Conceptually I understand that mass is a measure of the amount of stuff present in an object, while weight is a measure of the force of gravity applied to that object. An object of a given mass will have a bigger weight on Earth than on the moon because Earth’s gravity is stronger. But… mass is determined by weighing an object on a scale. And there is a simple mathematical conversion between grams (mass) and pounds (weight), implying that they’re just different units for the same concept. So what gives? (Also this is a question that applies to so many fields, I had no idea which flair to apply.)
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We can weigh something to determine its mass on Earth because we know how strong gravity is, therefore we can work backwards to calculate its mass from its weight. The mathematical conversion between pounds and grams that we use is only valid on Earth (or in another environment with the same gravitational pull as Earth), and we use it so commonly because the layperson doesn’t think of grams as a measure of mass. Most of us use it as a measure of weight, which is technically incorrect but practically speaking is fine, so long as we stay on Earth.
The specific equation used to turn mass to weight is the mass multiplied by the gravitational acceleration. Since we’re multiplying two different units together, the result is a different unit which is the product of the other two. Therefore grams and pounds are not different units for the same concept.
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