Every object that can vibrate has a natural frequency, the rate at which it swings freely when disturbed. Resonance happens when an outside push is applied at exactly that frequency. Each push arrives at the right moment to add energy, so the swings build up to a large amplitude.
A child on a swing feels this directly: push at the wrong time and the swing stumbles, but push once each cycle, in time with the motion, and it goes higher and higher with little effort.
Resonance can be useful or harmful. It lets a guitar body amplify a string's note and lets a radio tune in to one station. But it can also make a bridge sway dangerously if marching or wind matches its natural frequency, which is why soldiers break step on a bridge.
In SPM you should link resonance to matching the driving frequency with the natural frequency, and give both useful and unwanted examples.
Common misconceptions
- A bigger push always gives a bigger vibration -> Timing matters more; a small push at the natural frequency builds a large vibration.
- Resonance only happens in musical instruments -> It occurs in any object with a natural frequency, from swings to bridges.
- Resonance is always helpful -> It can also be destructive, such as making a structure vibrate dangerously.
The physics behind it
Every object that can vibrate has a natural frequency, written f₀, which is the frequency of its free oscillation once it is set going. Frequency is measured in hertz (Hz), the number of complete oscillations each second, and it is linked to the period T by f₀ = 1/T. Resonance occurs when an external driving force is applied at this same natural frequency.
When the driving frequency matches f₀, each push arrives exactly in step with the motion and feeds energy in at the right moment, so the amplitude grows steadily to a large value. Consider a simple pendulum with a period of T = 2 s.
Its natural frequency is f₀ = 1/T = 1 ÷ 2 s = 0.5 Hz.
Driving that pendulum with gentle pushes at 0.5 Hz makes it swing higher and higher, while pushing at 0.3 Hz or 0.8 Hz does little. The strength of the push matters less than its timing; a small force delivered in rhythm can build a large oscillation, which is the essence of resonance.
See it in daily life
A front-loading washing machine often gives a clear demonstration. As the drum speeds up during the spin cycle, there is one particular speed at which the whole machine suddenly shakes and thumps hard, then calms down again as the speed increases past it.
That noisy speed is where the forced vibration from the spinning drum matches the natural frequency of the machine on its feet, and resonance builds a large, rattling amplitude.
You may notice the same thing when a heavy lorry idles outside and the louvre glass in a window or a loose cupboard door begins to buzz. The engine vibration happens to hit the natural frequency of that pane or panel, so it vibrates far more than its neighbours.
A classroom version uses two identical tuning forks. Strike one, place it near the other, and the second begins to sound on its own, because the sound waves from the first drive it at exactly its natural frequency.
Energy passes across from one fork to the other through resonance.
How this comes up in SPM
In Paper 2 this idea is examined with command words such as define, explain, describe and state. You may be asked to define resonance, to explain how it builds a large amplitude, or to describe an experiment such as Barton's pendulum, where one driver pendulum sets others swinging.
Within the same Waves chapter, resonance is paired directly with damping, since a damped system loses energy and its resonant amplitude is limited. It builds on the fundamentals of waves and oscillations, where natural frequency, amplitude and period are defined, and it explains behaviour later seen in sound.
A frequent task is to describe Barton's pendulum and to state why only the pendulum with the same length, and therefore the same natural frequency, as the driver swings with the largest amplitude. Relate the length to the period, use f₀ = 1/T to find the natural frequency, and keep frequency in hertz throughout your reasoning.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)