Coupled small-oscillation normal modes
Near a stationary equilibrium, a conservative system with quadratic kinetic matrix M and potential Hessian K has Mq̈+Kq=0.
Key knowledge
Near a stationary equilibrium, a conservative system with quadratic kinetic matrix M and potential Hessian K has Mq̈+Kq=0. Normal modes solve K a=ω² M a; positive mode eigenvalues give oscillations, negative ones instability, while zero modes require further treatment.
Values and equations
Mq̈+Kq=0; Ka=ω²Ma
- M
- Mass matrix for translational coordinates kg
- K
- Potential Hessian/stiffness N m⁻¹
- q,a
- Displacement and mode-amplitude vector m
- ω
- Mode angular frequency rad s⁻¹
Conditions and scope
- Real symmetric K and positive-definite M, small displacements, no velocity-dependent/gyroscopic terms or damping in this model.
- Generalized coordinate units can differ; the displayed SI mass/stiffness units assume all q components are translational lengths. Linear stability alone does not decide nonlinear zero-mode stability.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Richard Fitzpatrick / University of Texas at Austin2 sources
Evidence and review
Newtonian Dynamics: Stability equations: Equations 773–788: equilibrium expansion and mass-weighted eigenproblem; K=−G changes the source's sign convention. Newtonian Dynamics: Normal modes: Equations 800–803: mode superposition and oscillatory/growing solutions; zero-eigenvalue case qualified separately.
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Entry details
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