Moment of inertia and rotational kinetic energy

Click For Summary

Discussion Overview

The discussion revolves around the concept of moment of inertia, its derivation, and its relationship to rotational kinetic energy. Participants explore theoretical aspects and mathematical formulations related to these concepts.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant expresses confusion about the meaning and derivation of moment of inertia and its connection to rotational kinetic energy.
  • Another participant explains that moment of inertia relates torque to rotational acceleration, drawing a parallel to mass in linear motion, and provides the equation τ = Iα.
  • A participant questions the reason for the term dm r^2 in the derivation of moment of inertia.
  • A subsequent reply attempts to clarify that dm r^2 arises from considering a point mass at distance r from the axis and relates torque and angular acceleration through the equation τ = Fr.
  • The explanation continues by linking the concepts of linear force and angular motion, ultimately leading to the integral form I = ∫ dm r^2.

Areas of Agreement / Disagreement

The discussion includes multiple viewpoints and clarifications, but no consensus is reached on the understanding of moment of inertia or its derivation.

Contextual Notes

Participants express varying levels of understanding and clarity regarding the mathematical derivation and physical significance of moment of inertia, indicating potential gaps in foundational knowledge.

CrazyNeutrino
Messages
99
Reaction score
0
I can't seem to understand moment of inertia. What does it mean and how is it derived ?
How does it relate to rotational kinetic energy.
 
Physics news on Phys.org
Hi CrazyNeutrino! :smile:

From the PF Library on moment of inertia

Moment of Inertia … relates rotational force (torque) to rotational acceleration in the same way that mass relates ordinary (linear) force to ordinary acceleration.​

ie τ = Iα just like F = ma

(and KE = 1/2 Iω2 just like 1/2 mv2)

it is derived from I = ∫ dm r2
 
Why dm r^2?
 
CrazyNeutrino said:
Why dm r^2?

because for a point mass m at distance r from the axis, subjected to a force F,

the torque is τ = Fr, and the angular acceleration is α = a/r

τ = Iα means Fr = Ia/r

but F = ma (good ol' Newton's second law)

so mar = Ia/r

so I = mr2

(and moment of inertia of the whole = sum of moments of inertia of the parts, giving us ∫ dm r2)
 

Similar threads

  • · Replies 40 ·
2
Replies
40
Views
5K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 138 ·
5
Replies
138
Views
9K
  • · Replies 60 ·
3
Replies
60
Views
7K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 5 ·
Replies
5
Views
1K
  • · Replies 4 ·
Replies
4
Views
2K