Using Bernonlli equation
\(\mathrm{A}\left(\mathrm{P}_2-\mathrm{P}_1\right)=\frac{1}{2} \rho\left(\mathrm{V}_1^2-\mathrm{V}_2^2\right) \mathrm{A}\)
\(\mathrm{mg}=\frac{1}{2} \times 1\left(70^2-50^2\right) \times 80\)
\(\mathrm{mg}=40 \times 2400\)
\(\mathrm{~m}=9600 \mathrm{~kg}\)