$\mathrm{T}_{2}=\mathrm{T}_{1} \sqrt{\frac{M_{2}}{M_{1}}}=\mathrm{T}_{1} \sqrt{\frac{2 M}{M}}$
$\mathrm{T}_{2}=\mathrm{T}_{1} \sqrt{2}=\sqrt{2} \mathrm{T}\left(\text { where } \mathrm{T}_{1}=\mathrm{T}\right)$
(where $g =$ acceleration due to gravity)