What is inherent natural frequency in structural dynamics?

• A. A frequency with which a structure can resonate; if it is in a range that can be created by naturally occurring forces, the structure is at risk.
• B. An object that dissipates energy in a spring-mass system.
• C. A device that moves liquid from low to high pressure.
• D. The torque produced by a rotating shaft.

Answer: (A)

Natural frequency is the frequency at which a structure tends to vibrate on its own when it is disturbed. It’s a property set by the structure’s mass distribution and stiffness (and it’s affected by damping, though the basic idea is about how the system would respond if nudged). If an external force contains energy at this frequency, the structure can experience resonance, where vibrations grow large unless damping or structural changes limit them. The described option captures this idea by defining the natural frequency as a resonant frequency of the structure and noting the risk when excitations fall into that range. The other options describe energy dissipation, a pump, or torque, none of which define the natural vibration characteristic of a structure.