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Past Discussion

There have been a lot of questions and discussions regarding determining the mass distribution of various bodies through non-destructive purposes. Here are a few:

Determination of mass density distribution of an object

How can I determine whether the mass of an object is evenly distributed?

The general conclusion that I reached after reading questions like these is that by mechanics alone, there is no way to positively determine the mass distribution of a 3-Dimensional object. The simplest arguments typically involved spherical masses and newton's shell theorem.

My Problem

My problem is a simplified case of this general problem that I hope can be solved more easily. I have a stick of balsa wood with dimensions 1/8" x 1/8" x 36", and I would like to know its mass distribution. Because it is so slender, it can be safely treated like a 1-Dimensional object with a density function $\rho(x)$ that depends only on variable. I can find the center of mass very easily just by balancing it, but beyond that I am stuck. I would really like to find an estimation for $\rho(x)$ that is accurate to about the 1" or 1/2" scale.

Solving It

One assumption that can be made is that the young's modulus, $E$, of the wood varies linearly with density. This might be useful for solutions that measure the deflection of the wood under different forces.

First, I am looking for solutions using the simplest methods and tools. I have a scale accurate to 0.01g (the whole stick weighs about 2.60g) and a micrometer that measures to 0.001". If a solution is possible that uses only these tools and other common and accessible ones, that would be best.

If that is not possible, I would still like to hear other more complex approaches that might require advanced equipment or might simply be unrealistic in practice.

EDIT:

The reason I need to solve this problem is that I am building an 18g, 8" tall structure for a competition that must hold over 600 pounds. I know the mass distributions of each specific stick can vary greatly and be quite unpredictable. Since the structure is subject to such extreme forces, small imperfections can significantly affect performance. If one of my 8" columns has 60% of its mass distributed to its lower half, then it will be weaker in the top half. For this reason it is beneficial for me to select wood that is nearly homogenous, and statistical methods (finding variance, etc...) are not particularly useful in this case because I need to be 100% sure that the wood I select is evenly distributed.

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  • $\begingroup$ If you have a bunch of sticks, you can take 5 or 6 of them, saw them into pieces, and weigh the pieces and measure their volumes separately. This should give you a rough idea of the statistical distribution of densities on a spatial scale similar to that which you cut them into, and from that you can infer what sort of variance you'd expect to find in the rest of the sticks. Also, if it's wood, I'll bet there are libraries of books from the early 1800's/1900's devoted to the properties of various species of wood, including density and density variance. $\endgroup$ – DumpsterDoofus Mar 30 '14 at 18:57
  • $\begingroup$ @DumpsterDoofus: My edit should explain why statistics and density variance won't really help me in this case. $\endgroup$ – Platatat Mar 30 '14 at 19:25
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The easiest way to determine the uniformity of your slabs of wood is to take an x-ray. Seriously. This image is of an x-ray of a piece of wood where the dark regions are cavities created by termites eating away at it (the termites are the white things inside the cavities).

enter image description here

If you don't want to take an x-ray (or can't afford one), then just assume that since your wood is 1/8" square thick, it's fairly unlikely that the density is that far off from uniform. If you measure the center of mass and it's pretty darn close to dead center, I'd say it's probably good enough to be about 80-90% certain (depending on proximity to center) that it's uniform.

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  • $\begingroup$ You're right that x-rays are the easiest way, but do you really think that's the simplest way? There's no way to do this with mechanical processes? Bummer. Also, in past experiments, I have found sticks whose center of mass was in the middle, but were not uniformly distributed. After cutting them into 6 inch segments, the masses of the segments varied by sometimes as much as 20% $\endgroup$ – Platatat Mar 30 '14 at 20:02
  • $\begingroup$ I cannot think of any other way to estimate the density of an object without destroying it. $\endgroup$ – Kyle Kanos Mar 30 '14 at 20:04
  • $\begingroup$ Alright, thank you. Could you tell me why my question got a minus 1? I'm trying to only post good questions, and I really think this one had merit. I just don't really understand why it would get a downvote, and I'd like my future questions to be better. $\endgroup$ – Platatat Mar 30 '14 at 20:08
  • $\begingroup$ No idea about the downvote, sorry. $\endgroup$ – Kyle Kanos Mar 30 '14 at 20:09
  • $\begingroup$ Hmm. I mean it obviously shows research effort, it is fairly clearly stated, and has not really been answered before on the site. Tsk. $\endgroup$ – Platatat Mar 30 '14 at 20:10

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