He then asserted that if we “consider exclusively the translational motion of the plate as a whole, we surely can view the plate as a material point of a certain mass M.” This assumption was necessary because the equations for relativistic energy and momentum were for point masses. Equating the plate to a material point brings to mind the conundrum of the work-energy theorem, which haunts many of these derivations, and again raises questions. Was the plate supposed to be rigid? If so, it offends relativity.