Answer 7_1_6
The key to this is the moment of inertia. The moment of inertia of the solid disk flywheel (ignoring the flange in the middle) is given by:
I = 1/2 Mr2
The second flywheel can, to a first approximation, be considered as a cylinder of mean radius, r. The assumptions are
that the thin part of the wheel has a mass that is very much smaller that the thick ring part.
the flange can be ignored.
The moment of inertia is given by:
I = Mr2
Therefore the second flywheel will have a moment of inertia that is double that of the first. Therefore it will store double the kinetic energy for any given angular velocity.