def critical_speed(_x):
    """Critical speed of rolling disc.

    _x is an array/list in the following order:
        g:  Gravitational constant
        r:  Radius of disc

    """
    # Unpack function arguments
    g, r = _x

    # Calculate return values
    cs = -3**(1/2)*(g/r)**(1/2)/3

    # Return calculated values
    return [cs]