The work done on a particle of mass $m$ by a force, $K\left[\frac{x}{\left(x^2+y^2\right)^{3 / 2}} \hat{i}+\frac{y}{\left(x^2+y^2\right)^{3 / 2}} \hat{j}\right]$ ( $K$ being a constant of appropriate dimensions), when the particle is taken from the point ( $a, 0$ ) to the point ( $0, a$ ) along a circular path of radius a about the origin in the $x-y$ plane is :