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Comment by AnimalMuppet | Hacker News Reader

Can you explain?

An ounce of gold is heavier than an ounce of feathers, because the "ounce of gold" is a troy ounce, and the "ounce of feathers" is an avoirdupois ounce. But that shouldn't be true between feathers and bricks - they're both avoirdupois.

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Note that a pound of gold is 12 troy ounces, while a pound of feathers is 16 avoirdupois ounces. So one of my favorite pedantic bits of trivia is that an ounce of gold is heavier than an ounce of feathers but a pound of gold is lighter than a pound of feathers.

Feathers are less dense so they have higher buoyancy in air, reducing their weight.

Pounds are a unit of weight, not of mass. 10 lbs of feathers is whatever amount of feathers causes a scale to display 10 lbs. If the scale also displays 10 lbs for the quantity of bricks, then they weigh the same, regardless of any differences in mass.

Is this still true? I thought pounds are now defined in terms of kilograms (about 0.453)? Because kilograms are definitely a unit of mass, not weight. Or is the pound defined as some amount of kilograms at a specific point on earth, in a specific phase of the moon?

It seems the pound has since been redefined and split into separate units: pound mass and pound force, the former in terms of kilograms (1 lb = 0.45359237 kg) and the latter in terms of the force exerted by one pound of mass in earth’s gravitational field (standard g = 9.80665m/s^2).

So using the word pound without qualification is ambiguous in contexts where it’s not clear whether mass or force is meant.

According to the dictionary, "heavier" can refer to weight or density. In their typical form, bricks are heavier (more dense) than feathers. But one should not make assumptions before answering the question. It is, as written, unanswerable without followup questions.

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