The shape of an orbit is often conveniently described in terms of relative distance r as a function of angle θ θ . For the Binet equation, the orbital shape is instead more concisely described by the reciprocal u = 1 / r as a function of θ θ . Define the specific angular momentum as h = L / m where L is the angular momentum and m is the mass. The Binet equation, derived in the next section, gives the force in terms of the function u ( θ θ ) : F ( u − − 1 ) = − − m h 2 u 2 ( d 2 u d θ θ 2 + u ) .