def g(r):
    """
    Initial velocity.  Radial symmetry so, for this problem, initial velocity
    is only a functin of radial position.

    Parameters
    ----------
    r : float64
        radial position.

    Returns
    -------
    float64 - initial velocity at a given radial position.

    """
    b = 0.2;
    v0 = 10.;
    retval = 0.;
    if r < b:
        retval = -v0;
    
    return retval;