PHY052 · Waves & Acoustics · Classes 8–11
Wave speed
Objective: To find the speed of a wave on a slinky or rope from its frequency and wavelength.
🧰 Apparatus
- Slinky spring or long rope
- Metre rule or measuring tape
- Stopwatch
- Chalk marks on the floor
📐 Formulas and SI units
v = f λ (m/s)
T = 1/f (s)
💡 Theory
A wave carries energy without carrying the medium along. In one period T the wave moves forward one wavelength λ, so its speed is v = λ/T = fλ. For a given medium the speed is fixed, so a higher frequency gives a shorter wavelength.
🪜 Procedure
- Stretch the slinky along the floor with a partner holding the far end.
- Shake one end steadily from side to side.
- Count the shakes in 10 s to find f.
- Measure the distance between neighbouring crests (λ).
- Calculate v = fλ.
- Shake faster and check that λ shortens while v stays about the same.
📋 Observations Simulated values
✅ Result and conclusion
A wave with f = 50 Hz and λ = 2 m travels at 100 m/s; each oscillation takes 20 ms.
🔬 In a real lab
Hand-shaking is not perfectly regular and the slinky loses energy, so the crests get smaller along its length.
⚠️ Safety
Do not let a stretched slinky snap back towards anyone; keep the floor area clear.
❓ Quick check
1. Wave speed equals…
2. Doubling f in the same medium makes λ…
3. A wave carries…
🏋️ Practice
Easyf = 5 Hz, λ = 0.4 m. v?
Answer: 2 m/s
MediumSound at 340 m/s, f = 680 Hz. λ?
Answer: 0.5 m
ChallengeT for 50 Hz?
Answer: 0.02 s