A spherically symmetric gravitational system of particles has a mass density ${\rm{\rho }} = \left\{ {\begin{array}{*{20}{c}} {{{\rm{\rho }}_0}\;for\;r \le R}\\ {0\;for\;r > R} \end{array}} \right.$ where ${{\rm{\rho }}_0}\;$ is a constant. A test mass can undergo circular motion under the influence of the gravitational field of particles. Its speed v as a function of distance $r\left( {0 < r < \infty } \right)$ from the centre of the system is represented by