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Angular Velocity Vector

When working with rotations, it is convenient to define the angular-velocity vector as a vector $ \underline{\omega}\in{\bf R}^3$ pointing along the axis of rotation. There are two directions we could choose from, so we pick the one corresponding to the right-hand rule, i.e., when the fingers of the right hand curl in the direction of the rotation, the thumb points in the direction of the angular velocity vector.B.18 The length $ \vert\vert\,\underline{\omega}\,\vert\vert $ should obviously equal the angular velocity $ \omega $. It is convenient also to work with a unit-length variant $ \underline{\tilde{\omega}}\isdeftext \underline{\omega}/ \vert\vert\,\underline{\omega}\,\vert\vert $.

As introduced in Eq.$ \,$(B.8) above, the mass moment of inertia is given by $ I=mR^2$ where $ R$ is the distance from the (instantaneous) axis of rotation to the mass $ m$ located at $ \underline{x}\in{\bf R}^3$ . In terms of the angular-velocity vector $ \underline{\omega}$, we can write this as (see Fig.B.6)

$\displaystyle I$ $\displaystyle =$ $\displaystyle mR^2
\eqsp m\cdot \left\Vert\,\underline{x}-{\cal P}_{\underline{\omega}}(\underline{x})\,\right\Vert^2$  
  $\displaystyle =$ $\displaystyle m\cdot \left\Vert\,\underline{x}-(\underline{\tilde{\omega}}^T\underline{x})\underline{\tilde{\omega}}\,\right\Vert^2
\protect$ (B.14)


$\displaystyle {\cal P}_{\underline{\omega}}(\underline{x}) \isdefs \frac{\under...
...ga}\eqsp (\underline{\tilde{\omega}}^T\underline{x})\underline{\tilde{\omega}}

denotes the orthogonal projection of $ \underline{x}$ onto $ \underline{\omega}$ (or $ \underline{\tilde{\omega}}$) [451]. Thus, we can project the mass position $ \underline{x}$ onto the angular-velocity vector $ \underline{\omega}$ and subtract to get the component of $ \underline{x}$ that is orthogonal to $ \underline{\omega}$, and the length of that difference vector is the distance to the rotation axis $ R$, as shown in Fig.B.6.

Figure: Mass position vector $ \underline{x}$ and its orthogonal projection $ {\cal P}_{\protect\underline{\omega}}(\underline{x})$ onto the angular velocity vector $ \underline{\omega}$ for purposes of finding the distance $ R$ of the mass $ m$ from the axis of rotation $ \underline{\tilde{\omega}}$.

Using the vector cross product (defined in the next section), we will show (in §B.4.17) that $ R$ can be written more succinctly as

$\displaystyle R \eqsp \left\Vert\,\underline{x}-{\cal P}_{\underline{\omega}}(\...
...\eqsp \left\Vert\,\underline{\tilde{\omega}}\times \underline{x}\,\right\Vert.

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Vector Cross Product
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Two Masses Connected by a Rod