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

[TastesLikeChicken]

Originally posted by: DrPizza
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)

Assuming the sphere was cast from molten metal (it would have to be created in two halves, then welded together and subsequently machined), the cooling process would have some effect on the final grain/crystalline structure of metal. ime though, most castings are allowed to air cool and are rarely quenched or artificially sustained at high temperatures. The tempering is then left up to subsequent heat treatments that alter the properties of the metal, which are applied based on the original alloy composition used in the casting.

That's assuming the spheres were cast in the first place. They could also be machined, spun, or forged.

 

[bobsmith1492]

Originally posted by: Born2bwire

Originally posted by: TastesLikeChicken

Originally posted by: Born2bwire

Originally posted by: TastesLikeChicken
Use a Rockwell hardness tester on one of the spheres. You'll get a definitive answer that way.

Is that included with the Rockwell Automation's Retro Encabulator?

Rockwell Automation's Retro Encabulator

I thought most engineers were familiar with this piece of equipment.

Sorry, I got it confused with the wire stretcher...

 

[jagec]

Originally posted by: DrPizza
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)

Well, if we allow for surface treatments, you can do all kinds of things. I suppose the next challenge is to come up with the least destructive test that is also the least sensitive to deliberate attempts to confuse the issue...in other words, hardness or electrical conductivity tests are out because they could easily be affected by surface treatment, moment of inertia could be affected IF we allow for various shenanigans on the inside of the sphere, but chemical methods would be a bit harder to fool.

A microprobe or spatially offset Raman would both be good, hard-to-fool tests which don't damage the spheres.

 

[Zepper]

It's a trick question as in the known universe, perfect spheres can't exist. They can be imagined and approximated, but can't be made (or found). There is no perfection in the known universe: no straight line, no circle, no square, no cube, etc. Being able to imagine perfection is one of the more unfortunate capacities of humans - that gets us into a lot of trouble not only scientifically, but socially and politically as well.

.bh.

 

[Born2bwire]

Originally posted by: jagec

Originally posted by: DrPizza
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)

Well, if we allow for surface treatments, you can do all kinds of things. I suppose the next challenge is to come up with the least destructive test that is also the least sensitive to deliberate attempts to confuse the issue...in other words, hardness or electrical conductivity tests are out because they could easily be affected by surface treatment, moment of inertia could be affected IF we allow for various shenanigans on the inside of the sphere, but chemical methods would be a bit harder to fool.

A microprobe or spatially offset Raman would both be good, hard-to-fool tests which don't damage the spheres.

Now where's the fun in that?

 

[jagec]

Originally posted by: Born2bwire

Originally posted by: jagec

Originally posted by: DrPizza
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)

Well, if we allow for surface treatments, you can do all kinds of things. I suppose the next challenge is to come up with the least destructive test that is also the least sensitive to deliberate attempts to confuse the issue...in other words, hardness or electrical conductivity tests are out because they could easily be affected by surface treatment, moment of inertia could be affected IF we allow for various shenanigans on the inside of the sphere, but chemical methods would be a bit harder to fool.

A microprobe or spatially offset Raman would both be good, hard-to-fool tests which don't damage the spheres.

Now where's the fun in that?

11th Commandment: Don't Get Caught.

 

[KIAman]

At first, I was pretty irritated at all the responses that were invasive to the spheres. I had thought there was an assumption the spheres were to be unharmed. Reading through the OP again, there was no such requirement.

Technical Answer: The simplest, easiest, most accurate answer is to burn them both and run them through a mass spectrometer.
Social Answer: Ask the guy who made them.

Now comes the real trick. I am going to make another assumption here that the testers somehow know which is aluminum and which is steel. If they both physically are identical and are small enough to manipulate, you could "shuffle" them around (behind your back or some other out of view) and then it doesn't matter which one you select because the testers won't know either.

 

[91TTZ]

look at them. Aluminum is a lighter color than SS.

 

[polarbear6]

well there are many ways

like

specific heat, specific resistivity, refractivi index(if possible) inertia denisity, (some times even half life period )

but rolling does work
to find out the moment of inertia u can like make it swing too

although i would obiously depend upon specific heat capacities

well u can do a test each from each branch of physics
namely optics, electrics, nuclear, mechanics, thermo and so on and so forth
and if possible we can even do some chemical tests )
the sky is the limit

 

[templar165]

Well, density of Aluminium is lesser than that of Steel. So one way you can make them out is by taking a liquid of intermediate density and put them both in there. Aluminium sphere will float, while the Steel sinks.

 

[Suspicious-Teach8788]

Why didn't anyone say to just stick them in an x-ray fluorescence machine? Duh? Press the button and you'll know within a second once the peaks start piling in.

You can use thermal conductivity also.

You can etch some of it for inorganic analysis like AA or ICP.

 

[bryanl]

Did anybody mention sniffing them? Aluminum always has a detectable scent; stainless steel does not.

 

[polarbear6]

Originally posted by: templar165
Well, density of Aluminium is lesser than that of Steel. So one way you can make them out is by taking a liquid of intermediate density and put them both in there. Aluminium sphere will float, while the Steel sinks.

we cant do that water i guess

i think we need a fluid who's density is more than that of steel and less than that of aluminum(or vice versa if steel is more dense than alu)

but one thing is for sure
i dont think we will have to encounter such kinda silly situations in our day to day lifes .........

 

[sao123]

Originally posted by: silverpig

Originally posted by: Capitalizt
I was asked the other day (in an in-house interview) about two perfect spheres, same diameter, same exact weight, same mass, one is made of stainless steel, one of aluminum. They're hollow, so the differing parameter is the shell thickness. The aluminum one will have a thicker internal shell, giving it the same weight as the steel sphere. The question was how to determine which one is which. One of the options was to roll it down a slope and see which one gets to the bottom first. Assuming a frictionless surface.

Any ideas?

*edit - solved!*

I asked on another forum and got this answer...sounds right to me

"Since they have the same weight, however the weight is distributed differently (in terms of average mass per unit radius), they will have different rotational inertia (how resistive it is to change in state (moving vs not moving)). roll them down the hill. the one that has more of its mass towards the center (the one with a greater density = thicker material = smaller inner radius) will reach the bottom first because it has less inertia. the one thats less dense, thinner material, larger inner radius, will have more inertia and will therefore take more time to roll down the hill."


Two more answers that might work also:

"Putting them in water may also do the trick, if we assume that there is air in the center of these hollow spheres. The one with a larger inner radius and thinner shell will have less apparent mass underwater because it will have more air wanting to push up (because air is less dense than water)."

"Put a heater on the top of both and the one with higher thermal conductivity would get hot at the bottom first. "

Those answers are kind of wrong.

Rolling it down the hill will work. The steel ball will get there last. Greater density = THINNER material (because you need less of it) = LARGER inner radius. The answer for this one was right, but they made a small mistake and mixed something up.

Putting them in water will NOT work. The mass of both balls is the same. Their volume is the same. Thus, their density is the same, and they will either float or sink together.

Thermal conductivity will work too.

You could stick them in water and use them to electrolyze the water. The one that rusts is your steel. Stainless will be rust resistant, but not necessarily rust proof... and if you electrolyze it, I think it'd rust pretty good.

Steel may be magnetic. If you stick a magnet to one of them it's your steel... if it doesn't stick to either, well then you need another test.

im not entirely convinced their volume & density would be the same... sure the appearance of the volume of the entire sphere would be the same...
However, since the volume would be the integral of the outer surface - the integral of the inner surface (after all they are hollow)...
Logically the one with the thinner shell (the only thing which differs) would have a higher density??

or does bouancy only matter on total water displacement (surface area instead?)

 

[KIAman]

If you treat the spheres are complete units, the densities are IDENTICAL. You cannot know the interior volume other than invasive methods.

Buoyancy only takes into account water displacement and mass and at the micro-scale, water tension.

 

[TuxDave]

Originally posted by: sao123

im not entirely convinced their volume & density would be the same... sure the appearance of the volume of the entire sphere would be the same...
However, since the volume would be the integral of the outer surface - the integral of the inner surface (after all they are hollow)...
Logically the one with the thinner shell (the only thing which differs) would have a higher density??

or does bouancy only matter on total water displacement (surface area instead?)

Yes the one with the thinner shell would compose of a material of higher density. But bouyancy only matters on total water displacement vs total mass.

Since the mass and water displacement is identical (barring any holes in the sphere), it will either float or sink together.

 

[CountZero]

Originally posted by: ganesh1

Originally posted by: templar165
Well, density of Aluminium is lesser than that of Steel. So one way you can make them out is by taking a liquid of intermediate density and put them both in there. Aluminium sphere will float, while the Steel sinks.

we cant do that water i guess

i think we need a fluid who's density is more than that of steel and less than that of aluminum(or vice versa if steel is more dense than alu)

but one thing is for sure
i dont think we will have to encounter such kinda silly situations in our day to day lifes .........

I dunno about that, as an EE working on ASICs barely a day goes by where someone doesn't bring me two identical spheres and I need to figure out which is made of which material.

 

[cusideabelincoln]

Heat them up and re-measure the diameter, using precision tools like a micrometer (or some other method) if necessary. The aluminum should expand more and have a larger diameter than the steel at over the same temperature increase.

And I (think I) have figured out why people are getting hung up on the water/buoyancy situation, because I spent a few minutes wrapping my head around it as well. First there is a mis-assumption: For these to spheres to be of the exact same mass (and weight), they need to not only have the same mass of each metal (SS/AL), but they also have to have the same mass of air inside the "hollow" cavity, assuming it isn't a vacuum. Now this would mean the air in the aluminum ball would need to be denser than the air in the stainless steel ball, since there is less volume available and assuming these balls were made at the same place with the same air composition and pressure.

Now people (and me, too) have probably assumed no special attention was paid to the air cavity inside each sphere. If this were true, and there was air of the same composition, pressure, and density inside the spheres, then the spheres would actually have different masses and thus different weights, and this weight difference would be more noticeable in water due to a stronger buoyant force if we were to "weigh" these spheres while they were submerged in water. This is the problem I was hung up on as well, but I finally forced myself to consider that the spheres are an enclosed environment. Thus, the following example would not apply, since it is an "open" envirnoment: http://metrology.burtini.ca/grav_air.html

As you can see in their hypothetical, the jars were exposed in the fluid air environment, so when they sucked the air out (which is correlative to moving something from a gaseous environment to a liquid one) the weight was shifted because the water displaced more fluid than the other object. However, in this thread's situation, both spheres are enclosed and the same size and the same mass, so moving it to a different environment should not show any differences in buoyancy or apparent weight.

 

[ubercaffeinated]

thermal, magnetic, or stress test. easy.

 

[jagec]

Originally posted by: cusideabelincoln
And I (think I) have figured out why people are getting hung up on the water/buoyancy situation, because I spent a few minutes wrapping my head around it as well. First there is a mis-assumption: For these to spheres to be of the exact same mass (and weight), they need to not only have the same mass of each metal (SS/AL), but they also have to have the same mass of air inside the "hollow" cavity, assuming it isn't a vacuum. Now this would mean the air in the aluminum ball would need to be denser than the air in the stainless steel ball, since there is less volume available and assuming these balls were made at the same place with the same air composition and pressure.

You're making it more complex than it has to be.

You don't have to pressurize the air to make up the difference. You can use a larger weight of aluminum than steel to make up for the smaller weight of air in the center of the aluminum sphere. End result = equal weights AND volumes without any pressurization.

Practically speaking, this could be achieved by simply weighing the molds as the spheres are being made. Since the volume of the molds does not change, and the weight of the molds includes both air and metal, the two will be the same once they come out.

 

[cusideabelincoln]

The point of my more complex scenerio was mainly to point out that the weight of air inside the spheres can't be overlooked. I, and I assume other people as well, initially took this scenario as both spheres having the same mass of each metal. And when the subject of bouyancy was introduced, they overlooked or forgot the fact the mass of gas inside each sphere is accountable for the total mass of the sphere as well as metals.

I just wanted to chime in because no one really offered a rebuttal or detailed explanation of the proposed bouyancy solution, and that was the best way I could explain it at the time.

 

[PolymerTim]

It's an interesting point to make. Weight and mass are not the same, even though we commonly assume they are. Your linked example is specifically referring to weight (net force of gravitational pull and buoyancy acting on an object of a given mass), but the original question specifically noted " two perfect spheres, same diameter, same exact weight, same mass" so we must assume that buoyancy is not going to help us here.

The simple rebuttal to the proposed buoyancy solution is that buoyancy is determined by the net density of the entire solid object and it really doesn't matter what the ratio of metal to air is if the two spheres have the exact same volume and mass. As TuxDave describes, the pertinent volume needed to determine the net density of the object is the volume of medium (air, water, etc) displaced by the object.

sao123 appears to be calculating the density of the metal shell and not the entire sphere. This would only matter if the displaced medium was able to penetrate the cavity of the sphere (again TuxDave referred to holes in the shell). I think it is safe to assume that a perfect sphere has no holes in it.

Since the OP defined the objects as perfect spheres with the same diameter and mass (including metal and air cavity), we must conclude that the volume and net density of each sphere is identical.

 

[Net]

apply heat and see which one melts the fastest

 

[TheLonelyPhoenix]

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.

I got a kick out of this answer.

Aluminum has lower resistance, assuming you knew the core material was air or something else non-conductive, you could just hook up a DC source and an ammeter and see which sphere flows more current.

Or, just roll them down a hill.