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The mass of a large lightweight object is not simple to determine. For you must allow for the buoyancy* of the volume of air displaced by the volume of the item.

But having done so, you will have the true mass, not merely its weight on Earth.

[Its weight will also vary according to the force of gravity at that point. Even in a country as small as New Zealand, the gravitational force varies by about one part in a thousand. Right at the limit for commercial weight precision. ]

* This buoyancy concept needs thought. Consider a fish, which you must not leave out of the water for more than 10 seconds. And of which you desire to know the mass.

Now, this fish, when neutrally buoyant, will neither rise, or sink. Therefore it is equally dense as is the water. So its mass is the same as the mass of the volume of water that it displaces. (This is Archimedes idea.) So by measuring the volume of the fish, and knowing the mass of that volume of water, we have the mass of the fish - not merely its weight.

So we quickly move the fish to a new tank, and measure the increase in the level of the water, to know the volume occupied by the fish.

[You may wish to conduct the same mental experiment (gedankenexperiment) to find the mass of a perfectly spherical balloon, filled with Helium.]

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12y ago

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