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Hydrostatic force

Problem 5.298 · easy

A vertical plate shaped like a semicircle of radius 4 m, flat side up is submerged in water with its top at the surface. Find the force of the water on one side. (Water weighs \( \displaystyle 9800\ \text{N}/\text{m}^3 \).)

Let y be depth below the surface. A horizontal strip at depth y has width w(y) = 2*sqrt(16 - y**2) and pressure 9800·y.
Reviewed
\[ \int\limits_{0}^{4} 19600 y \sqrt{16 - y^{2}}\, dy = \frac{1254400}{3} \]
Force = ∫ (pressure)(width) dy over the plate.✓ Proved

Answer \( \frac{1254400}{3} \approx 418133\ \text{N} \)

Lines: 1 proved, 1 reviewed. The answer was also checked a second way, without looking at the solution. The reviewers disagree about how one step is explained; every verdict is in the receipt.

The full receipt

Line	Status	Checked by	Detail
1	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	a sentence; read, not computed
2	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	numerical quadrature

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The setup correctly models the geometry of a semicircle with the flat side up, where the width at depth y is 2*sqrt(R^2 - y^2) and pressure is proportional to y. The integration limits and integrand are correct.

Every verdict on record (4)

gpt-oss:20b: pass 2026-10-06
qwen3.6:27b-mlx: pass 2026-10-06 — The setup correctly models the geometry of a semicircle with the flat side up, where the width at depth y is 2*sqrt(R^2 - y^2) and pressure is proportional to y. The integration limits and integrand are correct.
gpt-oss:20b: fail (error) 2026-10-06 — The width of a horizontal strip at depth y is not 2√(16−y²); the semicircle is centered at the top, so the correct width is 2√(16−(y−4)²). Using the wrong width makes the integral—and thus the force—incorrect.
qwen3.6:27b-mlx: pass 2026-10-06 — The setup correctly identifies the width of the semicircle at depth y and the hydrostatic pressure. The integration bounds and integrand are correct for the described geometry.

Proved: SymPy reduced the difference between the two sides to zero. Checked independently: a separate method, named above, confirmed it. Checked numerically: the two sides agree at every sampled point, which is evidence, not proof. Reviewed: a model or a person read it; that is all a sentence can have. Solution by generator:structured/hydrostatic_force, checked 2026-10-06 with SymPy 1.14.0.