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[deleted] t1_j1othwh wrote

Reply to comment by bobbyLapointe in How is density measured in porous materials? by That_Lego_Guy_Jack

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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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