$$\begin{aligned} \& v_1=\sqrt{2 g h_1} \\ \& =\sqrt{2 \times 9.8 \times 40} \\ \& v_1=\sqrt{784}=28 \mathrm{~m} \mathrm{~s}^{-1} \\ \& \text { and } v_2=\sqrt{2 g h_2}=\sqrt{2 \times 9.8 \times 10} \\ \& =\sqrt{196}=14 \mathrm{~m} \mathrm{~s}^{-1} \\ \& \text { Impulse }=\Delta \vec{p}=m\left(\vec{v}_f-\vec{v}_i\right)=m\left(\vec{v}_2-\vec{v}_1\right) \\ \& =\frac{1}{2}(14-(-28)) \\ \& =21 \mathrm{NS} \end{aligned}$$