How to plot solids of revolution

asked 2011-02-27 08:54:58 +0100

trk
67 ●2 ●3 ●10

Hi,

Is it possible to plot the solid generated by revolving a curve about a line?

For example, I want to see what kind of solid is generated from this question:

Use shells to find the solid generated from the region in the 1st quadrant bounded by $y=x$ and $y=x^2$, revolved about $x=-1$.

The volume of the solid is $\frac{\pi}{2}$. How can I view this solid? I checked the sage calculus tutorial for clues but couldn't find any.

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answered 2011-02-27 10:25:10 +0100

DSM

5242 ●21 ●68 ●112

updated 2011-02-27 10:53:53 +0100

It might take a bit of tweaking to get what you want, but one option is to take a look at the semi-secret function revolution_plot3d. The documentation (help(revolution_plot3d) or revolution_plot3d?) has an example which is similar to yours but still different:


        sage: line=u
        sage: parabola=u^2
        sage: sur1=revolution_plot3d(line,(u,0,1),opacity=0.5,rgbcolor=(1,0.5,0),show_curve=True,parallel_axis='x')
        sage: sur2=revolution_plot3d(parabola,(u,0,1),opacity=0.5,rgbcolor=(0,1,0),show_curve=True,parallel_axis='x')
        sage: (sur1+sur2).show()

image description

which produces the above picture. You could also do it with parametric_plot3d/implicit_plot3d, but you'd have to do the revolution manually (i.e. multiply x, y, and z by the appropriate trigonometric functions).

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answered 2011-02-27 09:21:36 +0100

niles
4529 ●23 ●72 ●133 http://nilesjohnson.net/

updated 2011-02-27 16:56:13 +0100

I think cylindrical_plot3d might be what you're looking for. Convert your equations to give x as a function of y, and then use this to compute the radius for each value of y. Of course you also need to compute where your equations intersect to get the appropriate bounds for y. I've omitted this step for now . . .

sage: var('y,theta')
(y, theta)
sage: s1 = cylindrical_plot3d(y+1,(theta,0,3*pi/2),(y,0,1), opacity=1, aspect_ratio=1)
sage: s2 = cylindrical_plot3d(sqrt(y)+1,(theta,0,3*pi/2),(y,0,1), opacity=.5, color='red')
sage: s1+s2

image description

(Note, in this version I just let theta run from 0 to 3*pi/2 so you could see just part of the revolved surfaces. Go all the way to 2*pi to see the entire object.)

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