Effective mass (solid-state physics) from both sides gives where is the th element of , is the th element of , and are the th and th elements of , respectively, and is the total energy of the particle according to the Planck–Einstein relation. The index is contracted by the use of Einstein notation (there is an implicit summation over ). Since Newton's second law uses the inertial mass (not the gravitational mass), we can identify the inverse of this mass in the equation above as the tensor This tensor expresses the change in group velocity due to a change in crystal momentum. Its
Front velocity any medium tends to one as the frequency tends to infinity, that the wave discontinuity, called the "front", propagates at a speed less than or equal to the speed of light "c" in any medium. In fact, the earliest appearance of the front of an electromagnetic disturbance (the precursor) travels at the "front velocity", which is "c", "no matter what the medium". However, the process always starts from zero amplitude and builds up. Front velocity In physics, front velocity is the speed at which the first rise of a pulse above zero moves forward. In mathematics, it is used to