The discussion revolves around the different units for moment of inertia (MI), specifically addressing the confusion between the area moment of inertia and the mass moment of inertia. Participants explore the implications of these different definitions and their respective units in the context of physics and engineering applications.
Participants express differing views on the definitions and applications of moment of inertia, indicating that multiple competing perspectives remain without a clear consensus.
Some participants highlight the importance of distinguishing between area moment of inertia and mass moment of inertia, noting that the assumptions regarding surface density and the context of application may affect the interpretation of units.
lingesh said:Hi,
I am a bit confused about the units of MI...I read that MI of rectangle is b*d^3/12,so unit is mm^4..Also i read in wiki that unit of MI is kg.mm^2...
Both are correct,but why this difference is?
Thanks...SteamKing said:You are getting two different moments of inertia confused.
The area moment of inertia, sometimes called the second moment of area, has units of L4. The second moment of area of a rectangle is bd3/12, when referenced to the centroid of the rectangle. The area moment of inertia is chiefly used to determine things like bending stresses in beams.
https://en.wikipedia.org/wiki/Second_moment_of_area
The mass moment of inertia, sometimes abbreviated MOI or MMOI, is a slightly different concept. The MMOI has units of M L2.
The MMOI relates the torque applied to a body to its angular acceleration, T = I α, and is analogous to F = ma for rectilinear motion.
https://en.wikipedia.org/wiki/Moment_of_inertia