# Calculating Surface area/volume from 2D cross section??

by SuperG
Tags: area or volume, cross, surface
 P: 5 I'm feeling a little stupid today and I need some help... Assume that I have a radially-symmetric 3D object (for example a candle stick or the base of a table lamp) and that I can calculate the surface area of the largest longitudinal cross section (ie, I split the object precisely along the axis of radial symmety and measure the area of the newly exposed surface). Also assume that the outside profile of this object can be described by a function (though I do not know what the function is) Is there any reason that I cannot then treat this object as the cross section of a simple cylinder, the product of whose length and diameter is equal to my object's cross sectional surface? In other words, if my object's cross sectional area turned out to be 10 square units with a length of 5), then can I say that my object's volume is equal to the volume of a cylinder with length 5 and diameter 2? EDIT: corrected an error
 P: 509 You're talking about a solid of revolution. In that case, take note of two different SORs, one cross-section has 2 unit circles each 1 unit symmetrically from the axis, and the other of 2 unit circles each 2 units symmetrically from the axis. The resulting torii have different volumes, but the same cross-sectional area. For objects which have the axis running through them, consider the unit circle and a square with sides of length $\sqrt{\pi}$. Then the volume of their SORs are $\frac{4\pi}{3}$ and $\frac{\pi^2\sqrt{\pi}}{2}$ even though their cross-sectional areas were the same. Also, for cross-sections of equal height and area, consider the absolute values of sine and cosine rotated.
P: 5
 Quote by hypermorphism You're talking about a solid of revolution. In that case, take note of two different SORs, one cross-section has 2 unit circles each 1 unit symmetrically from the axis, and the other of 2 unit circles each 2 units symmetrically from the axis. The resulting torii have different volumes, but the same cross-sectional area.
Assuming no hollow center though, would my method work?

I understand that what I'm assuming would not be universally applicable, but may rather be a special case.

I'm going to work on it some more to at least understand where this method fails (FWIW, this is not a class assignment but rather an applied problem--I'm trying to save my lab \$3000)

P: 509

## Calculating Surface area/volume from 2D cross section??

Hi Super,
See the rest of my reply.
 P: 5 On my drive home I convinced myself that my method is correct (even if my explanation may not have been very good ) I convinced myself with the following analogy. Imagine that the object in question--lets say it's a turned chair leg with lots of nice curves and a constantly changing diameter--is made not out of wood but out of something like an uncompressible yet malleable clay. Constraining its longitudinal dimension, it then stands to reason that that chair leg can be reformed into one and only one solid cylinder. The longitudinal cross section of that cylinder will then have an area which is, of course, symmetric around the chair leg's long axis. Obviously this area must then be the same as the area of the chair leg's longitudinal cross section. Therefore, if I can measure (or estimate) the area of the chair leg's longitudinal cross section-because maybe, all I have is a 2D image of the chair leg-I can then come up with a corresponding cylinder (again, holding length constant) which has the same longitudinal cross sectional area and hence, derive a volume for the leg. Which of course gets me nowhere because I want to ultimately figure out the surface area of the leg, not the volume. Oh well, at least my drive home wasn't boring
 P: 509 The sine and cosine counterexample proves the stated hypothesis wrong, unless you refine the hypothesis further.
 P: 5 I'm afraid I see that you are correct. Just shows that intuition is no match for actually crunching some numbers... Thanks for the coaxing.
 P: 509 No problem. There are a lot of hypotheses that seem like they should be true because they hold for a large amount of examples, but turn out to be false.
 P: 5 So how did you see that this was incorrect so quickly? Had you seen this balloon floated before?
 P: 509 I actually didn't see that it was incorrect at first, but I did look for a counterexample. If a nontrivial statement is false, one is sometimes able to find a counterexample faster than finding a proof if a statement is true (ie., why proof by contradiction is so satisfying). If I couldn't find a counterexample, I would've started to see how one would prove it. I also thought it strange that if it was true, I had never seen it as a cute problem in any of my myriad analysis texts.

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