D5. Inertia tensor

[math]\displaystyle{ \newcommand{\uvec}{\overline{\textbf{u}}} \newcommand{\vvec}{\overline{\textbf{v}}} \newcommand{\evec}{\overline{\textbf{e}}} \newcommand{\Omegavec}{\overline{\mathbf{\Omega}}} \newcommand{\velang}[2]{\Omegavec^{\textrm{#1}}_{\textrm{#2}}} \newcommand{\Alfavec}{\overline{\mathbf{\alpha}}} \newcommand{\accang}[2]{\Alfavec^{\textrm{#1}}_{\textrm{#2}}} \newcommand{\ds}{\textrm{d}} \newcommand{\ps}{\textrm{p}} \newcommand{\hs}{\textrm{h}} \newcommand{\Ns}{\textrm{N}} \newcommand{\Fs}{\textrm{F}} \newcommand{\ms}{\textrm{m}} \newcommand{\ts}{\textrm{t}} \newcommand{\us}{\textrm{u}} \newcommand{\vs}{\textrm{v}} \newcommand{\Rs}{\textrm{R}} \newcommand{\Ts}{\textrm{T}} \newcommand{\Ls}{\textrm{L}} \newcommand{\Bs}{\textrm{B}} \newcommand{\es}{\textrm{e}} \newcommand{\fs}{\textrm{f}} \newcommand{\is}{\textrm{i}} \newcommand{\Is}{\textrm{I}} \newcommand{\ks}{\textrm{k}} \newcommand{\js}{\textrm{j}} \newcommand{\rs}{\textrm{r}} \newcommand{\ss}{\textrm{s}} \newcommand{\Os}{\textbf{O}} \newcommand{\Gs}{\textbf{G}} \newcommand{\Cbf}{\textbf{C}} \newcommand{\Or}{\Os_\Rs} \newcommand{\Qs}{\textbf{Q}} \newcommand{\Cs}{\textbf{C}} \newcommand{\Ps}{\textbf{P}} \newcommand{\Ss}{\textbf{S}} \newcommand{\P}{\textrm{P}} \newcommand{\Q}{\textrm{Q}} \newcommand{\deg}{^\textsf{o}} \newcommand{\xs}{\textsf{x}} \newcommand{\ys}{\textsf{y}} \newcommand{\zs}{\textsf{z}} \newcommand{\dert}[2]{\left.\frac{\ds{#1}}{\ds\ts}\right]_{\textrm{#2}}} \newcommand{\ddert}[2]{\left.\frac{\ds^2{#1}}{\ds\ts^2}\right]_{\textrm{#2}}} \newcommand{\vec}[1]{\overline{#1}} \newcommand{\vecbf}[1]{\overline{\textbf{#1}}} \newcommand{\vecdot}[1]{\overline{\dot{#1}}} \newcommand{\OQvec}{\vec{\Os\Qs}} \newcommand{\QPvec}{\vec{\Qs\Ps}} \newcommand{\OPvec}{\vec{\Os\textbf{P}}} \newcommand{\OCvec}{\vec{\Os\Cs}} \newcommand{\OGvec}{\vec{\Os\Gs}} \newcommand{\abs}[1]{\left|{#1}\right|} \newcommand{\braq}[2]{\left\{{#1}\right\}_{\textrm{#2}}} \newcommand{\matriz}[9]{ \begin{bmatrix} {#1} & {#2} & {#3}\\ {#4} & {#5} & {#6}\\ {#7} & {#8} & {#9} \end{bmatrix}} \newcommand{\vector}[3]{ \begin{Bmatrix} {#1}\\ {#2}\\ {#3} \end{Bmatrix}} \newcommand{\vecdosd}[2]{ \begin{Bmatrix} {#1}\\ {#2} \end{Bmatrix}} \newcommand{\vel}[2]{\vvec_{\textrm{#2}} (\textbf{#1})} \newcommand{\acc}[2]{\vecbf{a}_{\textrm{#2}} (\textbf{#1})} \newcommand{\accs}[2]{\vecbf{a}_{\textrm{#2}}^{\textrm{s}} (\textbf{#1})} \newcommand{\accn}[2]{\vecbf{a}_{\textrm{#2}}^{\textrm{n}} (\textbf{#1})} \newcommand{\velo}[1]{\vvec_{\textrm{#1}}} \newcommand{\accso}[1]{\vecbf{a}_{\textrm{#1}}^{\textrm{s}}} \newcommand{\accno}[1]{\vecbf{a}_{\textrm{#1}}^{\textrm{n}}} \newcommand{\re}[2]{\Re_{\textrm{#2}}(\textbf{#1})} \newcommand{\psio}{\dot{\psi}_0} \newcommand{\Pll}{\textbf{P}_\textrm{lliure}} \newcommand{\agal}[1]{\vecbf{a}_{\textrm{Gal}} (#1)} \newcommand{\angal}[1]{\vecbf{a}_{\textrm{NGal}} (#1)} \newcommand{\vgal}[1]{\vecbf{v}_{\textrm{Gal}} (#1)} \newcommand{\fvec}[2]{\overline{\mathbf{F}}_{\textrm{#1}\rightarrow\textrm{#2}}} \newcommand{\mvec}[2]{\overline{\mathbf{M}}_{\textrm{#1}\rightarrow\textrm{#2}}} }[/math]

The Vector Theorems relate the external interaction torsor on a system ([math]\displaystyle{ \sum\overline{\mathbf{F}}_\mathrm{ext} }[/math], [math]\displaystyle{ \sum\overline{\mathbf{M}}_\mathrm{ext}(\Qs) }[/math]) to the change in time of vectors that depend on how the mass is distributed in the system (mass geometry) and on its motion. In the LMT, this vector is the linear momentum of the system, while in the AMT it its angular momentum (or kinetic momentum). This unit provides the tools necessary to describe the mass geometry of a rigid body and to calculate these two vectors.

---

---

D5.1 Centre of masses

The centre of mass of a mechanical system is a point whose kinematics is a weighted kinematics of all the elements of the system that have mass. In this course it is represented by the letter G.

Figure D5.1 Centre of mass of a system with constant mass

In a homogeneous rigid body S, the location of G is easy when the rigid body has important symmetries (Figure D5.2).

Figure D5.2 Centre of mass of rigid bodies with important symmetries

When this is not the case, integration must be carried out to obtain it. If M is the total mass of the rigid body: [math]\displaystyle{ \overline{\Os_\Rs\Gs}=\frac{1}{\mathrm{M}} \int_\mathrm{S}\mathrm{dm}(\Ps)\overline{\Os_\Rs\Ps} }[/math]

The Table shows the centre of mass of the most common geometries. From this information and for rigid bodies S formed by several of these elements [math]\displaystyle{ \mathrm{S}_\is }[/math], the position of the centre of mass can be found as a weighted average of the position of each [math]\displaystyle{ \mathrm{G}_\is }[/math].

✏️ Example D5.1: shell

The rigid body S is made up of a cylindrical shell and a spherical semi-shell, both homogeneous and with the same surface density [math]\displaystyle{ \sigma }[/math]. For symmetry reasons, the [math]\displaystyle{ (\xs,\ys) }[/math] coordinates of the centre of mass G are zero: [math]\displaystyle{ (\xs_\mathrm{G},\ys_\mathrm{G})=(0,0) }[/math]. The z coordinate of the cylindrical shell is [math]\displaystyle{ \zs_\mathrm{Gcil}=\Rs/2 }[/math]. That of the spherical semi-shell can be found from the Table:

[math]\displaystyle{ \zs_\mathrm{Gesf}=\Rs+(\Rs/2)=3\Rs/2 }[/math].

The mass of each element is the product of the surface density by the surface area of the element:

[math]\displaystyle{ \ms_\mathrm{cil}=\sigma 2 \pi \Rs^2 }[/math] , [math]\displaystyle{ \ms_\mathrm{esf}=\sigma \frac{1}{2} 4 \pi \Rs^2= \sigma 2 \pi \Rs^2 }[/math]
.

Hence: [math]\displaystyle{ \zs_\mathrm{G}=\frac{\ms_\mathrm{esf}\zs_\mathrm{Gesf}+\ms_\mathrm{cil}\zs_\mathrm{Gcil}}{\ms_\mathrm{esf}+\ms_\mathrm{cil}}=\frac{2\pi\Rs^2(3/2)\Rs +2\pi\Rs^2(1/2)\Rs}{2\pi\Rs^2+2\pi\Rs^2}=\Rs }[/math]

✏️ Example D5.2: folded plate

The rigid body S is a folded homogeneous triangular plate with a surface density of [math]\displaystyle{ \sigma }[/math]. The centre of mass can be found as the weighted average of the centre of mass of a square plate with a side length of 6L and two triangular plates with legs 6L:

[math]\displaystyle{ (\xs_1,\ys_1)=(3\Ls,3\Ls) \hspace{3cm} (\xs_2,\ys_2)=(8\Ls,2\Ls) \hspace{3cm} (\xs_3,\ys_3)=(2\Ls,4\Ls) }[/math]

[math]\displaystyle{ \hspace{1cm} \ms_1=(6\Ls)(6\Ls)\sigma=36\Ls^2\sigma \hspace{1.5cm} \ms_2=(1/2)(6\Ls)(6\Ls)\sigma=18\Ls^2\sigma \hspace{1cm} \ms_3=(1/2)(6\Ls)(6\Ls)\sigma=18 \Ls ^2 \sigma }[/math]

Therefore: [math]\displaystyle{ (\xs_\mathrm{G},\ys_\mathrm{G})=\frac{36\sigma(3,3)\Ls^3+ 18\sigma (8,2)\Ls^3 + 18\sigma (2,4)\Ls^3}{36\Ls^2\sigma+18\Ls^2\sigma+ 18 \Ls^2 \sigma}=(4,3)\Ls. }[/math]

✏️ Example D5.3: cylinder with a hole

---

---

---

---

---

---

---

---

D5.5 Change of vector basis

© Universitat Politècnica de Catalunya. All rights reserved

---

<<< D4. Vectorial theorems

D6. Examples of 2D dynamics >>>