389. Determine the force of water pressure on the gate wall, which is $20 \mathrm{~m}$ long and $5 \mathrm{m}$ high (assuming the gate is completely filled with water).
Solution. Here $y=f(x)=20, a=0, b=5(m), \gamma=1000$ kg $/ \mathbf{~}^{3}$. Using formula (5), we find

$$
P=9810 \int_{0}^{5} 20 x d x=\left.9810 \cdot 20 \frac{x^{2}}{2}\right|_{0} ^{5}=9810 \cdot 10 \cdot 25=2,45 \cdot 10^{6}
$$