def Drag(F, A, V, rho):
    """Calculates drag coefficient `Cd` for a given drag force `F`,
    projected area `A`, characteristic velocity `V`, and density `rho`.

    .. math::
        C_D = \\frac{F_d}{A\\cdot\\frac{1}{2}\\rho V^2}

    Parameters
    ----------
    F : float
        Drag force, [N]
    A : float
        Projected area, [m^2]
    V : float
        Characteristic velocity, [m/s]
    rho : float
        Density, [kg/m^3]

    Returns
    -------
    Cd : float
        Drag coefficient, [-]

    Notes
    -----
    Used in flow around objects, or objects flowing within a fluid.

    .. math::
        C_D = \\frac{\\text{Drag forces}}{\\text{Projected area}\\cdot
        \\text{Velocity head}}

    Examples
    --------
    >>> Drag(1000, 0.0001, 5, 2000)
    400.0

    References
    ----------
    .. [1] Green, Don, and Robert Perry. Perry's Chemical Engineers' Handbook,
       Eighth Edition. McGraw-Hill Professional, 2007.
    .. [2] Cengel, Yunus, and John Cimbala. Fluid Mechanics: Fundamentals and
       Applications. Boston: McGraw Hill Higher Education, 2006.
    """
    return F / (A * rho * V ** 2 / 2.0)
