PHYSICS

Pluck a guitar string, tap a bell, or feel a car engine hum and you are watching vibration in action. A vibration is a repeating back-and-forth motion around a resting position. Every object that can bend, stretch, or swing has a natural frequency: the rate it prefers to oscillate when bumped and left alone. A stiff steel beam vibrates fast. A long suspension bridge sways slowly. The size of each swing is called the amplitude. Engineers must understand both frequency and amplitude because uncontrolled vibrations can crack metal, shake buildings, and make passengers feel sick.