def arrow_polygon(size):
    """
    creates a polygon tuple with a centered arrow for use with sim.Animate

    Parameters
    ----------
    size : float
        length of the arrow
    """
    size /= 4
    return (-2 * size, -size, 0, -size, 0, -2 * size, 2 * size, 0, 0, 2 * size, 0, size, -2 * size, size)