Moment of Inertia and the IPS system.

  • Thread starter jasc15
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  • #1
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Main Question or Discussion Point

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)?
 

Answers and Replies

  • #2
478
32
1 lbf = 32.174 lbm*ft/s^2, which is really just F=mg.
 

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