Rigid body motion in special relativity

We study the acceleration and collisions of rigid bodies in special relativity. After a brief historical review, we give a physical definition of the term `rigid body' in relativistic straight line motion. We show that the definition of `rigid body' in relativity differs from the usual classical definition, so there is no difficulty in dealing with rigid bodies in relativistic motion. We then describe: 1. The motion of a rigid body undergoing constant acceleration to a given velocity. 2. The acceleration of a rigid body due to an applied impulse. 3. Collisions between rigid bodies.Comment: Extended the discussion,and added reference

The nature of electromagnetic energy

The nature of the electromagnetic (EM) energy for general charge and current distributions is analyzed. There are two well known forms for calculating EM energy as the integral over all space of either the electromagnetic fields: u_{\bf EB}=({\bf E\bcdot D+B\bcdot H})/8\pi, or the electromagnetic potentials and charge-current densities: u_{\rho{\bf A}}=1/2(\rho\phi+{\bf j\bcdot A}). We discuss the appropriate use of each of these forms in calculating the total EM energy and the EM energy within a limited volume. We conclude that only the form $u_{\bf EB}$ can be considered as a suitable EM energy density, while either form can be integrated to find the total EM energy. However, bounding surface integrals (if they don't vanish) must be included when using the $u_{\bf EB}$ form. Including these surface integrals resolves some seeming paradoxes in the energy of electric or magnetic dipoles in uniform fieldsComment: The discussion and conclusions have been modifie