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In [[analytical mechanics]], the '''mass matrix''' is a [[symmetric matrix|symmetric]] [[matrix (mathematics)|matrix]] ''M'' that expresses the connection between the time derivative <math>\dot q</math> of the [[generalized coordinates|generalized coordinate vector]] ''q'' of a system and the [[kinetic energy]] ''T'' of that system, by the equation
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:<math>T = \frac{1}{2} \dot q^\mathrm{T} M \dot q</math>
where <math>\dot q^\mathrm{T}</math> denotes the [[matrix transpose|transpose]] of the vector <math>\dot q</math>.<ref name=Riley/> This equation is analogous to the formula for the kinetic energy of a particle with mass <math>m</math> and velocity ''v'', namely
:<math>T \;=\; \frac{1}{2} m|v|^2 \;=\; \frac{1}{2} v\cdot m v </math>
and can be derived from it, by expressing the position of each particle of the system in terms of ''q''.


In general, the mass matrix ''M'' depends on the state ''q'', and therefore varies with time.
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[[Lagrangian mechanics]] yields an [[ordinary differential equation]] (actually, a system of coupled differential equations) that describes the evolution of a system in terms of an arbitrary vector of generalized coordinates that completely defines the position of every particle in the system.  The kinetic energy formula above is one term of that equation, that represents the total kinetic energy of all the particles.
==Examples==
===Two-body unidimensional system ===
[[File:Mass matrix masses in 1d.svg|thumb|System of masses in one spatial dimension.]]
 
For example, consider a system consisting of two point-like masses confined to a straight track. The state of that systems can be described by a vector ''q'' of two generalized coordinates, namely the positions of the two particles along the track.  
:<math>q=[x_1\, x_2]^\mathrm{T}</math>.
Supposing the particles have masses ''m''<sub>1</sub>, ''m''<sub>2</sub>, the kinetic energy of the system is
:<math>T = \sum_{i=1}^{2} \frac{1}{2} m_i \dot x_i{}^2</math>
This formula can also be written as
:<math>T=\frac{1}{2} \dot q^\mathrm{T} M \dot q</math>
where
:<math>M=\begin{bmatrix}m_1&0\\0 & m_2\end{bmatrix}</math>
 
===N-body system ===
More generally, consider a system of ''N'' particles labelled by an index ''i'' = 1, 2,...,''N'', where the position of particle number ''i'' is defined by ''n<sub>i</sub>'' free Cartesian coordinates (where ''n<sub>i</sub>'' is 1, 2, or 3). Let ''q'' be the column vector comprising all those coordinates. The mass matrix ''M'' is the [[diagonal matrix|diagonal]] [[block matrix]] where each in each block the diagonal elements are the mass of the corresponding particle:<ref name=Hand/>
 
:<math>M = \mathrm{diag}[ m_1 I_{n_1}, m_2 I_{n_2}, \cdots, m_N I_{n_N} ] </math>
 
where '''I'''<sub>''n i''</sub> is the ''n<sub>i</sub>'' × ''n<sub>i</sub>'' [[identity matrix]], or more fully:
 
<math>
M = \begin{bmatrix}
m_1 & \cdots & 0 & 0 & \cdots & 0 & \cdots & 0 & \cdots & 0 \\
\vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \ddots &\vdots & \ddots & \vdots \\
0 & \cdots & m_1 & 0 & \cdots & 0 & \cdots & 0 & \cdots & 0 \\
0 & \cdots & 0 & m_2 & \cdots & 0 & \cdots & 0 & \cdots & 0 \\
\vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \ddots &\vdots & \ddots & \vdots \\
0 & \cdots & 0 & 0 & \cdots & m_2 & \cdots & 0 & \cdots & 0 \\
\vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \ddots & \vdots & \ddots & \vdots \\
0 & \cdots & 0 & 0 & \cdots & 0 & \cdots & m_n & \cdots & 0 \\
\vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \ddots & \vdots & \ddots & \vdots \\
0 & \cdots & 0 & 0 & \cdots & 0 & \cdots & 0 & \cdots & m_n\\
\end{bmatrix}
</math>
 
=== Rotating dumbbell ===
 
[[File:Mass matrix rotating dumbbell.svg|thumb|Rotating dumbbell.]]
 
For a less trivial example, consider two point-like objects with masses ''m''<sub>1</sub>, ''m''<sub>2</sub>, attached to the ends of a rigid massless bar with length 2''R'', the assembly being free to rotate and slide over a fixed plane. The state of the system can be described by the generalized coordinate vector
:<math>q=[ x, y, \alpha]</math>  
where ''x'', ''y'' are the Cartesian coordinates of the bar's midpoint and ''α'' is the angle of the bar from some arbitrary reference direction.  The positions and velocities of the two particles are
:<math>
  \begin{array}{ll}
    p_1 = (x,y) + R(\cos\alpha, \sin\alpha) & v_1 = (\dot x,\dot y) + R\dot \alpha(-\sin\alpha, \cos\alpha) \\
    p_2 = (x,y) - R(\cos\alpha, \sin\alpha) & v_2 = (\dot x,\dot y) - R\dot \alpha(-\sin\alpha, \cos\alpha)
  \end{array}
</math>
and their total kinetic energy is  
:<math>T = m\dot x^2 + m\dot y^2 + mR^2\dot\alpha^2 + 2R d \cos\alpha \dot x \dot \alpha + 2R d  \sin\alpha \dot y \dot \alpha</math>
where <math>m  = m_1 + m_2</math> and <math>d = m_1 - m_2</math>.  This formula can be written in matrix form as
:<math>T=\frac{1}{2} \dot q^\mathrm{T} M \dot q</math>
where
:<math>M=\begin{bmatrix}m&0&R d \cos\alpha\\0 & m & R d \sin\alpha \\ R d \cos\alpha & R d \sin\alpha & R^2 m\end{bmatrix}</math>
Note that the matrix depends on the current angle ''α'' of the bar.
 
==Continuum mechanics==
 
For discrete approximations of [[continuum mechanics]] as in the [[finite element method]], there may be more than one way to construct the mass matrix, depending on desired computational and accuracy performance. For example, a lumped-mass method, in which the deformation of each element is ignored, creates a diagonal mass matrix and negates the need to integrate mass across the deformed element.
 
== See also ==
 
* [[Moment of inertia]]
* [[Stress tensor]]
* [[Stress-energy tensor]]
* [[Stiffness matrix]]
 
==References==
<references>
 
<ref name=Riley>
  Mathematical methods for physics and engineering, K.F. Riley, M.P. Hobson, S.J. Bence, Cambridge University Press, 2010, ISBN 978-0-521-86153-3
</ref>
 
<ref name=Hand>
  Analytical Mechanics, L.N. Hand, J.D. Finch, Cambridge University Press, 2008, ISBN 978 0 521 57572 0
</ref>
</references>
 
[[Category:Computational science]]

Revision as of 15:39, 25 February 2014

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