where otherwise stated, constants are denoted by the following symbols and may be taken to have the values below Elementary charge Speed of light in vacuum Planck constant Permeability of vacuum Permittivity of vacuum Unified atomic mass constant Rest mass of proton Rest mass of electron Boltzmann constant Avogadro constant Gravitational ... Two space craft are travelling in opposite directions towards each other, one at speed u and the other at speed v with respect to the Earth. ...