I get it that in general it's alright to refer to weight in pounds, kg's, or grams, but when dealing with tech specs, it really throws me off.
I'm looking up motor torque's right now, and they are listed in g-cm's. Although not SI, cm is obviously fine - but how do I deal with the grams? It's supposed to be a measurement of force! Am I to understand they refer to the force of "weight" and thus choose grams? If so, how should I convert to Newtons?
Convert mass to force (F = ma, F is force in Newtons, m is mass in kilograms, a is acceleration due to gravity = 9.8 m/s^2)
Calculate torque T = Fs (s being distance in metres)
You now have torque in S.I. units Newton-Metres. Of course if you're from the US of A you'll want the answer in gallons of BTU's per horsepower mile or some crazy crap like that and you'll have to use a conversion calculator to work it out! ;D
They are effectively the same thing on Earth. A weight is a mass acted on by gravity and since most people aren't going to experience other than Earth's gravity, a gram becomes a weight. Once outside the Earth's gravity. a gram remains a gram, a pound is meaningless. Strictly a torque should be expressed in newton metres. A newton is the force exerted by the mass of one Kilogram acted on by an acceleration (Einstein proved gravity is just an acceleration) of 1m per second per second. The earths gravity is 9.81 m/s/s hence the somewhat inconvenient figure for kg versus newtons. A newton metre is something of a large unit when applied to the typical small plastic servo, so rather than use millinewton metres or micronewton metres they use the incorrect gram cm which uses more familiar units. Read a gram as the force that is exerted by the mass of 1 gm under Earths gravity. In round terms at 9.81 apples to the Kg, a newton is the force exerted by 1 apple under Earths gravity.
Something I was taught at school. The weight of one apple exerts a force of ~1 Newton. Big apples more, little apples less, kind of an aide-mémoire considering what Newton was famous for.