Answer
$9187.5 \ N $
Work Step by Step
The width of the dam at height $y$ is given by $w(y)=0.5$
We need to use the formula such as: $F=\int_b^t \rho g (a-y) w(y) \ dy$.
Plug in the above formula the given values to obtain:
$F=\int_b^t \rho g (a-y) w(y) \ dy \\= \int_0^{0.5} \rho g (4-y) \times 0.5 \ dy\\=0.5 \times \rho g \times 1.875 \\=(0.5) (1000) (9.81) (1.875) \\=9187.5 \ N $
