Revision summary

Wave motion

A travelling disturbance transfers energy without matter travelling with it. Compare material vibration with propagation: perpendicular for transverse, parallel for longitudinal.

Frequency and period

f = 1/T. Frequency is complete cycles per second, in Hz. Period is time per complete cycle, in s. A complete cycle contains two equilibrium crossings, one in each direction.

Wave speed

v = fλ = λ/T. Use f in Hz and λ in m for v in m/s. Wave speed is the pattern's propagation speed.

Echo distance

d = vt/2 when the source and receiver are together and the outward and return paths match. Subtract emission time from reception time before converting units and calculating.

Choose the graph for the quantity

Keep the physical explanation

• A marked part of a rope or spring oscillates locally; a crest or compression travels.
• Sound comes from a vibrating source and needs a material medium. Compressions and rarefactions are regions of higher and lower local pressure/density, not air particles travelling from source to listener.
• A wavelength is one full spatial cycle; compression to neighbouring rarefaction is half of one. Amplitude is half the crest-to-trough displacement.
• Six consecutive crests span five wavelengths. Timing several cycles is different from timing a pulse's travel between two positions.
• A sound echo follows an outward and return path. Select the intended return, the correct time interval and the stated sound speed.

Units and measurement reminders

1 cm = 0.01 m; 1 ms = 0.001 s; 1 kHz = 1000 Hz. Match the units before substitution. Use a calibrated scale, count full wavelength intervals and complete cycles, keep propagation conditions steady, and choose timing resolution that can resolve the event. Repetition does not correct an incorrect scale or the wrong echo.