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Volume of 3 intersecting Cylinders

  • Thread starter davidjoey
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  • #1
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Homework Statement



I am trying to find the volume of three intersecting cylinders that intersect at right angles given that the radius is 5.

I have found many sites that state just the answer, but I am after the proof for it showing all of the working. I know how to prove the rule for 2 intersecting cylinders, I can't do it for 3 though.


Homework Equations



The rule I am after is 8(2-sqrt(2))r^3 or any similar form of that.

The Attempt at a Solution



V_3(r,r,r) = 16r^3int_0^(pi/4)int_0^1ssqrt(1-s^2cos^2t)dsdt

A site i have found begins solving with that equation, but I have completely no idea where the numbers come from.
 

Answers and Replies

  • #2
benorin
Homework Helper
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intersection3Cyl.jpg

The above is the looking-down view of the first octant. The next one is slightly cropped and from the z-axis looking out for getting the bounds of integration with the y=x plane included for reference.

intersection3Cyl_crop.jpg

Try setting up the integral now... I'll use cylinders of radius R.

In cylindrical coordinates [tex](r,\theta, z)[/tex]: Using symmetry (multiply by 16), [tex]0\le \theta\le \scriptstyle{\frac{\pi}{4}}[/tex]
here, z is bounded above by the red cylinder [tex]x^2+z^2=R^2[/tex] (second image) so [tex]0\le z\le \sqrt{R^2-r^2\cos^2\theta}[/tex]
and r is bounded by the blue cylinder so [tex]0\le r\le R[/tex]

so the integral is

[tex]16\int_{0}^{\scriptstyle{\frac{\pi}{4}}}\int_{0}^{R}\int_{0}^{\sqrt{R^2-r^2\cos^2\theta}}r\, dzdrd\theta = 16\int_{0}^{\scriptstyle{\frac{\pi}{4}}}\int_{0}^{R}r\sqrt{R^2-r^2\cos^2\theta}\, drd\theta[/tex]
[tex]= -{\scriptstyle{\frac{16}{3}}} R^3\int_{0}^{\scriptstyle{\frac{\pi}{4}}} \frac{\sin^3\theta -1}{\cos^2\theta}\, d\theta = 8R^3\left( 2-\sqrt{2}\right)[/tex]​
 
Last edited:

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