4
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I'm still struggling with RegionPlot3D. I need to find the volume of the solid generated by rotating the region bounded by $x=y^4+1$, $y=0$, and $x=2$ about the $y$-axis, the exact answer of which is $112\pi/45$.

Thanks to the help at RegionPlot3D example, I tried:

Clear[z1, z2, r]
z1[r_] := 0
z2[r_] := Power[r - 1, (4)^-1]
RegionPlot3D[
 z1[Sqrt[x^2 + y^2]] <= z <= z2[Sqrt[x^2 + y^2]] && 
  1 <= x^2 + y^2 <= 4, {x, -2, 2}, {y, -2, 2}, {z, 0, 1}]
Volume[ImplicitRegion[
  z1[Sqrt[x^2 + y^2]] <= z <= z2[Sqrt[x^2 + y^2]] && 
   1 <= x^2 + y^2 <= 4, {x, y, z}]]

The image was almost there but the Volume didn't work (probably because of taking the fourth root of a negative number somewhere along the line.

LessEqual::nord: Invalid comparison with 0.394101 +0.394101 I attempted.
General::stop: Further output of LessEqual::nord will be suppressed during this calculation.

Does anyone have a suggestion for getting the volume using the Volume command?

Update: Here is where this question came from originally. The problem was to find the volume of the solid obtained by rotating the region bounded by the curves $x=1-y^4$ and $x=0$ about the line $x=2$. I was able to get a sketch using RevolutionPlot3D thanks to help received at Region rotated about the line y=2.

Show[
 RevolutionPlot3D[{x - 2, (1 - x)^(1/4)}, {x, 0, 1}, 
  PlotStyle -> Opacity[0.5]],
 RevolutionPlot3D[{x - 2, -(1 - x)^(1/4)}, {x, 0, 1}, 
  PlotStyle -> Opacity[0.5]],
 RevolutionPlot3D[{-2, z}, {z, -1, 1}, PlotStyle -> Opacity[0.5]],
 PlotRange -> {{-2, 2}, {-2, 2}, {-1, 1}},
 Ticks -> {{{-2, 0}, {-1, 1}, {0, 2}, {1, 3}, {2, 4}}, Automatic, 
   Automatic}
 ]

enter image description here

So I did a similar shift to produce the following, thanks to m_goldberg's help.

Clear[z1, z2, r]
z1[r_] := -Surd[r - 1, 4]
z2[r_] := Surd[r - 1, 4]
RegionPlot3D[
 1.001 <= x^2 + y^2 <= 4 && 
  z1[Sqrt[x^2 + y^2]] <= z <= z2[Sqrt[x^2 + y^2]], {x, -2, 2}, {y, -2,
   2}, {z, -1, 1}, PlotStyle -> Opacity[0.5], BoxRatios -> Automatic]

enter image description here

Now what I am trying to do is to use RegionPlot3D without a shift. Here is what I am trying:

ContourPlot[{x == 1 - y^4, x == y^4 + 3, x == 2}, {x, 0, 4}, {y, -1, 
  1}, ImageSize -> Small]

enter image description here

So the projection onto the xy-plane is:

RegionPlot[1 <= (x - 2)^2 + y^2 <= 4, {x, 0, 4}, {y, -2, 2}, 
 ImageSize -> Small]

enter image description here

So I figured that $r=\sqrt{(x-2)^2+y^2}$ and tried the following:

Clear[z1, z2, r]
z1[r_] := -Surd[r - 3, 4]
z2[r_] := Surd[r - 3, 4]
RegionPlot3D[
 1.001 <= (x - 2)^2 + y^2 <= 4 && 
  z1[Sqrt[(x - 2)^2 + y^2]] <= z <= z2[Sqrt[(x - 2)^2 + y^2]], {x, 0, 
  4}, {y, -2, 2}, {z, -1, 1}, PlotStyle -> Opacity[0.5], 
 BoxRatios -> Automatic]

But I have been unsuccessful, getting the errors:

Surd::noneg: Surd is not defined for even roots of negative values.
General::stop: Further output of Surd::noneg will be suppressed during this calculation.

And:

Volume[ImplicitRegion[
  1.001 <= (x - 2)^2 + y^2 <= 4 && 
   z1[Sqrt[(x - 2)^2 + y^2]] <= z <= z2[Sqrt[(x - 2)^2 + y^2]], {x, y,
    z}]]

Gives an answer of zero.

Any further thoughts on how to fix this?

**Answer for m_goldberg's question:* Here is the image to help my students and I agree that we can use symmetry to find the upper volume then double it:

pts1 = Table[{1.55 Cos[t] + 2, 
    0.2 Sin[t] + (1 - 0.5)^(1/4)}, {t, \[Pi], 2 \[Pi], \[Pi]/20}];
pts2 = Table[{1.55 Cos[t] + 2, 
    0.2 Sin[t] - (1 - 0.5)^(1/4)}, {t, \[Pi], 2 \[Pi], \[Pi]/20}];
pts = Join[pts2, Reverse[pts1]];
Show[
 RegionPlot[0 <= x <= 1 - y^4 && 0 <= x <= 1, {x, 0, 1}, {y, -1, 1}, 
  Axes -> True, Frame -> None],
 ContourPlot[{x == 1 - y^4, x == 0, y == 0}, {x, -1, 2}, {y, -2, 2}, 
  PlotLegends -> "Expressions"],
 Graphics[{
   {GrayLevel[0.4], Opacity[0.5], EdgeForm[Black], 
    Rectangle[{0.45, -(1 - 0.5)^(1/4)}, {0.55, (1 - 0.5)^(1/4)}],
    Disk[{2, (1 - 0.5)^(1/4)}, {1.55, 0.2}],
    White, Disk[{2, (1 - 0.5)^(1/4)}, {1.45, 0.15}]},
   {GrayLevel[0.4], Opacity[0.5], EdgeForm[Black], Polygon[pts]},
   Arrow[{{0.5, -1.7}, {0.5, 1.7}}],
   Text[Style["(x,(1-x\!\(\*SuperscriptBox[\()\), \(1/4\)]\))", 12, 
     Background -> White], {0.5, 1.1}],
   Text[Style["(x,-(1-x\!\(\*SuperscriptBox[\()\), \(1/4\)]\))", 12, 
     Background -> White], {0.5, -1.1}],
   Red, PointSize[Large], 
   Point[{{0.5, (1 - 0.5)^(1/4)}, {0.5, -(1 - 0.5)^(1/4)}}],
   {Dashed, Line[{{2, -2}, {2, 2}}]},
   {Thick, Arrow[{{2, 0}, {0.5, 0}}]},
   Arrowheads[{-0.02, 0.02}], 
   Arrow[{{0.3, (1 - 0.5)^(1/4)}, {0.3, -(1 - 0.5)^(1/4)}}],
   Black, 
   Text[Style["2-x", 12, Bold, Background -> White], {1.25, 0.1}],
   Text[Style["2(1-x\!\(\*SuperscriptBox[\()\), \(1/4\)]\)", 12, Bold,
      Background -> White], {-0.2, 0.1}],
   Text[Style["dx", 12, Bold, Background -> White], {3.5, 1.1}],
   }],
 PlotRange -> {{-1, 5}, {-2, 2}}
 ]

enter image description here

And we can translate: I didn't translate the tube so folks can see the translation occurring.

z[r_] := Surd[r - 1, 4]

zPlot = RevolutionPlot3D[z[r], {r, 1, 2}];

Show[
 Graphics3D[Translate[zPlot[[1]], {2, 0, 0}]],
 Graphics3D[
  Translate[Rotate[zPlot[[1]], 180 \[Degree], {1, 0, 0}], {2, 0, 0}]],
  Graphics3D[{CapForm[None], Tube[{{0, 0, -1}, {0, 0, 1}}, 2]}],
 PlotRange -> All,
 BaseStyle -> Opacity[.5],
 Axes -> True,
 ImageSize -> Medium]

enter image description here

$\endgroup$
  • $\begingroup$ Try Surd in place of Power. $\endgroup$ – m_goldberg Dec 30 '16 at 1:32
  • 1
    $\begingroup$ Concerning your update: it is not clear to me what solid of revolution you are trying to create. Is it the spool-shaped solid? Or is it the solid that results from removing the spool from the cylinder containing it? $\endgroup$ – m_goldberg Dec 30 '16 at 16:32
  • 1
    $\begingroup$ Further, to make your problem simpler, you should make use of symmetry and only work in the half-space above the xy-plane. $\endgroup$ – m_goldberg Dec 30 '16 at 16:37
  • $\begingroup$ @m_goldberg I added an image from my notebook that shows the shaded region that is being rotated about the line $x=2$. I tried using the curves $x=1-y4$ and $x=0$ on the left with RegionPlot3D, but when it did not work, I am currently trying the reflection of that region about $x=2$, namely the curves $x=y^4+3$ and $x=4$. $\endgroup$ – David Dec 30 '16 at 16:59
  • 1
    $\begingroup$ Volume is invariant under rigid motions, so just double the result I got below; volume = (224 π)/45. $\endgroup$ – m_goldberg Dec 30 '16 at 18:46
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Try Surd in place of Power. Also, you don't need z1.

z2[r_] := Surd[r - 1, 4]
Volume[
  ImplicitRegion[0 <= z <= z2[Sqrt[x^2 + y^2]] && 1 <= x^2 + y^2 <= 4, {x, y, z}]]

(112 π)/45

Update

As I said in my answer to your previous question, it is always a good idea to look at the 2D cross-section of your region before trying work with the full 3D region. In this case, the 2D cross-section looks like this:

Plot[z2[r], {r, 1, 2}, PlotRange -> {{0, 2}, Automatic}]

plot

This shows the slope is infinite at r = 1. You have to deal with that by backing off a little. Also, note the range of z is {0, 1}, not {-1, 1}.

RegionPlot3D[
  1.001 <= x^2 + y^2 <= 4 && 0 <= z <= z2[Sqrt[x^2 + y^2]], 
  {x, -2, 2}, {y, -2, 2}, {z, 0, 1},
  PlotPoints -> 100,
  BoxRatios -> {4, 4, 1}]

region

Yet another update

To plot your solid, I strongly recommend plotting the surfaces that enclose it, because Mathematica is much better at plotting surfaces than solids (3D regions). Further, I will take advantage of symmetry.

z[r_] := Surd[r - 1, 4]

zPlot = RevolutionPlot3D[z[r], {r, 1, 2}];

Show[
 zPlot,
 Graphics3D[Rotate[zPlot[[1]], 180 °, {1, 0, 0}]],
 Graphics3D[{CapForm[None], Tube[{{0, 0, -1}, {0, 0, 1}}, 2]}],
 PlotRange -> All,
 BaseStyle -> Opacity[.5],
 ImageSize -> Large]

solid-1

solid-2

Back to basics

In my opinion, the best way to draw the solid you want in the position you want is to go back to basics (y[x_] := x^4 + 1) and draw the whole thing with a single Graphics3D expression.

y[x_] := x^4 + 1
spool = RevolutionPlot3D[y[x], {x, -1, 1}, RevolutionAxis -> "X"][[1]];
Graphics3D[
  Translate[
    {Rotate[spool, 90 °, {0, 1, 0}],
     {CapForm[None], Tube[{{0, 0, -1}, {0, 0, 1}}, 2]}}, 
    {2, 0, 0}],
  Axes -> True,
  BaseStyle -> Opacity[.5],
  ImageSize -> Large]

solid-3

solid-4

Note is there no gap at the narrowest point of the spool. This approach avoids the infinite slope problem.

$\endgroup$
  • $\begingroup$ Wow. Really quick answer. The Volume works, but RegionPlot3D[ 1 <= x^2 + y^2 <= 4 && 0<= z <= z2[Sqrt[x^2 + y^2]], {x, -2, 2}, {y, -2, 2}, {z, -1, 1}] spits out error messages like Surd::noneg: Surd is not defined for even roots of negative values and General::stop: Further output of Surd::noneg will be suppressed during this calculation. Do you have a suggestion for this or would it just be best to do a //Quiet at the end, because the image is not bad. By the way, I can't tell you how helpful a colleague you've been. Thanks. $\endgroup$ – David Dec 30 '16 at 1:51
  • $\begingroup$ I updated my question to show what the original problem was and to show what I am currently struggling with. Any thoughts? $\endgroup$ – David Dec 30 '16 at 7:07
  • $\begingroup$ Probably the best image yet. I learned a lot in your reply. I added a translation in my original post. Best way? $\endgroup$ – David Dec 30 '16 at 20:00
  • 2
    $\begingroup$ @David. Actually, the best way is go back to basics and work out the whole thing form there. see what I swear is my last update to this answer. $\endgroup$ – m_goldberg Dec 31 '16 at 1:50
  • 1
    $\begingroup$ This has been a wonderful interchange. I've learned so much from your examples. Thanks. $\endgroup$ – David Dec 31 '16 at 4:44

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