Integral over a rotating ellipsoid

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SUMMARY

The integral of ##\int x^2 dV## over a rotating ellipsoid with semi-axes a, b, and c, rotating around the z-axis at angular speed ##\omega##, can be approached by applying a 2-D rotation transformation. The initial result for a non-rotating ellipsoid is ##\frac{4\pi}{15}a^3bc##. For the rotating case, the integral results in a combination of terms: ##cos^2(\omega t) \int x^2 dV + sin^2(\omega t)\int y^2 dV##. A suggested method is to keep the ellipsoid fixed and rotate the ##x^2## term instead, leading to a mixture of ##x^2##, ##y^2##, and ##xy## terms, where the ##xy## integral cancels due to symmetry.

PREREQUISITES
  • Understanding of ellipsoidal coordinates and their equations, specifically ##\frac{x^2}{a^2}+\frac{y^2}{b^2}+\frac{z^2}{c^2}=1##.
  • Familiarity with 2-D rotation transformations and their application in integrals.
  • Knowledge of integration techniques in three-dimensional space.
  • Basic understanding of angular motion and its effects on integrals.
NEXT STEPS
  • Study the application of 2-D rotation transformations in multivariable calculus.
  • Learn about the properties of integrals over symmetric shapes like ellipsoids.
  • Explore the implications of angular velocity on integrals in rotating systems.
  • Investigate the cancellation of terms in integrals due to symmetry in three-dimensional shapes.
USEFUL FOR

Students and researchers in mathematics, physics, and engineering who are dealing with integrals over rotating bodies, particularly those studying ellipsoidal shapes and their properties in dynamic systems.

Silviu
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Homework Statement


Calculate ##\int x^2 dV## over an ellipsoid with semi-axes a, b and c along x, y and z. rotating around the z axis with an angular speed ##\omega##.

Homework Equations

The Attempt at a Solution


I managed to calculate this in the case when it is not rotating and I got ##\frac{4\pi}{15}a^3bc##. But I am not sure how to do it now I expected to get the same result as before times ##cos^2(\omega t)## but the result seems to be ##cos^2(\omega t) \int x^2 dV + sin^2(\omega t)\int y^2 dV##. Can someone help me with this? Thank you!
 
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This problem is new to me too, but to start with, the ellipsoid apparently has the equation ## \frac{x^2}{a^2}+\frac{y^2}{b^2}+\frac{z^2}{c^2}=1 ##. For rotation about the z-axis, the standard two dimensional rotation transformation for angle ## \theta=\omega t ## should apply. You rotate the axes with such a transformation by angle ## \theta ##, and you get the equation of the rotated ellipsoid in the form ## A(x')^2+B(y')^2 +C x'y'+z^2=1 ##. You then need to integrate ## \int (x')^2 \, dV ##. ## \\ ## Editing: Suggestion instead: Keep the ellipsoid fixed and rotate the ## x^2 ## term in the integral ## \int x^2 \, dV ## using the 2-D rotation transformation. The ## x^2 ## term will become a mix of ## x^2 ##, ## y^2 ## and ## xy ## terms. I think the ## xy ## integral over the ellipsoid will cancel from symmetry. Do you agree? And yes, I agree with your last statement in the OP. And since you solved it for the unrotated case, you should be able to write down the answer by inspection of the form you got for the unrotated case.
 
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