In a conservative field, the total mechanical energy (kinetic and potential) is conserved: E = 1 2 m | r ˙ ˙ | 2 + 1 2 I | ω ω | 2 + V ( r ) = constant (where ' ṙ' denotes the derivative of ' r' with respect to time, that is the velocity,' I' denotes moment of inertia of that body and ' ω' denotes angular velocity), and in a central force field, so is the angular momentum : L = r × × m r ˙ ˙ = constant because the torque exerted by the force is zero. As a consequence, the body moves on the plane perpendicular to the angular momentum vector and containing the origin, and obeys Kepler's second law. (If the angular momentum is zero, the body moves along the line joining it with the origin.)