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Identifying Materials Using Archimedes' Principle.
Pete W.:
Hi there, all,
Thank you all for your contributions. :nrocks: :nrocks: :nrocks:
I feel I need to reiterate my opening remarks as to the scope of this process/method.
First of all, while Marv's input is true, machining a precise sample of the material is more something for forensics than for diferentiating between two similar-looking materials prior to charging the foundry crucible or going to the scrap metal merchant.
Secondly, I used the word 'displaced' (adjective), not 'displacement' (noun). It's usually stated that Archimedes was inspired by seeing the water overflowing from his bath but the customary expression of his principle refers to 'displaced liquid', not 'overflowing liquid'.
I do agree that one could assess the specific gravity of the 'mystery object' by catching the overflowing liquid and measuring either its weight or its volume but, as Marv says, that is not without its difficulties. Ideally, you'd want to have a controlled size and position of overflow pipe or spout to simplify catching the overflowing liquid and the bath/bucket would need to be precisely pre-filled to the overflow level. But David A and, I think, Steve, have offered that process in conjunction with weighing the object dry. I submit that my method #1, comprising two weighings with the same instrument (one dry, the other wet) is much quicker, simpler and more in keeping with the overall aim.
My methods #2a & #2b were included to show how the result could be achieved without a quantitative weighing device. The use of a constant counter-weight plus a ruler or tape measure is an effective substitute, though I do agree that managing the things dangling from a stick would take a bit of practice.
As regards Steve's balloon or Russell's second tub, I'm retiring to the side-lines and changing to spectator-mode! :lol: :ddb: :lol: :ddb: :lol: :ddb:
vtsteam:
My thought on using the balloon if you didn't have a scale to weigh something would be to check the water level with nothing in the tub, then check with the mystery irregular object in the tub (I assumed it was a metal denser than water) and then attach the balloon to the object and fill the baloon with air untill the combo reached neutral buoyancy, and then check the water level again.
Divide the difference between the first and last measurements by the difference between the first two measurements. That will give you the specific gravity of the object. That's the density.
Multiply the specific gravity times the difference between the first and second measurements (the volume of the object) and the density of water (in whatever units you favor), and you have the weight
I can see Russell's method working, too. In this case you basically have a cylinder shaped tub, acting as a boat with what are esentially plimsoll marks, and you place the object in it, read the difference and get the weight.
But you haven't measured density of the irregular object. I guess you could first just use one tub for that as I did above.
DavidA:
Pete,
.. machining a precise sample of the material is more something for forensics than for diferentiating between two similar-looking materials prior to charging the foundry crucible or going to the scrap metal merchant...
That is exactly what I was engaged in at work. I was 'the sampler'. My job was to produce precise samples from workpieces and make them suitable for mechanical, chemical and spectrograph analysis.
Dave.
RussellT:
I thought about filling the balloon to reach neutral buoyancy and then decided it would be easier just to fully inflate the balloon so the object floated attached to the balloon and the right volume of water would be displaced.
My second tub method does the same thing but without having to tie things together. Fill the first tub and put the second tub in. Then add the mystery object and note the volume displaced (1 litre = 1 kg). That gives you the weight. Fill the first tub and put the mystery object in. Note the volume displaced. That gives you the volume. Density, SG etc can be calculated from there.
I have to say though that if you have a spring balance in the right weight range then Pete's method is quick and easy.
Now - on to making spring balances. If you could make a sping balance where you could adjust the scale (for example by having a scale on a piece of elastic or a spiral scale on a disc or drum) then you could weigh your mystery object, set the scale to read 100 and then dunk it in water and have a direct readout of the material.
Russell
vtsteam:
--- Quote from: RussellT on January 28, 2015, 10:45:29 AM ---I thought about filling the balloon to reach neutral buoyancy and then decided it would be easier just to fully inflate the balloon so the object floated attached to the balloon and the right volume of water would be displaced.
--- End quote ---
Good idea! :thumbup:
Homemade ordinary balances are fun, too. I once made one of paper to weigh paper airplanes with fractional gram accuracy. I calibrated it initially with coins. I made gram weights out of cardstock squares. The cost was a little paper, some fishing line a thumbtack and a paper clip.
You can do the same thing on a larger scale (pardon the pun) with water to calibrate initially.
edit: Oh yeah Russell, that just reminded me -- fifteen years ago I moved my houseboat up north on a trailer. I figured I needed about 500 pounds tongue weight a few minutes before taking off on the 1500 mile trip with a rental truck. How to weigh? Then it ocurred to me to just put a fulcrum under a 2 by 4 on edge, measure the distance to the tongue and figure out where I could press down with my 175 lb. human weight on the lever opposite to just lift and indicate 500 lbs tongue weight. I shifted the houseboat on the trailer slightly to adjust -- worked perfectly!
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