Wave Speed (Oxford AQA IGCSE Physics): Revision Note

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, $f=170\mathrm{Hz}$

• Wavelength, $\lambda =2\mathrm{m}$

Step 2: Write out the wave equation

$\nu =f\times \lambda$

Step 3: Substitute the known quantities into the wave equation  to calculate wave speed

$\begin{array}{rcl}\nu & =& 170\times 2\\ & & \\ \nu & =& 340\mathrm{m}/\mathrm{s}\end{array}$

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, $\nu =0.15\mathrm{m}/\mathrm{s}$ 

• Frequency, $f=0.5\mathrm{Hz}$ 

Step 2: Write out the wave equation

$\nu =f\times \lambda$

Step 3: Rearrange the equation to make wavelength the subject

$\lambda =\frac{v}{f}$

Step 4: Substitute the known values to calculate the wavelength

$\begin{array}{rcl}\lambda & =& \frac{0.15}{0.5}\\ & & \\ \lambda & =& 0.30\mathrm{m}\end{array}$

When stating equations make sure you use the right letters:

• For example, use $\lambda$ 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