Mammoth Memory

Wave speed

Wave speed is the speed (velocity) at which a wave is travelling.

The formula for calculating wave speed is:

zzz_wave speed-01.jpg

Wave speed = f λ  or  v = f λ     

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Speeding along she frequently (frequency) waved at length.

NOTE:

This does not only relate to waves in water but all waves, including light waves and electromagnetic waves.

v = f λ where:

v = wave speed (measured in m/s)

f = frequency (measured in hertz Hz)

λ pronounced lambda = wavelength (measured in metres)

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Example 1

A wave at sea travels with a velocity of 25m/s. If it has a wavelength of 10 m, what is its frequency?

Answer

Wave speed = frequency x wave length

Divide both sides by wavelength to make frequency the subject,

\frac{Wave\ speed}{wavelength}\ =\ \frac{\strike{wavelength\ }\times frequency}{\strike{wavelength}}

Frequency\ =\frac{Wave\ speed\ }{wavelength}=\ \frac{25}{10}\ =\ 2.5Hz 

 

Example 2

Lizzy reads the back label of her microwave oven. It says frequency = 2,450MHz. If the speed of microwaves is 3.00 x 108 m/s, what wavelength are they?

Answer

The first thing you must do to answer this question is convert the frequency to the standard SI units. So 2,450 MHz = 2,450 x 106 Hz

Wave speed = frequency x wave length

Divide both sides by frequency to make wavelength the subject,

\frac{Wave\ speed}{frequency}\ =\ \frac{\strike{frequency}\ \times wavelength}{\strike{frequency}}

wavelength=\frac{Wave\ speed\ }{frequency}=\frac{3\ \times10^{^8}}{2,450\ \times10^6}\ =\ 0.122m

0.122m = 12.2cm

 

Example 3

An electromagnetic wave travels at a speed of 300,000,000 m/s and has a wavelength of 15cm. Calculate the frequency of the electromagnetic wave.

Answer

Wave speed = frequency x wave length

Divide both sides by wavelength to make frequency the subject,

\frac{Wave\ speed\ }{wavelength}=\ \frac{\strike{wavelength}\times frequency}{\strike{wavelength}}

Frequency\ =\frac{\ Wave\ speed}{wavelength}

However the wavelength given in the question is not in the standard SI unit and so must first be converted from cm to metres.

15cm = 0.15m So:

Frequency\ =\ \frac{Wave\ speed\ }{wavelength}=\frac{300,000,000}{0.15}=2,000,000,000\ Hz\ =\ 2GHz

Example 4

A TV station broadcasts at 500,000 kHz. The waves travel through the air at 300,000,000 m/s, calculate the wavelength of the waves broadcast by this station.

Answer

Wave speed = frequency x wave length

Divide both sides by frequency to make wavelength the subject,

Wave\ speed\ =\ \frac{\strike{frequency}\ \times\ wavelength}{\strike{frequency}}

Wavelength\ =\ \frac{Wave\ speed}{frequency}

 

Beware, the question gives the frequency in kHz so you will need to convert this to Hz before you can put the figures into the formula.

500,000 kHz = 500,000,000 Hz So:

Wavelength\ =\frac{\ 300,000,000\ }{500,000,000}=\ 0.6m

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