Expansion and Collapse of Elastic Bodies Surrounded by Vacuum

A class of isotropic and scale invariant strain energy functions is given for which the corresponding spherically symmetric elastic motion includes bodies whose diameter becomes infinite with time or collapses to zero in finite time, depending on the sign of the  residual pressure. The body is surrounded by vacuum so that the boundary surface forces vanish, while the  density remains strictly positive.  The body is  subject only to internal elastic stress.