Answer
$\sqrt{\frac{Mrg}{m}}$
Work Step by Step
We can find the required speed as follows:
$T=Mg$
$T=\frac{mv^2}{r}$
Now $\frac{mv^2}{r}=Mg$
This can be rearranged as:
$v^2=\frac{Mgr}{m}$
$\implies v=\sqrt{\frac{Mrg}{m}}$
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