Moment of Inertia and the IPS system.

In summary, the formula for finding the MOI of an object with a known weight in lbf is I = 1/2*w/g*r2. However, confusion arises when converting the weight to mass and performing the calculation. It is important to remember to divide by the acceleration due to gravity, g, and the resulting units of lbm*in2 and lbf*in*s2 are numerically equivalent.
  • #1
jasc15
162
5
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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  • #2
1 lbf = 32.174 lbm*ft/s^2, which is really just F=mg.
 

1. What is moment of inertia?

Moment of inertia is a property of an object that describes its resistance to changes in rotational motion. It is calculated based on an object's mass and the distribution of that mass around its axis of rotation.

2. How is moment of inertia different from mass?

Moment of inertia and mass are related but distinct properties of an object. While mass describes an object's resistance to linear motion, moment of inertia describes its resistance to rotational motion.

3. What is the IPS system?

The IPS (International System of Units) system is a standardized system of measurement used in science and engineering. It includes units for fundamental quantities such as length, mass, time, and electric current.

4. How is moment of inertia calculated using the IPS system?

In the IPS system, moment of inertia is typically measured in units of kilogram meters squared (kg*m^2). The formula for calculating moment of inertia varies depending on the shape and distribution of mass of the object.

5. Why is moment of inertia important in physics?

Moment of inertia is important in physics because it helps us understand how objects behave when they are rotating. It is used to analyze the motion of objects such as wheels, gears, and pendulums, and is a key concept in fields such as mechanics and engineering.

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