Final answer:
A wave sustained by an external periodic force to maintain its oscillation despite energy loss due to damping forces is called a forced wave. The external force ensures the system's continuous oscillation by compensating for energy dissipated, typically as thermal energy or due to friction.
Step-by-step explanation:
A forced wave produced by the Transverse Periodic Force (TPF) is called so because the system is subjected to an external periodic force that maintains the oscillation despite damping forces. This scenario is akin to pushing a swing (Figure 15.24) where the act of pumping is required to counteract the dampening forces such as air resistance and friction that otherwise remove energy from the swing, eventually bringing it to a stop. The external force (TPF) compensates for the energy lost due to damping forces, thus forcing the system to continue oscillating at a certain amplitude.
Considering the example in Figure 15.25, if a mass attached to a spring oscillates in a fluid with viscosity, the amplitude of the oscillation would naturally decrease over time because the non-conservative damping force like viscosity dissipates energy, primarily as thermal energy. To maintain the oscillation with a specific amplitude, a periodic external force must be applied to the system to continually supply it with energy to counteract the energy losses. Thus, the wave maintained by this external force is appropriately termed a forced wave. A real-world example of this process includes the creation of interference patterns in a ripple tank as shown in Figure 3.2, where an external force such as the vibration of thin plungers produces and maintains circular water waves.