The distance of centre of mass from end A of a one dimensional rod (AB) having mass density $\rho=\rho_{0}\left(1-\frac{x^{2}}{L^{2}}\right) \mathrm{kg} / \mathrm{m}$ and length L (in meter) is $\frac{3 L}{\alpha} \mathrm{m}$. The value of $\alpha$ is ___________. (where x is the distance from end A)