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2 perfect spheres, same diameter, same weight...one aluminum, one steel. How can you tell them apart?

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My background was more with ceramic and glass materials, and virtually no metallurgy. However, to turn this topic back toward highly technical, to what degree would the rate of cooling affect either material? Would there possibly be overlap in the properties (i.e. the sound) due to the cooling rate?
 
Originally posted by: nevbie
Cut them. Divide each one to two half-ball pieces. Then measure the wall thickness. The one which has thicker wall, is Al.
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If you can't tell from the instant the saw touches the metal which one is aluminum, you really shouldn't be in charge of answering this question.😉

Originally posted by: silverpig
Originally posted by: Kyanzes

Easy. Over time the aluminum one will start to oxidize. You won't even have to touch the balls to tell them apart.

Edit: assuming the experiment is not time limited.
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Aluminum forms a native oxide that is maybe 2-3 nm thick and is transparent...
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Anyone who has ever worked in a machine shop can tell the difference between freshly machined aluminum, and aluminum after it's been sitting out for long enough to form its oxide layer.
 
Originally posted by: Modelworks
Originally posted by: Jimmah
Way too much thought you're all putting into this. Easiest thing is to feel the metal, maybe lick it (oddly enough was a test we did in College), hell you could even be simple and smack the two together and see which one marrs.
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Or if in a corporate environment, "What does upper management want each one to be ?"
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Priceless!

 
Aluminum is a better thermal conductor, it would feel cooler to the touch than steel (assuming both spheres are lower than body temp).

I'd use the scratch test, however.
 
Here is one that hasn't been mentioned... heat capacity. Since both spheres are the same mass, heat them both in an oven together to an arbitrary temperature. Then put each into an insulated container containing an identical volume of water (the two volumes of water should be the same temperature initially). Measure the temperature increase of the two volumes of water. Aluminum has a higher heat capacity than stainless steel (0.89 J/(g C) vs 0.5 J/(g C)), so the aluminum sphere will heat up the water more than the steel sphere.

I should note that this would work even if the spheres weren't of the same volume or mass, and/or if there was an undisclosed volume of dead space inside the spheres. Furthermore, this method would work if the objects weren't spheres.

I like the variety of answers here! We have newtonian physics, electricity/magnetism, chemistry, material science, and now thermodynamics. Very cool.
 
Putting them in water and seeing which hit bottom first should work silverpig, assuming that there is air in the cavity in the center of each, rather than a complete vacuum and that their weight is determined to be the same by measurement in atmosphere. They will have the same weight on a scale as the air inside will have no effect, but as soon as they are put in water, the greater volume of air inside the steel ball should exert a larger buoyant force on the ball, causing it to sink slower than the aluminum. I think. I had two years of physics in high school and that ended last May

Continuing under the assumption that there is air on the interior rather than a vacuum, weighing them both inside a vacuum should result in the steel being lighter. Wait, maybe not on that part - since the air would only make a difference in the weight if it can exert a buoyant force on the ball, which would require the weighing to take place in an environment denser than the air inside. Weighing them in water should also work.

And the rolling down the hill would most definitely work to any who are doubting, I've seen it in action 😉 Although one must keep in mind, when dealing with moments of inertia, that spheres and cylinders are NOT the same



While looking on youtube for some videos explaining the point, I stumbled across this video (http://www.youtube.com/watch?v...ECB2FF119649&index=24) of a MIT physics course lecture. I watched the first few minutes to refresh my knowledge (and to see if I had a clue what was being discussed) and ended up watching the whole video. I am AMAZED at how similar this professor (apparently one of the top MIT phys profs) was to my old physics prof, from OSSM (Oklahoma school of math and science). Their style, their demonstrations, the lecture dynamic, almost identical. I am only now coming to realize how great my professor was, and how lucky I was to have him in class.

Oh, and my old professor didn't even have a physics degree. He had his master's in chemistry and taught college level physics A+B and Calc I/II/III wonderfully. I may have to pay him a visit sometime
 
Originally posted by: yh125d
Putting them in water and seeing which hit bottom first should work silverpig, assuming that there is air in the cavity in the center of each, rather than a complete vacuum and that their weight is determined to be the same by measurement in atmosphere. They will have the same weight on a scale as the air inside will have no effect, but as soon as they are put in water, the greater volume of air inside the steel ball should exert a larger buoyant force on the ball, causing it to sink slower than the aluminum. I think. I had two years of physics in high school and that ended last May

Continuing under the assumption that there is air on the interior rather than a vacuum, weighing them both inside a vacuum should result in the steel being lighter. Wait, maybe not on that part - since the air would only make a difference in the weight if it can exert a buoyant force on the ball, which would require the weighing to take place in an environment denser than the air inside. Weighing them in water should also work.
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The OP states they have the same mass, weight and volume. They'll be equally buoyant as they displace an identical volume of water (or air...) with the same mass.
 
And let's complicate the things a bit more. What if the steel ball had aluminum interior and the AL ball had steel interior?

Which of the experiments will fail to tell the difference? 😉

Talk.....
🙂
 
Originally posted by: QuantumPion
You could put both spheres in a nuclear reactor. The neutron flux would activate nickle alloys in the stainless steel, transmuting them to Co-60 which gives off hard gammas. You could then give the spheres to your lab assistant and time how long it takes for him to start puking his guts out from the radiation exposure.
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Man, which planet do you come from???
 
Originally posted by: AstroGuardian
Originally posted by: QuantumPion
You could put both spheres in a nuclear reactor. The neutron flux would activate nickle alloys in the stainless steel, transmuting them to Co-60 which gives off hard gammas. You could then give the spheres to your lab assistant and time how long it takes for him to start puking his guts out from the radiation exposure.
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Man, which planet do you come from???
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Technically, you would want to only give one sphere to the lab assistant, but since you can only do this once, you'll need two lab assistants. In the interest of control, best if you had identical twins, since they have the same genes they should react to radiation poison at approximately the same rate.
 
Originally posted by: JTsyo
The way it works in our office is that you find someone that has dealt with the spheres before and ask them.
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even if they left the company 6 months prior to the question
 
Originally posted by: AstroGuardian
And let's complicate the things a bit more. What if the steel ball had aluminum interior and the AL ball had steel interior?

Which of the experiments will fail to tell the difference? 😉

Talk.....
🙂
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You mean, arranged so that their mass distribution was identical, as well as their size and density?

I don't think that anything changes except for the roll test...well, and the "neutron-activated-nickle-lab-assistant-killing" method. And the heat capacity method, assuming equal amounts of alminum and steel are in each sphere. And the grind-them-up-and-make-thermite method...

OK, fine, now we're just left with the surface-study methods. But those are the best methods anyway.
 
Heat will definitely work. CPU Heatsinks are made of aluminum for a reason. Aluminum transfers heat much quicker. Think aluminum foil. When you take something out of the oven that's been cooking for a long time with aluminum foil on top, you can touch the aluminum within seconds after taking it out. So heat them both up, the one that stays hot longer is steel.
 
Originally posted by: TastesLikeChicken
Use a Rockwell hardness tester on one of the spheres. You'll get a definitive answer that way.
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Is that included with the Rockwell Automation's Retro Encabulator?
 
Rolling them down a hill will not work. Though the shells are of different thicknesses, the COG and MOI are the same, as the material density is different, which causes shell thickness to differ.

Even though the steel shell is thinner, it's mass and density would roughly equal the mass and density of the thicker alluminum shell, and therefore have the same center of gravity and moment of inertia.

Dropping them in water would not work for reasons preveiously stated.

If you cannot damage/destroy the spheres, the easiest way to tell the difference is by asking the person who made them 🙂
 
Best answer is "send them off to a lab, when the company depends on the right answer I'm not going to guess".
 
Since someone allowed for oxides on the surface, you can greatly affect the hardness properties of the aluminum sphere by having it annodized. I pointed out earlier, but no one responded to it (and I'm not googling it), but wouldn't the processes of creating the sphere allow for a variety of characteristics. i.e. rapid cooling vs. slow cooling, etc. (i.e. tempered steel)
 
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