Grams as weight? Newton weeps.

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?


I agree. Maybe they didn’t know the difference between force and mass.

The cm is an SI unit; it is just not a primary unit like m, Kg, s, A.

What’s wrong with grams as weight?

1 kg = 1000 grams.
1000 grams = 2.20462262 pounds

1 gram-force/centimeter = 0.980665 newton/meter

I read this on Wikipedia, so it must be true. (Basically saying, g-cm is generally accepted as a non-SI unit.)

Other non-SI units of torque include “pound-force-feet”, “foot-pounds-force”, “inch-pounds-force”, “ounce-force-inches”, and “metre-kilograms-force”.

What’s wrong with grams as weight?

Because the (kilo)gramme is a unit of mass.

Should i just regard g as weight then? Strangely, theres a couple of sites that convert from grams to newtons

I would:

  • Convert grams to kilograms (/1000)
  • Convert centimetres to metres (/100)
  • 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

Wikipedia says “sometimes units of torque are expressed as “weight-force*meters”, and sometimes “force” is omitted.”

So it’s just used because grams are a convenient number for the usual applications: “how much will this servo lift at the end of the 2cm servo arm.”

9.8N per kg-force, .0098N per gram-force. If you’re wanting to deal in Newtons, that should be … obvious?

So you say that the grams mentioned are mass grams, not some pseudo weight grams?

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.

You lost me with the apples, but the rest makes sense. Thanks.

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.

Big apples more, little apples less, kind of an aide-mémoire considering what Newton was famous for.

Ah Newton! I bet Newton’s apple is where they got the idea to name the suspected ‘worm holes’ in space. :sunglasses:



F=FXXK shipping cost

m= weight of parts

a= the shipping method