Wave Speed (Oxford AQA IGCSE Physics)
Revision Note
Wave Speed
Wave speed is defined as:
The distance travelled by a wave each second
The wave speed can be calculated in a similar way to calculating the speed of moving objects:
Where:
Wave speed is measured in metres per second (m/s)
Distance travelled by the wave is measured in metres (m)
Time taken is measured in seconds (s)
All waves obey the wave equation, which is another way to calculate the wave speed:
Where:
= wave speed in metres per second (m/s)
= frequency in hertz (Hz)
= wavelength in metres (m)
You may need to rearrange the wave equation, which can be done using a formula triangle
Formula triangle relating wave speed, frequency and wavelength
Worked Example
Sound waves in air travel with a frequency of 170 Hz and a wavelength of 2 m.
Calculate the speed of sound waves in air.
Answer:
Step 1: List the known quantities
Frequency,
Wavelength,
Step 2: Write out the wave equation
Step 3: Substitute the known quantities into the wave equation to calculate wave speed
Worked Example
A wave in a pond has a speed of 0.15 m/s and a frequency of 0.5 Hz.
Calculate the wavelength of the wave.
Answer:
Step 1: List the known quantities
Wave speed,
Frequency,
Step 2: Write out the wave equation
Step 3: Rearrange the equation to make wavelength the subject
Step 4: Substitute the known values to calculate the wavelength
Examiner Tip
When stating equations make sure you use the right letters:
For example, use for wavelength, not L or W
If you can’t remember the correct letters, then just state the word equations
Be careful with units:
Wavelength is usually measured in metres and speed in m/s, but sometimes wavelength may be given in other units, such as cm or km. When calculating wave speed, check what units for speed the question requires and be prepared to convert the wavelength accordingly.
Likewise, watch out for frequency given in kHz: 1 kHz = 1000 Hz
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