The 4-velocity as a crucial node is linked to two aspects in the special relativity.One aspect is the relative 4-space-time due to the postulate of the constancy of the light speed,while the other aspect is the covariant mechanics and electrodynamics laws due to the pos...MORE The 4-velocity as a crucial node is linked to two aspects in the special relativity.One aspect is the relative 4-space-time due to the postulate of the constancy of the light speed,while the other aspect is the covariant mechanics and electrodynamics laws due to the postulate of the relativity.Teaching practice for a lon...