The problem asks us to find the moments of inertia $I_x$, $I_y$, and $I_z$ for the lamina bounded by the curves $y = x^2$ and $y = 4$, with density function $\delta(x, y) = y$.

Applied MathematicsCalculusMultivariable CalculusMoments of InertiaDouble IntegralsLamina
2025/5/25

1. Problem Description

The problem asks us to find the moments of inertia IxI_x, IyI_y, and IzI_z for the lamina bounded by the curves y=x2y = x^2 and y=4y = 4, with density function δ(x,y)=y\delta(x, y) = y.

2. Solution Steps

First, we need to find the intersection points of y=x2y = x^2 and y=4y = 4.
x2=4    x=±2x^2 = 4 \implies x = \pm 2. Thus, the region is bounded by x=2x=-2 to x=2x=2 and x2x^2 to 44.
Next, we calculate the mass mm:
m=22x24δ(x,y)dydx=22x24ydydx=22[12y2]x24dx=22(12(16x4))dx=22(812x4)dx=[8x110x5]22=(163210)(16+3210)=326410=32325=160325=1285m = \int_{-2}^{2} \int_{x^2}^{4} \delta(x, y) dy dx = \int_{-2}^{2} \int_{x^2}^{4} y dy dx = \int_{-2}^{2} [\frac{1}{2}y^2]_{x^2}^{4} dx = \int_{-2}^{2} (\frac{1}{2}(16 - x^4)) dx = \int_{-2}^{2} (8 - \frac{1}{2}x^4) dx = [8x - \frac{1}{10}x^5]_{-2}^{2} = (16 - \frac{32}{10}) - (-16 + \frac{32}{10}) = 32 - \frac{64}{10} = 32 - \frac{32}{5} = \frac{160 - 32}{5} = \frac{128}{5}.
Now, we calculate IxI_x:
Ix=22x24y2δ(x,y)dydx=22x24y3dydx=22[14y4]x24dx=22(14(256x8))dx=22(6414x8)dx=[64x136x9]22=(12851236)(128+51236)=256102436=2562569=23042569=20489I_x = \int_{-2}^{2} \int_{x^2}^{4} y^2 \delta(x, y) dy dx = \int_{-2}^{2} \int_{x^2}^{4} y^3 dy dx = \int_{-2}^{2} [\frac{1}{4}y^4]_{x^2}^{4} dx = \int_{-2}^{2} (\frac{1}{4}(256 - x^8)) dx = \int_{-2}^{2} (64 - \frac{1}{4}x^8) dx = [64x - \frac{1}{36}x^9]_{-2}^{2} = (128 - \frac{512}{36}) - (-128 + \frac{512}{36}) = 256 - \frac{1024}{36} = 256 - \frac{256}{9} = \frac{2304 - 256}{9} = \frac{2048}{9}.
Next, we calculate IyI_y:
Iy=22x24x2δ(x,y)dydx=22x24x2ydydx=22x2[12y2]x24dx=22x2(12(16x4))dx=22(8x212x6)dx=[83x3114x7]22=(64312814)(643+12814)=128325614=12831287=128(1317)=128(7321)=128(421)=51221I_y = \int_{-2}^{2} \int_{x^2}^{4} x^2 \delta(x, y) dy dx = \int_{-2}^{2} \int_{x^2}^{4} x^2 y dy dx = \int_{-2}^{2} x^2 [\frac{1}{2}y^2]_{x^2}^{4} dx = \int_{-2}^{2} x^2 (\frac{1}{2}(16 - x^4)) dx = \int_{-2}^{2} (8x^2 - \frac{1}{2}x^6) dx = [\frac{8}{3}x^3 - \frac{1}{14}x^7]_{-2}^{2} = (\frac{64}{3} - \frac{128}{14}) - (-\frac{64}{3} + \frac{128}{14}) = \frac{128}{3} - \frac{256}{14} = \frac{128}{3} - \frac{128}{7} = 128(\frac{1}{3} - \frac{1}{7}) = 128(\frac{7 - 3}{21}) = 128(\frac{4}{21}) = \frac{512}{21}.
Finally, we calculate IzI_z:
Iz=Ix+Iy=20489+51221=20487+512363=14336+153663=1587263I_z = I_x + I_y = \frac{2048}{9} + \frac{512}{21} = \frac{2048 \cdot 7 + 512 \cdot 3}{63} = \frac{14336 + 1536}{63} = \frac{15872}{63}.

3. Final Answer

Ix=20489I_x = \frac{2048}{9}
Iy=51221I_y = \frac{512}{21}
Iz=1587263I_z = \frac{15872}{63}

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