Diadem wrote:ps.02 wrote:And then there's kilograms as a force. I thought that was an artifact of people not understanding the difference, or of wanting to appear to use the metric system, while not really understanding it. But today I looked it up and learned that the kilogram-force is actually a thing - it is ~9.81 N. So I guess it's for people who like to use the metric system, but don't really want to have exact powers of 10 as conversion factors?

I guess you don't have a background in physics, or you'd have recognized 9.81 as earth's gravitational constant.

To be fair, little

g isn't really 9.81, it's 9.81N/kg=9.81m/s

^{2}=32.2 ft/s

^{2}. Someone used to imperial units may well not recognise the number 9.81. And anyway, it's

not a constant, either!

Diadem wrote:This makes the definition entirely logical: The weight of an object in kg is the same as its mass in kg. And you probably use this all the time, without even realizing it. For example when you say something like "he can bench press 100 kg".

But those barbells and things that you buy are really masses. What he can actually do is put 100kg of mass on the bench press in his local gym, and proceed to lift it chestily, by exerting whatever force that turns out to require.

Diadem wrote:I never use American units, but as far as I know the situation is the exact same for pound force and pound mass, isn't it? There's the same conversion factor there. A pound mass = 0.453592 kg and a pound force = 4.44822162 N, according to Google. That's a factor 9.81 difference too.

Well, again, it's not a scaling factor, it's a quantity with dimensions: 9.81N/kg, representing the gravitational field strength at the Earth's surface. So it's always going to work out at 9.81N/kg if you convert it to SI units. But in imperial units, it's 1lbf is the weight in normal gravity of 1lb; leaving things in imperial units we have

g=1lbf/lb. The point is that lbf and lb are

not the same unit and can't be cancelled. They've just been chosen for convenience when dealing with normal gravity. It's a bit like the way one fluid ounce of water has a mass of 1 ounce, i.e. the density of water is 1oz/floz. Unfortunately in the US the "fl" part is normally omitted, which is even more confusing, since the density of water would be 1oz/oz where the "oz" top and bottom aren't the same unit and can't be cancelled.