They were thinking that lbs and kgs are different types of measure:
Lbs measuring weight, which is characterized by the amount of gravitational force applied, and having SI units kg⋅m⋅s⁻² . And kg measuring mass, which is an intrinsic property characterized by the sum of all of an object’s atom’s masses, and having SI units kg.
But they realized in the U.S. a lb is now directly defined by a mathematical proportion to the kg, and therefore is now a measure of mass, not weight. Thus, they are indeed the same type of measure.
The confusion stems from the fact that laymen use force and mass interchangeably as they are always on earth and changes in altitude aren’t significant enough to worry about.
Standing on a European scale and seeing a measurement in kg isn’t entirely accurate- it’s actually measuring Newtons and implying your mass in kg from that. Standing on an American scale, however, is literally measuring your weight in lbf. However, there is also a confusing unit called lbm or pounds mass which measures the mass of a 1lbf weight object on earth. The average person will never use lbm realistically, but this is technically the unit that converts directly to kg.
That’s definitely part of it, but the lb dates way back to before there was any conception of difference between weight and mass. Nowadays, the kg, and thus the U.S. lb, is defined in terms of universal constants (the Planck constant and the speed of light), but traditionally the lb would have been defined by some sort of standard physical object, whereas the kg was defined as the mass of a litre of water. There was an implicit reliance on the force of earths gravity in the measure of the lb, which wasn’t part of the measure of the mass of a kg. So, I think historically speaking it’s understandable to think of the lb as a unit of weight, not mass.
If I’m understanding correctly does this imply that a space colony might measure things in kilograms but have an entirely different value due to differing gravity?
Edit: rereading I’m realizing that the opposite is true, lbs and kgs would be the same on and above every planet
They were thinking that lbs and kgs are different types of measure:
Lbs measuring weight, which is characterized by the amount of gravitational force applied, and having SI units kg⋅m⋅s⁻² . And kg measuring mass, which is an intrinsic property characterized by the sum of all of an object’s atom’s masses, and having SI units kg.
But they realized in the U.S. a lb is now directly defined by a mathematical proportion to the kg, and therefore is now a measure of mass, not weight. Thus, they are indeed the same type of measure.
The confusion stems from the fact that laymen use force and mass interchangeably as they are always on earth and changes in altitude aren’t significant enough to worry about. Standing on a European scale and seeing a measurement in kg isn’t entirely accurate- it’s actually measuring Newtons and implying your mass in kg from that. Standing on an American scale, however, is literally measuring your weight in lbf. However, there is also a confusing unit called lbm or pounds mass which measures the mass of a 1lbf weight object on earth. The average person will never use lbm realistically, but this is technically the unit that converts directly to kg.
That’s definitely part of it, but the lb dates way back to before there was any conception of difference between weight and mass. Nowadays, the kg, and thus the U.S. lb, is defined in terms of universal constants (the Planck constant and the speed of light), but traditionally the lb would have been defined by some sort of standard physical object, whereas the kg was defined as the mass of a litre of water. There was an implicit reliance on the force of earths gravity in the measure of the lb, which wasn’t part of the measure of the mass of a kg. So, I think historically speaking it’s understandable to think of the lb as a unit of weight, not mass.
If I’m understanding correctly does this imply that a space colony might measure things in kilograms but have an entirely different value due to differing gravity?
Edit: rereading I’m realizing that the opposite is true, lbs and kgs would be the same on and above every planet