Correct option is : (1) 21 NS
\(
v_{1}=\sqrt{2 g h_{1}}\)
\(
=\sqrt{2 \times 9.8 \times 40}\)
\(
v_{1}=\sqrt{784}=28 \mathrm{~m} \mathrm{~s}^{-1}\)
\(v_{2}=\sqrt{2 g h_{2}}=\sqrt{2 \times 9.8 \times 10}\)
\(
=\sqrt{196}=14 \mathrm{~m} \mathrm{~s}^{-1}\)
\(
\text{Impulse}\ =\Delta \vec{p}=m\left(\vec{v}_{f}-\vec{v}_{i}\right)=m\left(\vec{v}_{2}-\vec{v}_{1}\right)\)
\(\frac{1}{2}(14-(-28))\)
\( =21 \ NS\)
