Correction term while switching from inertial to body fixed

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Discussion Overview

The discussion centers on the transition from an inertial frame to a body-fixed frame in the context of dynamics, specifically addressing the necessity and derivation of a correction term involving angular velocity. Participants explore the implications of this correction factor on the equations of motion and the mathematical framework for frame transformations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents an equation in an inertial frame and questions the need for a correction factor when switching to a body-fixed frame, suggesting that it may relate to measuring angular momentum from different points.
  • Another participant asks about the consequences of omitting the correction factor and suggests starting with the relevant frames to derive the necessary conversion rules.
  • A further contribution discusses the use of rotation matrices for frame conversion and expresses confusion about the additional term in the equations, linking it to the concept of angular momentum.
  • One participant provides a detailed mathematical derivation of the time derivative of a vector in a rotating frame, introducing an axial vector related to angular velocity and demonstrating how this leads to the correction term in the equations.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding regarding the correction factor, with some questioning its necessity and others providing mathematical justifications. No consensus is reached on the implications of omitting the correction factor or the interpretation of the additional term.

Contextual Notes

Participants rely on specific mathematical properties of rotation matrices and the relationship between different frames, which may not be fully explored or agreed upon. The discussion includes assumptions about the nature of angular momentum and frame transformations that remain unresolved.

harmyder
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Suppose we have an equation in inertial frame A.
\begin{equation}\frac{{}^Ad\bf{H}_C}{dt} = \bf{M}_C\end{equation}
Now we want to switch to body fixed frame B. For this need to employ correction factor {}^A\bf\omega^B\times\bf{H}_B. Why do we have this correction factor? How to derive this correction factor?
\begin{equation}\frac{{}^Bd\bf{H}_C}{dt} + {}^A\bf{\omega}^B\times\bf{H}_C = \bf{M}_C\end{equation}
 
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Why do we have this correction factor?
What happens if you don't use a correction factor?
How to derive this correction factor?
... start with the frames you are interested in and write out the rules for converting between them.
 
Simon Bridge said:
What happens if you don't use a correction factor?
Simon Bridge said:
start with the frames you are interested in and write out the rules for converting between them.
The rule to convert between frames is multiply by rotation matrix. But here we add some term, which is strange for me. I can very well understand this in case of measuring angular momentum about different points... Oooo, maybe here they think of different frames also as diferent points to measure angular momentum?
 
Let ##\Sigma## be an inertial frame and ##\Sigma'## one, which rotates arbitrarily against ##\Sigma##. Then there's some rotation matrix ##\hat{D}(t) \in \mathrm{SO}(3)## for the components of vectors:
$$\vec{V}=\hat{D} \vec{V}',$$
where ##\vec{V}## are the components of an arbitrary vector wrt. the orthonormal basis at rest in ##\Sigma## and ##\vec{V}'## that wrt. to the one in ##\Sigma'##.

For the time derivative of ##\vec{V}## you get
$$\vec{A}:=\dot{\vec{V}}=\dot{\hat{D}} \vec{V}'+\hat{D} \dot{\vec{V}}'.$$
For the components of ##\vec{A}## wrt. ##\Sigma'## it follows
$$\vec{A}'=\hat{D}^{-1} \vec{A}=\hat{D}^{T} \vec{A}=\hat{D}^T \dot{\hat{D}} \vec{V}'+\dot{\vec{V}}'.$$
Now since ##\hat{D} \in \mathrm{SO}(3)## we have
$$\hat{D}^T \hat{D}=1 \; \Rightarrow \; \hat{D}^T \dot{\hat{D}}=-\dot{\hat{D}}^T \hat{D}=-(\hat{D}^T \dot{\hat{T}})^T.$$
i.e., ##\hat{D}^T \dot{\hat{D}}## is antisymmetric, and thus you can introduce an axial vector ##\vec{\omega}## such that
$$(\hat{D}^T \dot{\hat{D}})_{ij}=-\epsilon_{ijk} \omega_k.$$
So you get
$$A_i'=\dot{V}_i'-\epsilon_{ijk} \omega_k V_j = \dot{V}_i' + \epsilon_{ikj} \omega_k V_j'.$$
In vector notation that reads
$$\vec{A}''=\dot{V}'+\vec{\omega} \times \vec{V}',$$
which proves your formula.
 
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