Why halve the distance when timing an echo?

The sound travels out and back, covering the same distance twice.

Why does changing the frequency not change the speed?

Speed follows from the stiffness and inertia of the medium, not from how fast it vibrates.

Why is the fundamental of a fixed string half a wavelength?

Both ends are nodes, so the shortest standing wave fits half a wave.

S5 Physics: Waves and Sound Practice Paper 1

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper one opens with a 170 Hz wavelength, a 50 Hz period and a 0.5 m wave, compares sound in air and water, then times thunder, halves a sonar echo and doubles a frequency, before a string fundamental and the audible limits, ending with the faster-sound mix-up.

Quick answer　2 m, 0.02 s, 300 m/s, faster in water, 1 360 m, 450 m, halved wavelength, 1.6 m with 320 m/s, 17 m with 0.017 m, and only a shorter wavelength.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Waves and Sound – Tutor Lesson Plan
Level	S5

02　Questions (10 items)

FoundationA 170 Hz sound wave travels at 340 m/s in air. Find its wavelength.
FoundationA wave has a frequency of 50 Hz. Find its period.
FoundationA wave of wavelength 0.5 m has a frequency of 600 Hz. Find its speed.
FoundationDoes sound travel faster in air or in water?
IntermediateThunder is heard 4 s after lightning is seen, with sound at 340 m/s. Find the distance.
IntermediateA sonar pulse returns after 0.6 s with sound at 1 500 m/s in water. Find the depth.
IntermediateIn the same medium, what happens to the wavelength if the frequency doubles?
ChallengeA 0.8 m string is fixed at both ends. Find the fundamental wavelength, and the speed if the frequency is 200 Hz.
ChallengeWith sound at 340 m/s, find the wavelengths at 20 Hz and at 20 000 Hz.
Error redoA student claims a higher frequency means a faster sound. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	2 m (v = f lambda, 340 / 170)	Speed equals frequency times wavelength, so two of the three give the third.
2	0.02 s (1 / 50)	Period and frequency are reciprocals, and the period is in seconds.
3	300 m/s (0.5 x 600)	Substitute into v = f lambda with consistent units.
4	Faster in water, about 1 500 m/s, against about 340 m/s in air	The closer the particles, the faster sound passes through.
5	1 360 m (340 x 4)	Light arrives almost at once, so the delay is all sound travel.
6	450 m (1 500 x 0.6 / 2)	The echo covers the trip twice, so halve the distance.
7	The wavelength halves, since the medium fixes the speed	With v fixed, f and lambda are inversely proportional.
8	Wavelength 1.6 m and speed 320 m/s	At the fundamental the string holds exactly half a wavelength.
9	17 m and 0.017 m	Across the audible range the wavelengths differ a thousandfold.
10	The speed is fixed by the medium; a higher frequency only shortens the wavelength	Speed depends on the medium, not on the frequency.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.