The present study is divided into six articles.Part-5:Some efforts have been made to prove negative mass behavior through some experiments performed in mechanics [1], and other disciplines [9], as well as some theories in electrostatics [2,3,4,5,6,7,8], but I haven’t found research about similar effects at the atomic level, where the most elementary mass given by the atomic nucleus is to be found.Is the second Newton’s law still valid with negative mass?What could happen if we make the atom behave in a negative mass regime?Is the negative refractive index related to negative mass?Are we able to control the magnitude of mass?Are we able to control the sign of mass?The answers to these questions are given through this series of papers, with results that are coincident with experimental data, except for the negative mass regime. Experiments must be done to confirm or invalidate the theory developed in these articles.In This Paper:Following the analysis made in Part-3, the third remaining external force is evaluated in this study.The nuclear response to external forces is analyzed with the aim to observe any changes in the nuclear mass and study the behavior of the refractive index under such changes.The analysis will be performed in the time domain as well as in the frequency domain by means of the Fast Fourier Transform (FFT) method. The external forces applied to the nucleus were classified into three types:The force originated by a polarized transverse electromagnetic wave (TEM) (see Part- 3).The force originated by a polarized TEM plus a static electric field (see Part-4).The force originated from a signal plus a static electric field.