AQA A-Level Physics Paper 1, November 2020: Question 15

1 mark · Easy difficulty · Multiple Choice

Identify the correct physical descriptions for displacement variables w and z shown on a progressive wave graph representing displacement against distance.

Practise this question

Question

A displacement-distance graph of a progressive water wave showing displacement on the y-axis and distance on the x-axis. The arrow labeled 'w' indicates the vertical height from the central equilibrium line to a crest, and the double-headed arrow labeled 'z' indicates the horizontal distance spanning exactly one full wavelength from one zero-crossing to the next corresponding zero-crossing after one complete cycle. Below the graph, a multiple-choice table gives four options A to D, asking which row correctly identifies w as amplitude or half-amplitude, and z as wavelength or period.
Question text

15 A wave travels across the surface of water.

The diagram shows how the displacement of water particles at the surface varies with

distance.

Which row correctly describes both w and z?

[1 mark]

w z

A amplitude wavelength

B half-amplitude period

C half-amplitude wavelength

D amplitude period

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 15 is option A.

15 A

How to answer it

Identifying Wave Parameters from Displacement-Distance Graphs

What this question tests

This question assesses your ability to interpret graphical representations of progressive waves, specifically distinguishing between displacement-distance graphs and displacement-time graphs, and correctly identifying fundamental wave properties such as amplitude and wavelength.

Question 15 (Multiple Choice)

Identifying w and z

✅ Correct Answer: A

Row A correctly identifies w as the amplitude and z as the wavelength.

Marks available: 1 mark

💡 Key Knowledge

  • X-axis label: The graph shows distance on the horizontal axis, not time. Therefore, properties read off this axis relate to spatial dimensions (wavelength), not temporal dimensions (period).
  • Amplitude ( w ): The maximum displacement of a particle from its equilibrium (zero) position.
  • Wavelength ( z ): The minimum distance between two points in phase with each other (e.g., peak-to-peak or start of one full cycle to the start of the next). Here, z spans exactly one complete wave cycle (one full oscillation).

🧠 Exam Technique

  • Check the axes first: Always look at the label on the x-axis before answering questions about waves. Distance = wavelength ( λ ); Time = period ( T ).
  • Eliminate options quickly: Since the x-axis is distance, period-based options (B and D) can be ruled out immediately.

❌ Common Errors

  • Confusing displacement-distance graphs with displacement-time graphs, leading students to mistake wavelength ( z ) for the time period ( T ).
  • Mistaking w for peak-to-trough distance (which would be 2 × amplitude ). Here, w is measured from the equilibrium line to the peak, making it correctly defined as the amplitude.

Topics

Physics · 3.3 Waves

Question and mark scheme from the AQA A-Level Physics examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.