$(3$ marks$)$ Prove that the force

$F=(\frac{x}{\sqrt[\phantom{2}]{x^2+y^2+z^2}}(hat(x))+\frac{y}{\sqrt[\phantom{2}]{x^2+y^2+z^2}}(hat(y))+\frac{z}{\sqrt[\phantom{2}]{x^2+y^2+z^2}}(hat(z)))$

with the constant $>0$ is conservative. Find the potential energy $U(r).$