3.1.3Waves
Wave Speed — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 4 of 13
Part 1 of 5Two ways to find wave speed
| Equation | Symbols and units |
|---|---|
| wave speed = distance ÷ time | Speed in metres per second (m/s), distance in metres (m), time in seconds (s) |
| wave speed = frequency × wavelength, or v = f × λ | v in m/s, f in hertz (Hz), λ in metres (m) |
Part 2 of 5Understanding the quantities
Wave speed
- Wave speed The distance travelled by a wave each second.
- It depends on the medium the wave is travelling through.
Frequency
- Frequency The number of complete waves passing a point each second. 1 Hz is one wave per second.
Wavelength
- Wavelength The distance from a point on one wave to the same point on the next wave, such as crest to crest.
Why the equation works
- Why the equation works In one second, f waves pass, and each is λ long.
- So the wave moves a distance of f × λ every second.
- All waves obey this equation, whether they are sound, light or water waves.
Part 3 of 5Worked example: finding speed
1
Sound in air has a frequency of 170 Hz and a wavelength of 2 m. Find its speed.
2
List what you know: f = 170 Hz and λ = 2 m.
3
Write the equation: v = f × λ.
4
Substitute: v = 170 × 2 = 340 m/s.
Part 4 of 5Worked example: finding wavelength
1
A pond wave has speed 0.15 m/s and frequency 0.5 Hz. Find its wavelength.
2
Start with v = f × λ and rearrange to make wavelength the subject: λ = v ÷ f.
3
Substitute: λ = 0.15 ÷ 0.5.
4
Calculate: λ = 0.30 m.
5
A formula triangle with v on top and f and λ below gives the same rearrangements: cover the quantity you want.
Part 5 of 5Avoiding unit errors
Check units before substituting
Wavelength units
- Wavelength units If a wavelength is in cm or km, convert to metres first when the speed must be in m/s.
- For example, 50 cm = 0.50 m.
Frequency units
- Frequency units 1 kHz = 1000 Hz and 1 MHz = 1 000 000 Hz. Convert before calculating.
Symbols
- Symbols Use λ for wavelength, not L or W. If unsure of a symbol, write the word equation instead.
Key terms
| Term | Meaning |
|---|---|
| Wave speed | The distance a wave travels each second, measured in m/s. |
| Frequency | The number of waves passing a point per second, measured in hertz (Hz). |
| Wavelength | The distance between a point on one wave and the same point on the next, measured in metres. |
| Wave equation | v = f × λ, which links wave speed, frequency and wavelength. |
| Hertz (Hz) | The unit of frequency; one wave per second. |
Avoid these mistakes
1
Using the wrong symbol for wavelength, such as writing L or W instead of λ.
2
Leaving wavelength in centimetres or kilometres when the answer must be in m/s.
3
Forgetting to convert kHz to Hz, which makes the answer 1000 times too small.
4
Mixing up the rearrangement. To find wavelength you divide speed by frequency; you do not multiply.
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