The initial velocity of the block to undergo a complete vertical circular motion is $u=\sqrt{5gR}$

Let us assume that the speed of the block when it is moving in the vertical direction is $v$, then using conservation of mechanical energy we get

$\frac{1}{2}m{\left(\sqrt{5gR}\right)}^2=\frac{{\displaystyle 1}}{{\displaystyle 2}}m{v}^2+mgR$

$\Rightarrow v^2=\sqrt{3gR}$

The tangential and centripetal accelerations of the block are

$a_{\mathrm{t}}=g \left(\downarrow \right)$ and  $a_{\mathrm{c}}=3g\mathit{ }\left(\mathit{\leftarrow }\right)$

$a_{\mathrm{net}}=\sqrt{a_{\mathrm{c}}^2+a_{\mathrm{t}}^2}=g\sqrt{10}$