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- In modal analysis, why is there no force excitation given as input? If there is no input how can the software or method predict a vibration mode?

My understanding in modal analysis is very limited. All I know is it helps to find a specific mode of vibration and the natural frequency corresponding to it.

While I was discussing about this with my NVH team colleague, he told me that there is no force input or excitation input given to a modal analysis. I also saw a modal analysis report and it gave a lot of different modes with corresponding natural frequency, without indication any direction or magnitude of load input.

These are my questions:

Assuming modal analysis of a simple shaft is to be carried out,

While I was discussing about this with my NVH team colleague, he told me that there is no force input or excitation input given to a modal analysis. I also saw a modal analysis report and it gave a lot of different modes with corresponding natural frequency, without indication any direction or magnitude of load input.

These are my questions:

Assuming modal analysis of a simple shaft is to be carried out,

- What are the typical boundary conditions given (For example, radially two ends fixed, rotary motion constrained etc.)?
- If there is no force input given, why should a shaft vibrate and how is it captured as a mode?