Physics 9702 · Paper 2

Describing a Wave

A few words describe any wave: displacement, amplitude, wavelength, period and frequency.
Learn each with its unit; the mark scheme is strict about them.

The five words to know

A wave frozen in space. Amplitude A is the height of a crest above the middle line; wavelength λ is the distance from one crest to the next.

Displacement x: how far a point has moved from its rest position, and in which direction (metres, m).
Amplitude A: the largest displacement, that is, the height of a crest above the middle line (m).
Wavelength λ: the distance from one point to the next identical point, e.g. crest to crest (m).
Period T: the time for one full oscillation (s).
Frequency f: the number of oscillations per second (hertz, Hz). 1 Hz is one cycle per second.

f = \dfrac{1}{T}

Symbols

$f$ = frequency (Hz)
$T$ = period, the time for one cycle (s)

Frequency and period describe the same motion: a high frequency means a short period, and a low frequency means a long period.

Two graphs that look the same but are not

A displacement-distance graph is a snapshot of the whole wave frozen in space. Read the wavelength λ off it.
A displacement-time graph follows one point as time passes. Read the period T off it.

Two graphs that look identical. The left one is a snapshot along the wave (read the wavelength λ). The right one follows a single point over time (read the period T).

Common mistake

The two graphs look identical, so students read the wrong quantity. Check the horizontal axis first: if it says distance you get λ; if it says time you get T.

Worked example

Frequency from a period

A point on a wave takes 91 ms to complete one oscillation. Find the frequency.

One oscillation is one period, so $T = 91\text{ ms} = 0.091\text{ s}$ .
$f = \dfrac{1}{T} = \dfrac{1}{0.091} = 11\text{ Hz}$ .

Answer

f = 11\text{ Hz}

(9702/11/O/N/24 Q24)

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