Measuring the Speed of Waves
Build confidence in Measuring the Speed of Waves for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Measure the speed of waves on a stretched string and of ripples on the surface of water using v = fλ.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
This required practical measures the speed of waves in a solid, in water and in air.
For ripples on water, a ripple tank with a strobe light is used. The wavelength is measured by freezing the pattern with the strobe, and the frequency is read from the signal generator driving the dipper. Speed is then calculated using v = fλ.
For waves on a string, a vibration generator produces a standing wave. The wavelength is found by measuring the distance across several loops and dividing, and the frequency is read from the signal generator. Measuring across several waves and dividing reduces the percentage uncertainty in the measurement.
Revision notes
Waves in water
Use a ripple tank with a dipper driven by a signal generator.
A strobe light freezes the pattern so the wavelength can be measured with a ruler. The frequency is read from the signal generator. Calculate the speed using v = fλ.
Waves on a string
A vibration generator attached to a string produces a standing wave with visible loops.
Measure the distance across several loops and divide to find the wavelength, remembering that one wavelength is two loops. Read the frequency from the signal generator.
Reducing uncertainty
Measuring across several waves and dividing gives a more accurate wavelength than measuring one.
The absolute uncertainty in the ruler reading is the same either way, so spreading it over a larger distance reduces the percentage uncertainty.
Key points
- Speed is calculated using v = fλ.
- A ripple tank is used for water waves.
- A strobe light freezes the wave pattern.
- Frequency is read from the signal generator.
- Measure across several waves and divide.
- This reduces the percentage uncertainty.
Worked examples
Example 1
A wave on a string has a frequency of 30 Hz and a wavelength of 0.4 m. Calculate its speed. [2 marks]
Model answer
Wave speed = frequency × wavelength = 30 × 0.4 (1 mark) = 12 m/s (1 mark).
Example 2
Explain why the wavelength is found by measuring across several waves and dividing. [2 marks]
Model answer
The uncertainty in the ruler reading is the same whether one wave or several are measured (1 mark), so measuring a longer distance and dividing reduces the percentage uncertainty in the wavelength (1 mark).
Example 3
State the purpose of the strobe light in a ripple tank experiment. [2 marks]
Model answer
It freezes the apparent motion of the wave pattern (1 mark), so the wavelength can be measured accurately with a ruler (1 mark).
Common mistakes
Measuring a single wavelength.
Measuring several and dividing gives a smaller percentage uncertainty.
Forgetting that one wavelength is two loops on a string.
Count carefully when converting loop measurements to wavelength.
Estimating the frequency instead of reading it.
It is read directly from the signal generator.
Using the wrong units.
Wavelength in metres and frequency in hertz give speed in m/s.
Exam tips
- Measure across several waves to reduce uncertainty.
- Read the frequency from the signal generator.
- Remember two loops make one wavelength on a string.
- Use v = fλ and check the units.
Key terms
- Ripple tank
- Apparatus for producing and observing water waves.
- Strobe light
- A flashing light that freezes wave motion.
- Signal generator
- Apparatus setting the frequency of vibration.
- Percentage uncertainty
- Uncertainty expressed as a proportion of the measurement.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.