What is a key characteristic of forced vibration in structures?

Forced vibration in structures is characterized primarily by its origin in external forces acting upon the system. This type of vibration occurs when an external disturbance, such as a machine, seismic activity, or waves in a fluid, applies energy to a structure, resulting in induced vibrations. This is distinct from free vibrations, which emerge from a system's inherent properties when it oscillates on its own after being displaced from its equilibrium position.

The notion that forced vibrations arise from external forces emphasizes their dependency on these influences, underscoring the nature of their origin and how they affect the design and analysis of structures. Understanding this characteristic is critical for engineers when designing buildings, bridges, and other infrastructure to ensure they are resilient against vibrations that could potentially lead to failure or other structural issues.

In this context, the other options do not accurately describe forced vibrations. For instance, stating that forced vibration occurs naturally without external influence misrepresents its definition, while the implication that it only occurs in liquids is overly restrictive and incorrect, as forced vibrations can happen in solids, liquids, and gases. Additionally, the notion that forced vibrations have a constant frequency does not hold true, as the frequency can vary depending on the nature of the external forces and the system's response. This variation