Simplified treatment of mechanical forces on materials in an electromagnetic field

The paper is an attempt to discuss, in very simple terms, some of the concepts used to interpret mechanical forces on a material medium in an electromagnetic field, as a consequence of a rate of change of electro magnetic momentum. Energy gradients arising from conversion to heat, or from progressive changes in the storage fields associated with ponderable matter, produce such forces, and so do reflections at an interface between two different media. In the paper the macroscopic picture of matter is mostly used, but attention is drawn to the microscopic one and the consequent duality of treatment that comes from this. It is concluded that a coherent and logical explanation of the various component forces may be obtained from the relevant rate of change of electromagnetic momentum in the material medium. Special consideration is given to the force that develops from a gradient of stored energy, because this does not seem to have received, in the past, all the attention it deserves.