- #1
wadawalnut
- 14
- 0
Homework Statement
Find the volume of the solid lying inside both the sphere [itex]x^2 + y^2 + z^2 = 4a^2[/itex] and the cylinder [itex]x^2 + y^2 = 2ay[/itex] above the xy plane.
Homework Equations
Polar coordinates:
[tex] r^2 = x^2 + y^2 [/tex]
[tex] x = r\cos(\theta)[/tex]
[tex] y = r\sin(\theta)[/tex]
The Attempt at a Solution
So I tried this problem and got it almost perfectly correct. My result was: [itex]V = \frac{8\pi a^3}{3}[/itex], and the right answer is [itex]V = \frac{8\pi a^3}{3} - \frac{32 a^3}{9} [/itex], so I'm just missing one term. The first thing that crossed my mind was that one of my integrals evaluated to 0, so obviously that's where I went wrong. I checked the solution and noticed the only difference from my work was that the book solution used symmetry and integrated [itex]\theta[/itex] from 0 to [itex]\frac{\pi}{2}[/itex], and I just integrated from 0 to [itex]\pi[/itex]. This is what I did:
[itex]x^2 + y^2 = 2ay[/itex] describes a cylinder with a circle base that is centered at (0,a) and has radius a. Therefore, this cylinder lies entirely in the first and second quadrants, where [itex]\theta[/itex] ranges from [itex](0,\pi)[/itex]. Using cylindrical coordinates, the sphere has the equation [itex] z = \sqrt{4a^2 - r^2} [/itex] and the cylinder has the equation [itex] r^2 = 2ar\sin(\theta) [/itex], or [itex] r = 2a\sin(\theta)[/itex].
[tex] V = \int_{0}^{\pi}\int_{0}^{2a\sin(\theta)}\sqrt{4a^2 - r^2}rdrd\theta[/tex]
Making the substitution [itex] u = 4a^2 - r^2, du = -2rdr[/itex]
[tex] V = \frac{-1}{2} \int_{0}^{\pi}\int_{r = 0}^{r = 2a\sin(\theta)}\sqrt{u}dud\theta[/tex]
[tex] V = \frac{-1}{3} \int_{0}^{\pi}(4a^2 - r^2)^{3/2}\bigg|_{0}^{2a\sin(\theta)}d\theta[/tex]
[tex] V = \frac{-1}{3} \int_{0}^{\pi}\left[(4a^2(1 - sin^2(\theta))^{3/2} - (4a^2)^{3/2}\right]d\theta[/tex]
[tex] V = \frac{-1}{3} \int_{0}^{\pi}\left[(4a^2)^{3/2}(cos^2(\theta))^{3/2} - 8a^3\right]d\theta[/tex]
[tex] V = \frac{-8a^3}{3} \int_{0}^{\pi}cos^3(\theta)d\theta + \frac{8\pi a^3}{3}[/tex]
Now, the remaining integral evaluates to 0 because it will be a function of [itex]\sin(\theta)[/itex] where [itex]\theta \in (0,\pi)[/itex].
I see that if I were to use symmetry like it was done in the book ([itex] \theta \in (0,\frac{\pi}{2})[/itex]) and multiply everything by 2, my integral should return the right answer. I just don't understand why what I did was wrong.