The radius of gyration is defined as (I/M)^1/2, or the square root of the moment of inertia divided by mass. The moment of inertia is an important thing to know when solving problems that have to do with how things rotate. The equation for moment of inertia is different depending on the shape of the object, but for a flywheel (basically a solid disk), it’s I=(1/2)MR^2, where M is the mass and R is the radius of the flywheel. Putting this into the equation from before, we find that the radius of gyration for a flywheel is R*(1/2)^1/2, or 0.707*R.
The units of moment of inertia are mass*distance^2, so if you divide by mass and take the square root, you get something with units of just distance (a.k.a. the radius of gyration). Conceptually, the radius of gyration is the distance that, if the entire mass of the object were all packed together at only that radius, would give you the same moment of inertia. That is, if you were to take the entire mass of the disk-shaped flywheel with some radius and pack it into a narrow donut whose radius is the flywheel’s radius of gyration, they’d both have the same moment of inertia. With the same moment of inertia, they will behave very similarly when you spin them.
(republished on 07/13/06)