Moment of inertia changes during rotation -- Calculate work?

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Discussion Overview

The discussion revolves around the concept of moment of inertia and its changes during rotation, specifically focusing on calculating the work associated with changes in kinetic energy. The inquiry involves theoretical aspects of angular momentum and kinetic energy in a rotating system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • A participant presents a scenario involving a point mass rotating along an axis and discusses how moving the mass affects angular momentum and kinetic energy.
  • The participant proposes that the angular momentum must be conserved, leading to a relationship between initial and final angular velocities.
  • There is a calculation of the change in kinetic energy, suggesting that work done by internal forces results in a positive change in kinetic energy.
  • The participant questions what specific force performs the work and during which displacement this occurs.
  • There is a request for methods to calculate work without relying on the work-energy theorem.

Areas of Agreement / Disagreement

The discussion does not reach a consensus, as it includes a mix of exploratory questions and clarifications without definitive answers or resolutions to the posed problems.

Contextual Notes

Participants do not provide specific definitions or assumptions regarding the forces involved or the nature of the work done, leaving these aspects unresolved.

Gian_ni
Hi everyone, i have a question

Moment of inertia changes during rotation. Calculate the work that changes kinetic energy?
Angular moment (along the axis of rotation) L = I * w
A point mass M rotates along an axis attached to a mass-negligible rod, of length r.
If someone moves the mass M at distance r / 2, the angular moment must conserve ( so
L1 = I2 w2 -> w2 = 4w1) , but kinetic energy is changed: ΔK = 0.5M (w2 ^ 2 * (r / 2) - w1 ^ 2 * r) = 0.5M * w1 ^ 2 * 7r
Since the work performed by the internal force (?) has increased, ΔK = W is positive.
- But what force in this case did the work and during which displacement?
- Is there a way to calculate the Work W without the work-energy theorem? Calculations?

Thank you
 
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Please submit HW questions in the Homework and Coursework Questions forum and read the guidelines before submitting.
 
jambaugh said:
Please submit HW questions in the Homework and Coursework Questions forum and read the guidelines before submitting.
Thank you for reply. I''ll post, though it's not an homework but a ''problem'' made by myself to understand.
 

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