Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked equations: v = fλ, T = 1/f
Notes
What waves do
Waves transfer energy from one place to another. They do not transfer matter. A cork floating on a ripple tank just bobs up and down as the waves pass, and air particles in a sound wave vibrate back and forth but don’t travel with the wave.
Transverse and longitudinal waves
- Transverse: the oscillations are perpendicular (at 90°) to the direction of energy transfer. Examples: ripples on water, all electromagnetic waves (including light), S-waves.
- Longitudinal: the oscillations are parallel to the direction of energy transfer. They have compressions (particles squashed together) and rarefactions (particles spread out). Examples: sound waves, P-waves.
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- Amplitude: the maximum displacement of a point on a wave away from its undisturbed position.
- Wavelength (λ): the distance from a point on one wave to the equivalent point on the next wave. Unit: m
- Frequency (f): the number of waves passing a point each second. Unit: Hz
- Period (T): the time for one complete wave to pass a point. T = 1 ÷ f. Unit: s
- Wave speed (v): the speed at which energy is transferred through the medium. v = f × λ. Unit: m/s
Reading a wave diagram
- Amplitude is measured from the middle (rest position) to the top of a crest, not from crest to trough.
- Wavelength can be measured from crest to crest, trough to trough, or across one full cycle.
- For accuracy, measure across several wavelengths and divide by the number of waves.
Measuring the speed of sound in air
Stand a measured distance (e.g. 100 m) from a partner. The partner bangs two cymbals together. Start a stopwatch when you see the cymbals clash and stop it when you hear the sound. Speed = distance ÷ time. Repeat and take a mean to reduce the effect of reaction time.
Questions
Q1 (F) What do waves transfer from one place to another?
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Energy (not matter).
Q2 (F) Describe the difference between a transverse and a longitudinal wave.
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In a transverse wave the oscillations are perpendicular to the direction of energy transfer. In a longitudinal wave the oscillations are parallel to the direction of energy transfer.
Q3 (F) Give one example of a transverse wave and one example of a longitudinal wave.
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Transverse: water ripples, light or any EM wave. Longitudinal: sound waves.
Q4 (F) A wave measures 8 cm from the top of a crest to the bottom of a trough. What is its amplitude?
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4 cm. Amplitude is measured from the rest position, so it’s half the crest-to-trough height.
Q5 (F) 30 waves pass a point in 10 seconds. Calculate the frequency.
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Frequency = 30 ÷ 10 = 3 Hz
Q6 (F/H) Five complete waves fit into a distance of 2.0 m. They have a frequency of 3 Hz. Calculate the wave speed.
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Wavelength = 2.0 ÷ 5 = 0.40 m
F: v = f λ
I: v = 3 × 0.40
A: 1.2 m/s
Q7 (F/H) A wave has a frequency of 50 Hz. Calculate its period.
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F: T = 1 ÷ f
I: T = 1 ÷ 50
A: 0.02 s
Q8 (H) A student measures the speed of sound using cymbals 100 m away. Her mean time is 0.31 s. Calculate the speed of sound and explain one way to improve the accuracy of the experiment.
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Speed = 100 ÷ 0.31 = 320 m/s (2 s.f.)
Improvement: use a larger distance, so the time is longer and human reaction time makes up a smaller fraction of it. Or repeat and take more readings for the mean.
Q9 (H) Describe how you could show that it is the wave and not the water that travels in a ripple tank.
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Float a small object such as a cork on the water. As the ripples pass, the cork moves up and down but does not travel along with the waves, so the water itself isn’t moving along.
