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Identifying Materials Using Archimedes' Principle.
DavidA:
VT,
Thanks for that. But that is confirming what I said.
Maybe it is the way that Pete phrased it that is causing my confusion.
...What Archimedes realised was that an object immersed in a liquid experiences a reduction in weight equal to the weight of the liquid it displaces, i.e. a quantity of the liquid that has the same volume as the immersed object.
This seems to imply that two object with equal volume but made of different metals will displace different weights of water even though the displacement will be the same.
A cubic foot of lead weigh much more than a cubic foot of aluminium, but they both displace the same cubic foot of water.
Dave.
Pete W.:
Hi there, David,
--- Quote from: DavidA on January 27, 2015, 09:44:38 AM ---VT,
Thanks for that. But that is confirming what I said.
Maybe it is the way that Pete phrased it that is causing my confusion.
...What Archimedes realised was that an object immersed in a liquid experiences a reduction in weight equal to the weight of the liquid it displaces, i.e. a quantity of the liquid that has the same volume as the immersed object.
This seems to imply that two object with equal volume but made of different metals will displace different weights of water even though the displacement will be the same.
A cubic foot of lead weigh much more than a cubic foot of aluminium, but they both displace the same cubic foot of water.
Dave.
--- End quote ---
With all respect, I challenge your implication. The sentence of mine that you quoted is entirely consistent with your own last sentence.
Archimedes Principle is explained well on Wikipedia, try http://en.wikipedia.org/wiki/Archimedes%27_principle
If you have a spring balance, try it out for yourself.
mklotz:
If it's practical to cut a small sample from the subject material it can be machined into a convenient shape (eg, cylinder, cube) and the volume calculated. Once weighed this sample's density can be computed and found in online tables.
Once the density is known, the volume of the subject material can be found from its weight.
vtsteam:
The confusion comes I believe, because of the different uses of the word displacement. In naval architecture it is equal to the weight of a floating object. Thus two objects of the same shape and size (and overall volume) may have different displacements. They would have correspondingly different weights out of water, as well. And different effective densities.
But in other uses the word displacement is a volume measure, and is always the same for the same size and shape object. It's equal to the volume of water that an object displaces when submerged. Two objects of different weights and effective densities would always have the same volume and the same displacement.
In fact, this kind of displacement has nothing to do with water, or any fluid. You can measure an object's displacement in alcohol, mercury, or sand, and it will not vary. And the object itself be made of lead or balsa, and it would still have the same displacement.
But, you certainly can determine an object's density using this fact IF you also weigh it, and then divide the weight by the displacement.
vtsteam:
To add to the fun, here, I can think of a way of determining the weight and density of a material of irregular shape with a calibrated tub, some fresh water, and a balloon, without a scale. Can you?
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