  We present a ballistic description of the formation and propagation of the
working surface of a relativistic jet. Using simple laws of conservation of
mass and linear momentum at the working surface, we obtain a full description
of the working surface flow parametrised by the initial velocity and mass
injection rate. This allows us to compute analytically the energy release at
any time in the working surface. We compare this model with the results
obtained numerically through a new hydrodynamical code applied to the
propagation of a relativistic fluid in one dimension in order to test the
limits of our study. Finally, we compare our analytical results with observed
light curves of five long gamma ray bursts and show that our model is in very
good agreement with observations using simple periodic variations of the
injected velocity profiles. This simple method allows us to recover initial
mass discharge and energy output ejected during the burst.
