Direct answer: Resonance is useful when a large, controlled oscillation is desired in musical instruments and radio tuning. It can be destructive when large amplitudes overstress bridges, buildings, or machinery.
SL and HL: C.4 Standing waves and resonance
The Reasoning and Mechanism
Resonance occurs when the frequency of a periodic driving force equals or is close to a system's natural frequency. Energy is then transferred efficiently from the driver to the oscillating system, producing a large amplitude.
The amplitude does not increase indefinitely. Damping removes energy and limits the maximum amplitude. Light damping produces a larger, sharper resonance peak, while heavy damping reduces and broadens the response.
| Application or situation | Why resonance is useful or destructive |
|---|---|
| Musical instruments | A string or air column drives the instrument body at a natural frequency, producing a louder sound. Standing waves determine the allowed harmonics. |
| A child on a swing | Regular pushes at the swing's natural frequency transfer energy efficiently, increasing amplitude. |
| Radio tuning circuits | The circuit resonates at the desired station's frequency, strengthening that signal. |
| Buildings during earthquakes | Ground motion near a building's natural frequency can produce large oscillations and structural damage. Dampers reduce the response. |
| Bridges and footbridges | Wind, traffic, or synchronized pedestrians can excite structural oscillations. Insufficient damping may allow dangerous amplitudes. |
| Rotating machinery | An unbalanced motor can drive components near their natural frequency, causing vibration, fatigue, and eventual failure. |
A common misconception is that resonance always causes destruction. It only produces a large response when the driving frequency is suitable, and the response depends strongly on damping.
Exam Technique
For an IB Explain question, link ideas: driving frequency close to natural frequency, efficient energy transfer, and increased amplitude. For a destructive example, add that oscillations create stress or fatigue, and identify damping as a way to reduce the effect.