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3.1.2Waves

Describing Wave Motion

5 min read 5 parts Lesson 3 of 20 1 quiz

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Section 1 of 5Key Quantities

3.1.2 Describing Wave Motion

Each property of a wave is measured in a standard unit, and these definitions appear in both diagrams and calculations.

Amplitude
  • The distance from the undisturbed (rest) position to a peak or trough of the wave, measured in metres (m).
Wavelength
  • The distance from one point on a wave to the same point on the next wave, measured in metres and written as λ.
Frequency
  • The number of complete waves passing a point each second, measured in hertz (Hz) and written as f.
Time period
  • The time taken for one complete wave to pass a point, measured in seconds (s) and written as T.

Section 2 of 5Wavefronts

Wavefronts
Wavefront
  • A line representing one wave as seen from above, drawn perpendicular to the direction in which the wave travels.
Ray
  • An arrow drawn across the wavefronts to show the direction in which the wave is moving.
Close wavefronts
  • Wavefronts that are close together represent a wave with a short wavelength.
Distant wavefronts
  • Wavefronts that are far apart represent a wave with a long wavelength.

Section 3 of 5Reading a Wave Graph

1
On a displacement-distance graph, measure the amplitude from the rest line up to the top of a crest.
2
Measure the wavelength from one crest to the next crest, or between any two matching points on successive waves.
3
On a displacement-time graph, measure the time period from one point on the wave to the same point on the next wave.
4
Use the time period to find the frequency, because frequency is the number of waves that pass each second.

Section 4 of 5Worked Example

Question

  • A water wave makes 4 complete waves pass a point in 2 seconds.
  • Calculate its frequency and its time period.

  1. Frequency is the number of waves per second: f = 4 waves / 2 s = 2 Hz.
  2. Time period is the reciprocal of frequency: T = 1 / f = 1 / 2 Hz.
  3. CalculateT = 0.5 s.

Answer The frequency is 2 Hz and the time period is 0.5 s.

Section 5 of 5Exam Checklist & Tips

3.1.2 – Key Points for the Exam

  • Amplitude The maximum displacement of a wave from its rest position.
  • Wavelength (λ) The distance between two matching points on adjacent waves.
  • Frequency (f) The number of complete waves passing a point per second.
  • Time period (T) The time for one complete wave to pass a point.
  • Wavefront A line drawn to represent the crests of a wave as seen from above.

Common Mistakes

  • Measuring amplitude from a crest to a trough;
    • amplitude is measured from the rest position to a crest or trough.
  • Measuring wavelength from a crest to a trough;
    • it should be measured between matching points on successive waves.
  • Confusing frequency with time period; frequency is waves per second, while period is seconds per wave.
  • Giving a wavelength without its unit, which must be metres.
Physics 0625 & 9702 SUVAT Mastery Graph Interpretation

Constant Acceleration Equations (SUVAT) & Slopes

v = u + at  |  s = ut + ½at2  |  v2 = u2 + 2as  |  s = ½(u + v)t

  • Displacement-Time Graph: Gradient = Velocity (curve = accelerating).
  • Velocity-Time Graph: Gradient = Acceleration; Area under curve = Distance travelled / Displacement.
  • Free Fall in vacuum: a = g ≈ 9.81 m/s2 downwards (air resistance negligible).