The potential energy of a particle in a certain field has the form \(\mathrm{U}=\frac{\mathrm{a}}{\mathrm{r}^{2}}-\frac{\mathrm{b}}{\mathrm{r}},\) where a and b are positive constants, r is the distance from the centre of the field. The maximum magnitude of the attractive force is \(-\frac{n b^{3}}{81 a^{2}}\). the value of n is