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.)
In: 53
Edit: Bonus ELI2 version
Gravity pushes against stuff. Objects with more mass have more stuff for gravity to push against. Weight is a measurement of how hard gravity is pushing against a given amount of stuff. Without gravity to do the pushing, there is no weight; but the stuff (mass) is still there.
TL;DR: Mass refers to how much “stuff” is in a given object. Weight refers to how hard that object pushes against another object (like a planet) due to its mass and gravity. More mass = more stuff being pushed by gravity = more weight.
Long version:
Let’s say you have two bricks; one is made of lead, and the other is made of foam. They are solid and identical in size, but the lead brick has more mass.
In the presence of an equal gravitational field (say, sitting next to one another on two identical scales in your kitchen) gravity pushes both bricks “down” against the scales. The objects’ *weights* are a function of the strength of the field and the *mass* of each object. So, the object with more mass pushes harder on the scale, which displays an accordingly larger number.
In the *absence* of a gravitational field (say, in deep space) neither one would have any weight. However, they still have mass. If you were to hypothetically try to push each object (and assuming you had something to brace yourself against) it would be much harder to get the lead brick moving compared to the foam one, because of its greater mass. It would also be much harder to stop.
Regarding your observation about weights being convertible, you’re correct–because scales actually display mass, not weight. They are generally calibrated to the gravitational field of Earth, and do a little math (or are mechanically designed) to factor out local (Earth) gravity and display only the mass.
If you’re interested, the math would basically be…
Mass = (push strength) ÷ (gravity²)
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