Show that a solid cylinder of mass M and radius R is equivalent to a thin hoop of mass M and radius R/sqrt(2) , for rotation about a central axis. (b) The radial distance from a given axis at which the mass of a body could be concentrated without altering the rotational inertia of the body about that axis is called the radius of gyration. Let k represent the radius of gyration and show that k = sqrt(I*M) . This gives the radius of the "equivalent hoop" in the general case.