def sefl(c, f, f0):
    """
    Semi-empirical normalized force limit.

    Parameters
    ----------
    c : scalar
        Constant based on experience, typically around 1.5
    f : scalar
        Frequency of interest, typically lower end of band.
    f0 : scalar
        Fundamental frequency in direction of interest.

    Returns
    -------
    nfl : scalar
        The normalized force limit:

        .. code-block:: none

            nfl = c                 f <= f0
            nfl = c / (f/f0)        f > f0

    Notes
    -----
    See reference [#fl1]_ for more information on force limiting.

    References
    ----------
    .. [#fl1] Scharton, T.D. (1997). 'Force Limited Vibration Testing
        Monograph'. NASA. Reference Publication RP-1403, JPL.

    See also
    --------
    :func:`ntfl`, :func:`ctdfs`, :func:`stdfs`.

    Examples
    --------
    Compute force limit for a s/c attached to a launch vehicle, where
    the interface acceleration specification level is 1.75 g, and:

     - frequency of interest starts at 75 Hz
     - fundamental axial mode of s/c is 40 Hz

    >>> from pyyeti import frclim
    >>> m = 6961    # s/c mass
    >>> faf = 40    # fundamental axial frequency of s/c
    >>> spec = 1.75
    >>> m*spec*frclim.sefl(1.5, 75, faf)
    9745.4
    """
    if f <= f0:
        return c
    return f0 * c / f