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
Mass: how much stuff there is combined in the object youre talking abouy. Stuff here means atomic bits. Mass is just counting how many atomics you have.
Weight: how much gravity effects your atomic bits. This changes when gravity effects you less, like being further away from big objects when in space.
Could be wrong but this is how I think about it
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.
The other answers have addressed most of your question, but I wanted to draw attention to:
> implying that they’re just different units for the same concept
While they’re not the same (as others have said), you did hit on something really important here: inertial mass (how much something resists acceleration) and gravitational mass (how susceptible something is to a gravitational field) _are_ always equal, and this is not an obvious thing! For example, there’s no connection between inertial mass and how susceptible something is to an _electromagnetic_ field. We’ve always measured the two kinds of masses (inertial and gravitational) to be equal, but there’s no inherent reason to assume they should be, absent those many observations. Einstein called the assumption that they’re equal the [equivalence principle](https://en.wikipedia.org/wiki/Equivalence_principle), and it’s an important part of general relativity.
tl:dr: your confusion is in large part due to the fact that these two logically-independent definitions of mass are always equal, and that turns out to be a non-obvious yet very important fact about the universe.
Grams and pounds are just the metric and imperial units of mass. They measure the same thing.
Bathroom scales don’t actually measure mass, they measure pressure. They’re marked in kg or lb because they’re calibrated for Earth’s gravity and the conversion is simple between kg (mass) and N/m2 (pressure). They could be marked in N/m2 (but then you’d need to account for the area of the person’s footprint), or just Newtons. We’re used to using units of mass, so they’re marked in those.
Since weight is a measure of the gravitational attraction between two masses and because Earth’s mass is relatively constant (and inescapable in our localized setting), it is the varied masses of the objects attracted to Earth that determine their weight.
So, while you only have to be able to lift (and balance) your own weight to lift the entire earth over your head (AKA, do a handstand), you would have to be able to lift about 2 tons to get between a car and the earth to separate them.
As others have said, weighing an object on a spring or digital scale measures the weight (down force) that is applied to the scale and tells you the mass provided you are somewhere with the same gravity as the scale was made for. If you took the same scale and put it on the moon or mars, it would incorrectly tell you that your mass is a lot lower than it actually is, likewise, if you stood on the scales in an elevator and read out while it was accelerating, it would incorrectly read a higher mass because it is actually measuring down force and converting it to a mass reading
Note the a balance scale doesn’t have this problem as it compares 2 objects so both are affected equally when gravity changes.
Part of the problem is that we are a bit casual about mixing terms for weight and mass. Strictly speaking, your weight in the SI system should be measured in Newtons (units of force) and you mass in kilograms (units of mass) but we don’t for historical reasons and because so long as we don’t have to have equipment that measures in varying gravity, it doesn’t really matter.
It’s also worth noting that converting from grams to pounds isn’t converting mass to force – it’s converting SI to customary (imperial) and unfortunately, the imperial system uses pounds for both force and mass so unless you specify which you mean it can get confusing
Mass is your resistance to acceleration (without friction.) Weight is how hard gravity is pulling you down.
Wow, so my weight won’t change even if I binge on Mars snacks? Best diet plan ever!
Weight is the force mass makes in combination with gravity.
Mass is the same everywhere but we measure it in earth relative units.
Correct me if I’m wrong 😅
>there is a simple mathematical conversion between grams (mass) and pounds (weight), implying that they’re just different units for the same concept.
Yes, but no. There’s two different pound units, there’s pound (mass) and pound (weight). You can do calculations to get from grams (mass) to pounds (mass) then to pounds (weight), but that doesn’t mean that mass and weight are the same.
Summarily, you can use a distance (miles) to calculate your speed (mph) , but that doesn’t mean that distance and speed are the same thing. They are two different units that describe different things.
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