Amplitude
Activity 1: Amplitude
Fill in the table below by measuring the distance between the equilibrium and each crest
and trough in the wave above. Use your ruler to measure the distances.
Crest/Trough
a
b
c
d
e
Measurement (cm)
f
Table 1
1. What can you say about your results?
2. Are the distances between the equilibrium position and each crest equal?
3. Are the distances between the equilibrium position and each trough equal?
4. Is the distance between the equilibrium position and crest equal to the distance
between equilibrium and trough?
As we have seen in the activity on amplitude, the distance between the crest and the
equilibrium position is equal to the distance between the trough and the equilibrium position.
This distance is known as the amplitude of the wave, and is the characteristic height of the
wave, above or below the equilibrium position. Normally the symbol A is used to represent
the amplitude of a wave. The SI unit of amplitude is the metre (m).
Definition 1: Amplitude
The amplitude of a wave is the maximum disturbance or displacement of the
medium from the equilibrium (rest) position.
Quantity: Amplitude (A)
Unit name: metre
Figure 1
Interesting Fact:
Unit symbol: m
A tsunami is a series of sea waves caused by an underwater earthquake, landslide, or
volcanic eruption. When the ocean is deep, tsunamis may be less than 30 cm high on the
ocean's surface and can travel at speeds up to 700 km·hr−1. In shallow water near the
coast, it slows down. The top of the wave moves faster than the bottom, causing the sea to
rise dramatically, as much as 30 m. The wavelength can be as long as 100 km and the
period as long as a hour.
In 2004, the Indian Ocean tsunami was caused by an earthquake that is thought to have
had the energy of 23 000 atomic bombs. Within hours of the earthquake, killer waves
radiating from away from the earthquake crashed into the coastline of 11 countries, killing
150 000 people. The final death toll was 283 000.
Example 1: Amplitude of sea waves
Question
If the crest of a wave measures 2 m above the still water mark in the harbour, what is the
amplitude of the wave?
Answer
Analyse the information provided
We have been told that the harbour has a still water mark. This is a line created when there
are no disturbances in the water, which means that it is the equilibrium position of the water.
Determine the amplitude
The definition of the amplitude is the height of a crest above the equilibrium position. The
still water mark is the height of the water at equilibrium and the crest is 2 m above this, so
the amplitude is 2 m.
Activity 2: Wavelength
Figure 2
Fill in the table below by measuring the distance between crests and troughs in the wave
above.
Distance(cm)
a
b
c
d
Table 2
1. What can you say about your results?
2. Are the distances between crests equal?
3. Are the distances between troughs equal?
4. Is the distance between crests equal to the distance between troughs?
As we have seen in the activity on wavelength, the distance between two adjacent crests is
the same no matter which two adjacent crests you choose. There is a fixed distance
between the crests. Similarly, we have seen that there is a fixed distance between the
troughs, no matter which two troughs you look at. More importantly, the distance between
two adjacent crests is the same as the distance between two adjacent troughs. This
distance is called the wavelength of the wave.
The symbol for the wavelength is λ (the Greek letter lambda) and wavelength is measured
in metres (m).
Figure 3
Example 2: Wavelength
Question
The total distance between 4 consecutive crests of a transverse wave is 6 m. What is the
wavelength of the wave?
Answer
Draw a rough sketch of the situation
Determine how to approach the problem
From the sketch we see that 4 consecutive crests is equivalent to 3 wavelengths.
Solve the problem
Therefore, the wavelength of the wave is:
3λλ===6m6 m32 m
(1)
Quote the final answer
The wavelength is 2 m.