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Homework Help: Solid of Revolution

  1. Jun 30, 2008 #1
    1. The problem statement, all variables and given/known data

    The area enclosed between the ellipse [itex]4x^2 + 9y^2 = 36[/itex] and its auxiliary circle [itex]x^2 + y^2 = 9[/itex] is rotated about the y-axis through [itex]\pi[/itex] radians. Find, by integration, the volume generated.

    This is the whole question. I assume it means bounded by the x-axis, but even if this isn't the case, my answer is wrong. :(

    Solution: [itex]12\pi[/itex]

    2. Relevant equations

    [tex]V = \int_a^b{A(y)\,dy}[/tex]

    3. The attempt at a solution

    [tex]x^2 + y^2 = 9 \Rightarrow x^2 = 9 - y^2[/tex]

    [tex]4x^2 + 9y^2 = 36 \Rightarrow x^2 = \frac{36-9y^2}{4}[/tex]

    [tex]A(y) = \pi \left [ \left ( 9-y^2 \right ) - \left ( \frac{36-9y^2}{4} \right ) \right ] = \frac{5\pi}{4}y^2[/tex]

    [tex]\int_0^3{\frac{5\pi}{4}y^2\,dy} = \frac{5\pi}{4} \left [ \frac{y^3}{3} \right ]^3_0 = \frac{45\pi}{4}[/tex]

    I hope someone can help me find where I went wrong! Thanks very much.
  2. jcsd
  3. Jun 30, 2008 #2


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    Science Advisor

    I see no reason to assume "bounded" by the y-axis. There are "natural" bounds at x= -3 and x= 3 and if nothing else is said, I think those should be used.

    Of course, x2+ y2= 9 is a cirlcle with center at (0,0) and radius 3. Rotated around any axis it makes a sphere with center at (0,0,0) and radius 3. Its volume is [itex](4/3)\pi 3^3= 36\pi[/itex].

    [itex]4x^2+ 9y^2= 36[/itex] is an ellipse with center at (0,0), semi-major axis along the x-axisd of length 3 and semi-minor axis along the y-axis of length 2. Rotating around the y-axis makes an ellipsoid with center at (0,0,0) and semi-axes of length 2, 3 and 3. Its volume is given by [itex](4/3)\pi (3)(3)(2)= 24\pi[/itex].

    The volume between the two figures is [itex]36\pi- 24\pi= 12\pi[/itex]. However, if it is only rotated by [itex]\pi[/itex] radians, that is only half a circle so we should ony get 1/2 the volume: [itex]6\pi[/itex]. Are you sure about that? Of course, that's not "by integration" but serves as a check.

    The problem is that the ellipse does not extend above y= 2. I think you need to do this in parts: for 0< y< 2, you do, in fact, have [itex]\pi [(9- x^2)- (36- 9x^2)/4]= (5/4)\pi x^2[/itex]. But for 2< y< 3, you have the full [itex]\pi(9- x^2)[/itex].
  4. Jun 30, 2008 #3
    I *think* I get it now. Because [itex]4x^2+9y^2=36[/itex] doesn't extend above [itex]y=2[/itex], I needed to "partition" the volume and evaluate
    [tex]\pi \left ( \frac{5}{4}\int_0^2{x^2\,dx} + \int_2^3{9-x^2\,dx} \right ) = 6\pi[/tex]
    and then either double for the part below the x-axis, or work it out by hand using the same idea.

    Thanks very much!
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