OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe how ripples on water surfaces are used to model transverse waves…
Full specification wording
Wave behaviour: describe how ripples on water surfaces are used to model transverse waves whilst sound waves in air are longitudinal waves, and how the speed of each may be measured
Ripples on a water surface are used as a model of transverse waves: the water surface moves up and down while the ripple travels along, so the oscillation is perpendicular to the direction of travel.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 11 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Ripples on a water surface are used as a model of transverse waves: the water surface moves up and down while the ripple travels along, so the oscillation is perpendicular to the direction of travel. Sound in air is longitudinal: air particles vibrate back and forth along the direction the sound travels, forming compressions and rarefactions. To measure ripple speed, record the time for a ripple to travel a measured distance across a ripple tank and use speed = distance ÷ time, or use the wavelength and frequency. To measure the speed of sound in air, measure a known distance between two points, time a sound travelling between them with a stopwatch or data logger, and divide distance by time; a common method uses echoes or a clap timed against a distant reflection.
What earns the marks
- Describes water ripples as a model of transverse waves in which the surface oscillates perpendicular to the direction of travel.
- Describes sound in air as a longitudinal wave in which particles oscillate along the direction of travel, forming compressions and rarefactions.
- Measures ripple speed by timing a ripple over a measured distance and using speed = distance ÷ time.
- Measures the speed of sound in air by timing a sound over a measured distance and dividing distance by time.
Common mistakes and how to put them right
Describes sound as a transverse wave.
Put it right by stating that sound in air is longitudinal with compressions and rarefactions.
Treats the water itself as travelling along with the ripple.
Put it right by noting that the water oscillates while the wave pattern travels.
Omits the distance measurement when calculating speed.
Put it right by recording both distance and time before dividing.
Exam tips
- State clearly that water ripples model transverse waves and sound is longitudinal.
- Use the equation speed = distance ÷ time and show the measured distance and time.
- Describe the practical method, including what is measured and with what instrument.
Try the lesson
The start of the Wave behaviour: describe how ripples on water surfaces are used to model transverse waves whilst sound waves in air are longitudinal waves, and how the speed of each may be measured lesson, made from checked teaching notes for this specification point.
Have a go first
In a ripple tank, a small floating cork bobs up and down as a water wave passes. What does this show about the direction of movement of the water particles?
Listen
Ripples on a water surface are used as a model of transverse waves: the water surface moves up and down while the ripple travels along, so the oscillation is perpendicular to the direction of travel. Sound in air is longitudinal: air particles vibrate back and forth along the direction the sound travels, forming compressions and rarefactions. To measure ripple speed, record the time for a ripple to travel a measured distance across a ripple tank and use speed = distance ÷ time, or use the wavelength and frequency. To measure the speed of sound in air, measure a known distance between two points, time a sound travelling between them with a stopwatch or data logger, and divide distance by time; a common method uses echoes or a clap timed against a distant reflection. A strong answer earns marks for these points. Describes water ripples as a model of transverse waves in which the surface oscillates perpendicular to the direction of travel. Describes sound in air as a longitudinal wave in which particles oscillate along the direction of travel, forming compressions and rarefactions. Measures ripple speed by timing a ripple over a measured distance and using speed = distance ÷ time. Measures the speed of sound in air by timing a sound over a measured distance and dividing distance by time. A common slip: Describes sound as a transverse wave. Instead, stating that sound in air is longitudinal with compressions and rarefactions. Exam tip: State clearly that water ripples model transverse waves and sound is longitudinal.
Key words
Say what each word means before you turn the card.
Transverse waves
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Longitudinal waves
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Compressions and rarefactions
Try answering aloud or in your head before turning the card.
Speed
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See every definition
- Transverse waves
- Waves modelled by ripples on a water surface, where the surface moves up and down while the ripple travels along, so the oscillation is perpendicular to the direction of travel.
- Longitudinal waves
- Waves such as sound in air, where air particles vibrate back and forth along the direction the sound travels, forming compressions and rarefactions.
- Compressions and rarefactions
- What is formed in sound in air as air particles vibrate back and forth along the direction the sound travels.
- Speed
- Calculated as distance divided by time, used for measuring ripple speed and the speed of sound in air over a measured distance.
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