Learning Objectives:Calculate how

fast a wave travels

P1.5 – Measuring Waves

Skills objective:

Checking units

How big??

33 Hertz 10m 5m/s

100km 9KHz 4mHz

3mm 0.03mm/s 8km/s

Units and size

Frequency

speed distance

Big

Small

SI Units

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)Centimeters

(cm)Hundredths

Millimeters (mm)Thousandths

Kilometers (km)Thousands

Megameters (Mm)Millions

Meters (m)

Changing between prefixes

• 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

Converting between pre-fixes

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?

Converting between pre-fixes

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?

Converting between pre-fixes

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?

Converting between pre-fixes

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?

Converting between pre-fixes

Try these together1.How many m in 1000km?

2.How many cm in 5m?

3.How many mm in 500m?

4.How many m in 2000Mm?

Wave speed

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?

Putting it all together

• 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 λv = 1000 x 2v = 2000m/s

2. The speed of a wave with a wavelength of 50m and a frequency of 4Hz

v = f x λv = 4 x 50v = 200m/s

3. The frequency of a wave travelling at 500m/s with a wavelength of 25m

f = v ÷ λf = 500 ÷ 25f = 20Hz

4. The speed of a radio wave with a wavelength of 3000m and a frequency of 100Hz

v = f x λv = 100 x 3000v = 300,000 m/s

5. The speed of a wave with a frequency of 30kHz and wavelength of 10cm

v = f x λv = 30,000 x 0.1v = 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 ÷ 5,500λ = 2m

7. The frequency of the wave below, which is travelling at 9km/s

f = v ÷ λ f = 9,000 ÷

0.03f = 300,000Hz

or 300kHz

3cm = 0.03m

Plenary

• Make a flow chart for the steps you would take to solve this problem

A wave is travelling at 5mm/s and it has a frequency of 4KHz. What is the wavelength of this wave?

Q1.

Write the equation that I need to answer this question