Moment of Inertia and the IPS system.

AI Thread Summary
The discussion revolves around calculating the moment of inertia (MOI) for a flat disk using the formula I = 1/2 * w/g * r^2. There is confusion regarding the relationship between weight in pounds-force (lbf) and mass in pounds-mass (lbm), particularly how to handle the gravitational constant (g) in the calculations. It is clarified that a weight of 2.6 lbf corresponds to a mass of 2.6 lbm on Earth, leading to the calculation of MOI as 130 lbm*in^2. The participant questions whether lbm*in^2 is equivalent to lbf*in*s^2, highlighting the conversion factors involved. Understanding these relationships is crucial for accurate MOI calculations in the context of the IPS system.
jasc15
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IPS drives me nuts. If I know an object's weight in lbf (a flat, 10" radius disk for simplicity's sake), and want to find it's MOI, the formula is I = 1/2*w/g*r2. Now here's where i get confused:

Since I have the object's weight in lbf, it's mass is numerically equal, so a weight of 2.6lbf on Earth has a mass of 2.6lbm (right?) and the MOI is I = 1/2 * 2.6lbm * 102 = 130lbm*in2.

So how do I get around dividing by g? Also, are the units of lbm*in2 numerically equivalent to lbf*in*s2 (i.e., 130lbm*in2 = 130 lbf*in*s2)?
 
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1 lbf = 32.174 lbm*ft/s^2, which is really just F=mg.
 

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