This book is dedicated to Einstein's special relativity. The main topics are: postulates of relativity, events and Minkowski space-time, Lorentz transformations, metrics in scalar products (with an analysis on the metric tensor in an Euclidean and Minkowski space), intervals and their classification, effects of relativity such as time dilation and length contraction, speed transformations, equation of motion in relativistic dynamics, relativistic Lagrangian, kinetic, mass and total energy and their non-relativistic limits, four-momentum conservation.
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In non-relativistic mechanics we know that for a free system, i.e. not subject to forces, the total momentum and the energy are conserved. In special relativity these two conservation laws become only one conservation law: the four-momentum

conservation law.
The conservation, for a certain physical process, such as a decay of a particle, implies that the four-momentum before and after the process is the same

where we have indicated with capital letter the total final four-momentum, while with lowercase the initial one.
Let us consider, as an example of application, the decay of a particle X into two particles Y and Z. We can write the decay as
