A Lagrangian based method is used to derive an analytical model for studying the dynamics of matter-wave bright soliton created in a harmonic potential which is attractive in the transverse direction and expulsive in the longitudinal direction. By means of sech trial functions and a Ritz optimization procedure, evolution eqautions are constructed for width, amplitude and nonlinear frequency chirp of the propagating soliton of the atomic condensate. Our eqaution for the width is an exact agreement with that of Carr and Castin $[ Phys. Rev. A {bf{66}}, 063602 (2002)]$, obtained by more detailed analysis. In agreement with the experiment of Paris group $[ Science {bf{296}}, 1290 (2002)]$, the expulsive potential is found to cause the soliton to explode for $N|a_s|=0.98$, $N$ being the number of atoms in the condensate and $a_s$, the scattering length of the atom-atom interaction.
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