About torque and moment of inertia

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Discussion Overview

The discussion revolves around the concepts of torque and moment of inertia, particularly in the context of the human body and its limbs. Participants explore the relationships between these two concepts, their definitions, and how they apply to physical scenarios involving rotation and acceleration.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that torque is defined as force times the length of the lever arm, while moment of inertia is mass times the square of the distance from the axis of rotation, suggesting that moment of inertia may be larger than torque in certain contexts.
  • Another participant argues that comparing moment of inertia to torque in terms of size is inappropriate, as they are different dimensions, similar to comparing area to volume.
  • A participant provides an example involving two rings of different radii to illustrate how torque and moment of inertia interact, noting that a larger radius results in greater torque but a smaller acceleration.
  • One participant emphasizes the relationship between torque and moment of inertia through the equation \(\vec{\tau}=I\vec{\alpha}\), likening it to Newton's second law.
  • Another participant expresses confusion about the implications of longer lever arms and moment of inertia, questioning whether a longer lever arm increases torque while also requiring more torque due to increased moment of inertia.
  • A participant seeks clarification on the implications of longer arms in terms of torque and moment of inertia, suggesting that it may require more torque to achieve the same rotational acceleration with a longer radius.
  • Further clarification is requested regarding the mechanics of lifting an arm in different positions and how this affects torque and moment of inertia.

Areas of Agreement / Disagreement

Participants generally agree that torque and moment of inertia are distinct concepts that relate to each other, but there is disagreement on how to compare them and the implications of their relationship in practical scenarios. The discussion remains unresolved regarding the specific effects of arm length and position on torque and moment of inertia.

Contextual Notes

The discussion includes various assumptions about the definitions and relationships between torque and moment of inertia, as well as the specific conditions under which these concepts are applied, particularly in the context of human anatomy and movement.

  • #31
ggl205 said:
Historically, sprinters favor shorter crank arms while pursuit and time trial racers like longer crank arms. The physics apply but one variable I find missing or should I say underestimated, is how the longer crank arm affect human energy to move that longer arm through a single spindle rotation. Leaving acceleration aside, increased torque of a longer crank arm will require more energy per revolution than a smaller one. So, if cadence remains the same for longer and shorter arms, energy required to move the longer arm should be greater. If this is true, the commonly held notion that longer crank arms are better for time trial racers may not hold water.
No, the power required to move the bike is the same either way. A longer crank arm means a smaller force to achieve the same torque. A longer crank arm would allow you to provide more torque if needed for acceleration.
 

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