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In the eigenvalue problem \( (K - \omega^2 M)\phi = 0 \), the non-trivial solutions require that

  • A.the stiffness matrix be equal to the mass matrix multiplied by a scalar
  • B.the determinant of \( K - \omega^2 M \) vanish
  • C.the mass matrix be singular for the system under consideration
  • D.the mode shape vector be identically zero at every degree of freedom

Answer: B

Correct answer: the determinant of \( K - \omega^2 M \) vanish

The equation is homogeneous, so a non-zero \( \phi \) exists only if the matrix is singular:

\[ \det(K - \omega^2 M) = 0 \]

Expanding the determinant gives the characteristic polynomial in \( \omega^2 \), whose \( n \) roots are the natural frequencies. Substituting each root back gives the corresponding mode shape, determined only up to a scale factor - which is why mode shapes are normalised rather than absolute.

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