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bobbyLapointe t1_j1nnak2 wrote

Take you dry sponge, put it on a scale, you have its mass. Take a graduated glass recipient with exactly 1L of water in it, put the sponge in it, read the new volume indicated, substract 1L to it, you have the volume of your sponge. Divide the mass by the volume and you have the volumic mass density of the sponge. Divide it again by the volumic mass of pure water (1000kg/m3 or 1kg/L) and you obtain its density.

Edit: corrected as I didn't get the correct english terms.

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whiteknives t1_j1owipp wrote

> Divide the mass by the volume and you have the volume mass of the sponge.

This calculates the sponge’s density. Mass divided by volume. Nothing further needed.

> Divide it again by the volumic mass of pure water (1000kg/m3 or 1kg/L) and you obtain its density.

This is totally useless and wrong.

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Graham146690 t1_j1p2stm wrote

Could be a language barrier. The second step calculates Specific Gravity in English.

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whiteknives t1_j1q4f8l wrote

You nailed it! And now I know just a little bit more than I did yesterday, thanks!

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bobbyLapointe t1_j1pblw2 wrote

I made the mistake of not checking the exact english terms. In my language we differentiate density and relative density with two different terms, litteraly volumic mass and density. I thought op wanted the relative density

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whiteknives t1_j1q44tt wrote

Awesome! Yeah I could tell you knew what you were talking about but that last line made me go whaaa? Makes sense!

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bobbyLapointe t1_j1qg8ub wrote

I'm glad your took it that way after my explanation, in opposite to the last sentence of your initial response which I found a bit agressive :)

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BrokenHS t1_j1ol22p wrote

Why do you divide it by the density of water? Isn't the mass divided by the volume the density? What is "volumic mass"?

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bobbyLapointe t1_j1pbcbf wrote

Density is an absolute ratio, where the volumic mass of the water is the reference (water density =1). By dividing the volumic mass of the sponge by the water's you compare both, and will easily know if it's denser (desnity > 1) than water or not.

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fishman1287 t1_j1o6xmr wrote

Does this account for the sponge absorbing the water?

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Macemore t1_j1okhki wrote

That is why you use the water, it's a tool in this case to measure the sponge's cavities. By using a known metric (water) and volume (1L) and calculating the differences before/after the absorption you can determine the quantity of water in the sponge. If you know how much water is in there, and you know how much space water takes, you know how much space is empty in the sponge.

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[deleted] t1_j1othwh wrote

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PA2SK t1_j1oxt59 wrote

The air wouldn't contribute anything to the weight so it shouldn't matter

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[deleted] t1_j1pb5mc wrote

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PA2SK t1_j1pbpmi wrote

Just put it in a jar of water and squeeze all the air out of it, then measure the change in volume of the water in the jar. That delta is the volume of the sponge material.

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[deleted] t1_j1pdvph wrote

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PA2SK t1_j1qexc4 wrote

The air is neutrally buoyant. It's not that it contributes a small amount to the weight, it contributes zero. For example if you had a cup filled with nothing but air and weighed it the air won't affect your reading at all because the weight of that air is 0 when you're in Earth's atmosphere.

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orijing t1_j1qswfx wrote

The buoyancy force would reduce the apparent mass of the cup by the mass of the displaced air. It may not be significant but if you want to be precise, you need to account for it.

Just like how a helium balloon isn't negative mass, it just appears to be because of the buoyancy force, it's important to account for the mass of the displaced air to be precise.

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PA2SK t1_j1qtx4l wrote

I see what you're saying. If you wanted to be as precise as possible you could calculate the mass of displaced air and add it to the weight of the sponge.

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bobbyLapointe t1_j1pbf19 wrote

In deed, you should try to press the sponge to evacuate all the air trapped inside.

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