Moment of Inertia and the IPS system.

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SUMMARY

The discussion focuses on calculating the Moment of Inertia (MOI) for a flat disk using the formula I = 1/2 * w/g * r², where w is the weight in pounds-force (lbf) and r is the radius in inches. The participant confirms that a weight of 2.6 lbf corresponds to a mass of 2.6 lbm on Earth, leading to a calculated MOI of 130 lbm*in². The confusion arises regarding the division by g and the equivalence of units lbm*in² and lbf*in*s², with clarification that 1 lbf equals 32.174 lbm*ft/s², reinforcing the relationship between force and mass.

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  • Understanding of Moment of Inertia (MOI) calculations
  • Familiarity with units of force (lbf) and mass (lbm)
  • Basic knowledge of physics principles, specifically F=mg
  • Concept of gravitational acceleration (g) and its role in weight and mass conversion
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  • Research the derivation of the Moment of Inertia formula for various shapes
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  • Learn about the implications of using different units in engineering calculations
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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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