  The solution of the classic problem of stress in a rotating elastic disk or
cylinder, as solved in standard texts on elasticity theory, has two features:
dynamical equations are used that are valid only in an inertial frame of
reference, and quadratic terms are dropped in displacement gradient in the
definition of the strain. I show that, in an inertial frame of reference where
the dynamical equations are valid, it is incorrect to drop the quadratic terms
because they are as large as the linear terms that are kept. I provide an
alternate formulation of the problem by transforming the dynamical equations to
a corotating frame of reference of the disk/cylinder, where dropping the
quadratic terms in displacement gradient is justified. The analysis shows that
the classic textbook derivation of stress and strain must be interpreted as
being carried out in the corotating frame of the medium.
