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Author Topic: Can volume and weight measurements be converted one for one? Or at all?  (Read 2418 times)

Offline annie123

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Re airline limits - 100ml containers for carry on baggage. When something like toothpaste is measured in grams how can this be converted into litres? Surely it isn't one for one as the security people tell me. I asked about a 125gm item and was told this is 125 ml.How can a volume measurement be described in units of weight?


Offline damocles

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Volume and weight(mass) are connected by "density": density = mass /volume.

For things that are mostly water, it is a fairly good and fairly common approximation to take the density as being 1 g/mL, so that 125 g = 125 mL.

But toothpaste is not "mostly water". If a dollop of toothpaste floats on water (as with some gel-type toothpastes) then the number of mL will be larger than the number of g. If it sinks (as with most toothpastes) the number of mL will be smaller than the number of g.

However, airline security personnel probably are not trained to make these distinctions. Their name is either Sir or Madam, and they need to be treated with kid gloves. It is much less hassle to buy a new tube of toothpaste at your destination.

By the way, there is a very real distinction between "weight" and "mass". When your flight to the moon reaches its destination, you will find that 5/6 of the weight of your luggage has disappeared (effect of gravity), but that it still has the same mass (amount of stuff).

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