Answer
$1.65Kg$
Work Step by Step
We can find the required mass of the bob as follows:
$M=m(\frac{v_{\circ}}{\sqrt{2gh}}-1)$
We plug in the known values to obtain:
$M=(8.1\times 10^{-3}Kg)(\frac{320m/s}{\sqrt{(2)(9.81m/s^2)(0.125m)}}-1)$
$M=1.65Kg$
