Then Lagrange's equations of motion d d t ( ∂ ∂ L ∂ ∂ r ˙ ˙ ) = ∂ ∂ L ∂ ∂ r take the form m r ¨ ¨ = m r φ φ ˙ ˙ 2 − − d U d r = m h 2 r 3 + F ( r ) since the magnitude F (r) of the radial force equals the negative derivative of the potential energy U (r) in the radial direction.