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Because the characteristic of a backpack in question is volume or how much content it can hold, not mass. An extreme example: If I fill lead into a 20l pack, I get ~225kg, if I fill styropor into a...
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Because the characteristic of a backpack in question is volume or how much content it can hold, not mass. An extreme example: If I fill lead into a 20l pack, I get ~225kg, if I fill styropor into a...