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The central difference method for solving the equation of motion is

  • A.conditionally stable, with a limit on the time step
  • B.unconditionally stable for any time step whatsoever
  • C.exact for all forcing functions and all step sizes
  • D.applicable only to undamped single-degree systems

Answer: A

Correct answer: conditionally stable, with a limit on the time step

The central difference method is explicit, and explicit schemes are conditionally stable: the step must satisfy \( \Delta t \le T_{min}/\pi \), where \( T_{min} \) is the shortest period in the model.

The limit is set by the highest mode, which for a fine mesh may be of no interest whatever - so a model refined for accuracy forces very small steps. That is the trade against implicit methods such as Newmark's, which are unconditionally stable but require an equation solution at every step.

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