An astrophysical disk can be modeled as a fluid with negligible mass compared to the central object (e.g. a star) and with negligible pressure. We can suppose an axial symmetry such that Φ Φ ( r , z ) = Φ Φ ( r , − − z ) . Starting from the equations of movement in cylindrical coordinates : r ¨ ¨ − − r θ θ ˙ ˙ 2 = − − ∂ ∂ r Φ Φ r θ θ ¨ ¨ + 2 r ˙ ˙ θ θ ˙ ˙ = 0 z ¨ ¨ = − − ∂ ∂ z Φ Φ