Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 16 - Multiple Integration - 16.5 Applications of Multiple Integrals - Exercises - Page 892: 40

Answer

${I_0} = \frac{1}{{240}}{a^2}{b^2}\left( {5{a^2} + 4{b^2}} \right)$

Work Step by Step

We have the domain ${\cal D}$, between the line $y = \frac{b}{a}x$ and the parabola $y = \frac{b}{{{a^2}}}{x^2}$, where $a,b > 0$. The mass density is $\delta \left( {x,y} \right) = xy$. We find the intersection of the line $y = \frac{b}{a}x$ and the parabola $y = \frac{b}{{{a^2}}}{x^2}$: $y = \frac{b}{a}x = \frac{b}{{{a^2}}}{x^2}$ Multiplying both sides by $\frac{{{a^2}}}{b}$ gives $ax = {x^2}$ ${x^2} - ax = 0$ $x\left( {x - a} \right) = 0$ So, the intersections occur at $x=0$ and $x=a$. We describe ${\cal D}$ as a vertically simple domain such that the description is given by: ${\cal D} = \left\{ {\left( {x,y} \right)|0 \le x \le a,\frac{b}{{{a^2}}}{x^2} \le y \le \frac{b}{a}x} \right\}$ 1. Evaluate the moment of inertia relative to the $x$-axis: ${I_x} = \mathop \smallint \limits_{}^{} \mathop \smallint \limits_{\cal D}^{} {y^2}\delta \left( {x,y} \right){\rm{d}}A$ $ = \mathop \smallint \limits_{x = 0}^a \mathop \smallint \limits_{y = b{x^2}/{a^2}}^{bx/a} x{y^3}{\rm{d}}y{\rm{d}}x$ $ = \frac{1}{4}\mathop \smallint \limits_{x = 0}^a x\left( {{y^4}|_{b{x^2}/{a^2}}^{bx/a}} \right){\rm{d}}x$ $ = \frac{1}{4}\mathop \smallint \limits_{x = 0}^a \left( {\frac{{{b^4}}}{{{a^4}}}{x^5} - \frac{{{b^4}}}{{{a^8}}}{x^9}} \right){\rm{d}}x$ $ = \frac{{{b^4}}}{{4{a^4}}}\left( {\left( {\frac{1}{6}{x^6} - \frac{1}{{10{a^4}}}{x^{10}}} \right)|_0^a} \right)$ $ = \frac{{{b^4}}}{{4{a^4}}}\left( {\frac{{{a^6}}}{6} - \frac{{{a^6}}}{{10}}} \right) = \frac{{{a^2}{b^4}}}{{60}}$ 2. Evaluate the moment of inertia relative to the $y$-axis: ${I_y} = \mathop \smallint \limits_{}^{} \mathop \smallint \limits_{\cal D}^{} {x^2}\delta \left( {x,y} \right){\rm{d}}A$ $ = \mathop \smallint \limits_{x = 0}^a \mathop \smallint \limits_{y = b{x^2}/{a^2}}^{bx/a} {x^3}y{\rm{d}}y{\rm{d}}x$ $ = \frac{1}{2}\mathop \smallint \limits_{x = 0}^a {x^3}\left( {{y^2}|_{b{x^2}/{a^2}}^{bx/a}} \right){\rm{d}}x$ $ = \frac{1}{2}\mathop \smallint \limits_{x = 0}^a \left( {\frac{{{b^2}}}{{{a^2}}}{x^5} - \frac{{{b^2}}}{{{a^4}}}{x^7}} \right){\rm{d}}x$ $ = \frac{{{b^2}}}{{2{a^2}}}\left( {\left( {\frac{1}{6}{x^6} - \frac{1}{{8{a^2}}}{x^8}} \right)|_0^a} \right)$ $ = \frac{{{b^2}}}{{2{a^2}}}\left( {\frac{{{a^6}}}{6} - \frac{{{a^6}}}{8}} \right) = \frac{{{a^4}{b^2}}}{{48}}$ Since, ${I_0} = {I_x} + {I_y}$, so ${I_0} = {I_x} + {I_y} = \frac{{{a^2}{b^4}}}{{60}} + \frac{{{a^4}{b^2}}}{{48}} = \frac{1}{{240}}{a^2}{b^2}\left( {5{a^2} + 4{b^2}} \right)$
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