Science 7–10 · Year 9

Slinky waves: transverse and longitudinal pulses, wavelength, frequency and speed

Physical sciences — Waves and motion: waves, sound and light (NSW Stage 5 focus area)

Practical, model not builtLow risk

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The idea

A wave carries energy along a medium while the medium's parts only move about their rest positions, either across the direction of travel (transverse) or along it (longitudinal).

What you need

long steel slinky spring, 1
ribbon or masking-tape flag tied to one coil, 1
tape measure 5 m, 1
stopwatch or phone video, 1
smooth floor 5 m long

How to do it

Stretch the slinky to 4.0 m along the floor with a learner holding each end. Flick one end sideways once and watch the pulse travel and reflect; watch the flag to see that a coil moves sideways and returns.
Time a pulse travelling 4.0 m and back (8.0 m); repeat five times and average. Speed = 8.0 m / time.
Push one end sharply forward along the line of the slinky and watch the compression travel; watch the flag to see that a coil moves forward and back along the line.
Send a steady transverse wave by moving the end from side to side twice per second; measure the distance between crests (wavelength) from a photo, and check that frequency x wavelength matches the pulse speed.
Stretch the slinky to 5.0 m and repeat the pulse timing; compare speeds.
Move the end faster and then with larger swings; record which of wavelength, frequency and amplitude changes each time.

What you should see

Speed = 8.0 m divided by the average round-trip time, and the speed rises when the slinky is stretched further: once the stretch is much longer than the coiled spring, a spring of stiffness k and mass m stretched to length L carries pulses at L x sqrt(k / m), so going from 4.0 m to 5.0 m raises the speed by about a quarter. The flag shows the coil moving sideways for transverse pulses and along the slinky for longitudinal pulses, and returning to its place after the pulse passes. For a steady transverse wave, frequency x wavelength matches the pulse speed measured on the same stretch: a slinky whose 8.0 m round trip takes 2.4 s carries pulses at 3.3 m/s, so shaking the end twice a second gives crests about 1.7 m apart and only two or three of them fit on the 4.0 m stretch. Crests 0.75 m apart at 2.0 Hz would mean a speed of 1.5 m/s and a round trip of 5.3 s, far slower than a stretched slinky gives. Faster shaking shortens the wavelength while the speed stays unchanged; bigger swings change only the amplitude.

What changes

What you change: stretch of the slinky (m), or frequency of shaking (Hz)
What you measure: pulse speed (m/s), wavelength (m)
What you keep the same: same slinky
pulse size for speed timing
ends held fixed

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The coils travel along with the wave.
Shaking faster makes the wave travel faster.
Bigger waves travel faster.

Safety card

Low riskLearners carry it out

Hazards

slinky snapping back when released under tension
tangling

Controls

release tension slowly and together
do not let go of a stretched end

Note

No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-WAM-01 SC5-WS-01 SC5-WS-05
Australian Curriculum v9AC9S9U04AC9S9I03AC9S9I04

Sources

The pages the author read to write this activity.

All Concept Studio activities