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 parametrized by the initial velocity and mass injection rate. We use this model to reproduce the gamma-ray light curve of the blazar PKS 1510-089 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,the mass ejection rate, and energy output ejected during the burst among other parameters.