The breaking strength of a string is 55 kg wt. The maximum permissible speed of a stone of mass 5 kg which revolved in a vertical circle of radius 4 m with the help of is the string is g=10 ${s}^{2}$.

$T_{\text {max }}=55$

$\begin{aligned} m & =5+g \\ R & =4 \mathrm{~m} \\ g & =10 \mathrm{mgs} \\ T_{\text {max }} & =55 \times 10 \mathrm{~N} \\ & =550 \mathrm{~N} \end{aligned}$

$\begin{array}{l} T_{m a x}=m g+\frac{m v^{c}}{r} \\ \Rightarrow 550=5 \times 10+\frac{5 \times v^{2}}{4} \\ \therefore v=20 \mathrm{~m} / \mathrm{s} \end{array}$