m 2 m 1 + m 2