def calc_force(df, close, volume, col_name='force'):
    """Calculate force indicator (Elder).

    :param df: Data frame to do and add calculations to
    :param close: Pandas dataframe column name close price
    :param volume: Pandas dataframe column name volume
    :param col_name: Pandas dataframe column name for new force calculations
    :type df: ``pandas.DataFrame()`` [``float``]
    :type close: ``str``
    :type volume: ``str``
    :type col_name: ``str``
    :returns: Input dataframe with added force column
    :rtype: ``pandas.DataFrame()`` [``float``]
    """
    df[col_name] = df[close].diff() * df[volume]
    return df
