P1.5 – Measuring Waves
Learning
Objectives:
Calculate how fast a wave travels
Skills objective:
Checking units

33 Hertz
10m 5m/s
100km 9KHz 4mHz
3mm 0.03mm/s 8km/s

Big
Frequency speed distance
Small

To avoid problems like these we came up with the International System of Units
(SI units)
These are a set of units that everyone
must use when carrying out calculations.

Meters, kilometers or centimeters?
Units tell us what type of number we are talking about.
For example, 2m means we are talking about a distance.
Sometimes that unit has a prefix. This tells us about how big or small the value is.
For example, 2mili-meters is a smaller distance than
2kilo-meters.
The most common prefixes are, (in order of size)
Millimeters
(mm)
Thousandths
Centimeters
(cm)
Hundredths
Meters
(m)
Kilometers
(km)
Thousands
Megameters
(Mm)
Millions

• It’s hard to remember all of the rules
(do you divide by 1000 or times? Is it
100 or 1000?)
• Instead, try to remember what each means

Kilo = thousand
, so 1km is “1 thousand meters”
Or 2.5km is “2.5 thousand meters”
1 km = 1000m
What do you have to do to get from m to km?
What do you have to do to get from km to m?

Mega = a million
, so 1Mm is “1 million meters”
Or 2.5Mm is “2.5 million meters”
1 Mm = 1,000,000m
What do you have to do to get from m to Mm?
What do you have to do to get from Mm to m?

Milli is a thousandth
, so 1mm is “1 thousandth of a meter”
Or 1000mm is meter
1000mm = 1m
What do you have to do to get from m to mm?
What do you have to do to get from mm to m?

Centi is a hundredth
, so 1cm is “1 hundredth of a meter”
Or 100cm is meter
100cm = 1m
What do you have to do to get from m to cm?
What do you have to do to get from cm to m?

Try these together
1. How many m in 1000km?
2. How many cm in 5m?
3. How many mm in 500m?
4. How many m in 2000Mm?

You can also calculate how fast a wave will travel using this equation:
• Wavespeed = wavelength x frequency
(m/s) (m) (Hz)
If a wave has a wavelength of 2m and a frequency of 1Hz, what will the wave speed be?

• We are going to apply all of these new rules to a new formula
• The Silly Wave Formula

Wavespeed = wavelength x frequency
(m/s) (m) (Hz)
What is the frequency of a wave of wavelength 2km travelling at 0.5m/s.
1. Are there any difficult units?
2. Do I need to rearrange the equation?
3. Now I can put the numbers in!

1. The speed of a wave with a frequency of 1kHz and a wavelength of 2m.
v = f x λ v = 1000 x 2 v = 2000m/s

2. The speed of a wave with a wavelength of 50m and a frequency of 4Hz v = f x λ v = 4 x 50 v = 200m/s

3.
The frequency of a wave travelling at
500m/s with a wavelength of 25m f = v ÷ λ f = 500 ÷ 25 f = 20Hz

4.
The speed of a radio wave with a wavelength of 3000m and a frequency of 100Hz v = f x λ v = 100 x 3000 v = 300,000 m/s

5.
The speed of a wave with a frequency of 30kHz and wavelength of 10cm v = f x λ v = 30,000 x 0.1
v = 3,000 m/s

6.
The wavelength of a wave travelling at
11km/s with a frequency of 5.5kHz
λ = v ÷ f
λ = 11,000 ÷ 5,500
λ = 2m

7.
The frequency of the wave below, which is travelling at 9km/s
0.03
f = v ÷ λ f = 9,000 ÷ f = 300,000Hz or
300kHz
3cm = 0.03m

• Make a flow chart for the steps you would take to solve this problem
Q1.
A wave is travelling at 5mm/s and it has a frequency of 4KHz. What is the wavelength of this wave?
Write the equation that I need to answer this question