juicemouse

Impressed by the slug reference? Oh jeez...

The kilogram (kg) is indeed the SI unit of mass. The SI unit of force is the Newton (N). But when you bring kgm, kgf, lbm, and lbf (not to mention the beloved slug) into the picture, it gets real confusing real quick. First of all, the weight of something is how much force it exerts on whatever it's sitting on, so weight is a force. It is defined as the mass of an object multiplied by the force of gravity on the object. Mass is a constant, but since the force of gravity can change from planet to planet (and even slightly at different elevations on Earth), weight is not a constant.

Now, what's the difference between kg and kgm and kgf and N, or between lb and lbm and lbf and slug? And why to Europeans commonly measure their weight in kilos (short for kilograms) instead of Newtons? Well, for one thing, since most people only travel into space a few times a year or so, we tend to use an "Earth-centric" system of measurements that differs somewhat from the "standard" SI and English systems. What happens is that we tend to ignore the fact that the force of gravity is not a constant, because it is so close to being a constant as long as we stay on Earth. Believe it or not, it makes things easier to understand, because it takes the variable force of gravity out of the equation. But again, the requirements are that you have to be dealing with objects on the surface of the Earth, and it's a good idea to stay within one unit system without converting back and forth (it can be done but it's a bit more of a PITA looking up obscure conversion factors between kgf and N and lbf, for example). Here's how it works:

1 kgm is equal to the SI standard 1 kg. It is a unit of mass. We stick the "m" on the end so we don't get it confused with the kgf when we're dealing with both.

1 kgf is defined as the force exerted on Earth on an object that has a mass of 1 kgm.

1 kgf absolutely does not equal 1 Newton. 1 Newton is defined as 1 kgm times the force of gravity on earth (9.81 meters per second squared).

So how much does a 1 kgm object weigh on Earth? Either 1 kgf, or 9.81 N. Both answers are correct.

The kilo that Europeans commonly use as a measure of weight is actually the kgf. Are we having fun yet?

1 lbf is equal to the English standard 1 lb. It is a unit of force. Again, we stick the "f" on the end so we don't get it confused with the lbm when we're dealing with both.

1 lbm is defined as the mass of an object that weighs 1 lbf on Earth.

1 lbm absolutely does not equal 1 slug (the English standard unit of mass). 1 lbf is defined in the English system as 1 slug times the force of gravity on earth (32.2 feet per second squared).

So what is the mass of an object that weighs 1 lbf on Earth? Either 1 lbm, or 0.031 slugs (1/32.2 = 0.031). Both answers are correct.

That felt way too much like freshman year.