It has been proposed that large flywheels could be used to store energy. Consider a flywheel made of concrete (density 2060 kg/m3) in the shape of a solid disk, with a radius of13.7 meter and a thickness of 2.5 meter. If it's rotational kinetic energy is 693 MJ (6.93 x108 Joule), find what is the angular velocity of the flywheel?