Mass and center of mass of a lamina
Problem 11.267 · medium
A lamina occupies the triangle with vertices \( \displaystyle (0, 0) \), \( \displaystyle (2, 0) \), \( \displaystyle (0, 2) \) with density \( \displaystyle \rho(x, y) = 5 x \). Find its mass and center of mass.
- \[ \int\limits_{0}^{2}\int\limits_{0}^{2 - x} 5 x\, dy\, dx = \frac{20}{3} \]Mass m = ∬ ρ dA.✓ Proved
- \[ \left[\begin{matrix}\int\limits_{0}^{2}\int\limits_{0}^{2 - x} 5 x^{2}\, dy\, dx\\\int\limits_{0}^{2}\int\limits_{0}^{2 - x} 5 x y\, dy\, dx\end{matrix}\right] = \left[\begin{matrix}\frac{20}{3}\\\frac{10}{3}\end{matrix}\right] \]Moments M_y = ∬ xρ dA and M_x = ∬ yρ dA.✓ Proved
- \[ \left[\begin{matrix}1\\1 \cdot \frac{1}{2}\end{matrix}\right] = \left[\begin{matrix}1\\\frac{1}{2}\end{matrix}\right] \]x̄ = M_y/m, ȳ = M_x/m.✓ Proved
Answer \( m = \frac{20}{3},\ (\bar x, \bar y) = \left(1, \frac{1}{2}\right) \)
✓ Nihil obstat Every line of this solution was proved by the computer algebra system SymPy. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation.
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 3 | ✓ 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 double integrals |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass
Every verdict on record (4)
gpt-oss:20b: pass 2026-10-06qwen3.6:27b-mlx: pass 2026-10-06gpt-oss:20b: pass 2026-10-06qwen3.6:27b-mlx: pass 2026-10-06 — The solution correctly sets up the integrals for mass and moments over the triangular region, computes them accurately, and derives the center of mass coordinates.
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/lamina_mass_center, checked 2026-10-06 with SymPy 1.14.0.