# Moment of Inertia (Units)

1. Aug 27, 2009

### GreenLRan

1. The problem statement, all variables and given/known data

I am asked to convert moment of inertia of a spacecraft from slug-ft^2 to N-m^2.

2. Relevant equations

3. The attempt at a solution

Is this a trick question? Usually moment of inertia is expressed in kg-m^2 not N-m^2.

1 slug = 1 lbf-s^2/ft --> therefore 1 slug-ft^2 = 1 lbf-s^2-ft = 1.3558 N-s^2-m = 1.3558 (kg-m/s^2)-s^2-m = 1.3558 kg-m^2

2. Aug 28, 2009

### CompuChip

That is correct. However, often when converting from such units to SI units, we "smuggle" in an extra factor. (I'm not familiar with imperial units, but I believe something similar happens in mechanics, where lbs can be converted to N).

So I don't know how appropriate this is for a spacecraft, but usually we assume that 1 kg of mass corresponds to g N of force (where g is the gravitational constant, about 9,8 depending on where on earth you are). This actually corresponds to "normal" language usage: we usually say we "weigh" 60 kg, while we actually mean that our mass is 60 kg, and we "weigh" 60g (about 600 N).