In [[analytical mechanics]], the '''mass matrix''' is a [[symmetric matrix|symmetric]] [[matrix (mathematics)|matrix]]

M

that expresses the connection between the time derivative

\dot q

of the [[generalized coordinates|generalized coordinate vector]]

q

of a system and the [[kinetic energy]]

T

of that system, by the equation :

T = \frac{1}{2} \dot q^\top M \dot q

where "

q^\top

" denotes the [[matrix transpose|transpose]] of the vector

q

. This equation is analogous to the formula for the kinetic energy of a particle with mass

m

and velocity

v

, namely :

T \;=\; \frac{1}{2} m|v|^2 \;=\; \frac{1}{2} v\cdot m v

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. [[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 === 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. :

q=[x_1\, x_2]^\top

. Supposing the particles have masses

m_1,m_2

, the kinetic energy of the system is :

T = \sum_{i=1}^{2} \frac{1}{2} m_i \dot x_i{}^2

This formula can also be written as :

T=\frac{1}{2} \dot q^\top M \dot q

where :

M=\begin{bmatrix}m_1&0\\0 & m_2\end{bmatrix}

===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_i'' free Cartesian coordinates (where ''n_i'' 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: :

M = \mathrm{diag}[ m_1 I_{n_1}, m_2 I_{n_2}, \cdots, m_N I_{n_N} ]

where '''I'''_{''n i''} is the ''n_i'' × ''n_i'' [[identity matrix]], or more fully:

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}

=== Rotating dumbbell === For a less trivial example, consider two point-like objects with masses

m_1,m_2

, attached to the ends of a rigid massless bar with length

2R

, 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

q=[ x, y, \alpha]

where

x, y

are the Cartesian coordinates of the bar's midpoint and

\alpha

is the angle of the bar from some arbitrary reference direction. The positions and velocities of the two particles are :

\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}

and their total kinetic energy is :

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

where math>m = m_1 + m_2 and

d = m_1 - m_2

. This formula can be written in matrix form as :

T=\frac{1}{2} \dot q^\top M \dot q

where :

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}

Note that the matrix depends on the current angle

\alpha

of the bar. ==Applications== ===Rigid-body dynamics=== For applications in which mass is distributed such as [[rigid-body dynamics]], there may be off-diagonal terms. For example, in one dimension if two particles with mass are connected by an ideal spring with a uniformly distributed mass, the effective acceleration of all points along the spring would correspond to differential mass elements, the acceleration of which would interpolate between the velocities of the two particles.{{Citation needed|date=July 2009}} ===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== 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 Analytical Mechanics, L.N. Hand, J.D. Finch, Cambridge University Press, 2008, ISBN 978 0 521 57572 0 [[Category:Computational science]] [[zh:质量矩阵]]