# Alfriston College Year 12 Physics - Muriwai Beach Fieldtrip

## Wavelengths

Work along the road below the North carpark down to the rocks at the end of the road. Start on the road just below the gate where the water comes to the highest point on the beach. Mark out 90m with 30m intervals along the way. This will give you three sections - Section A (0-30m), Section B (30-60m), Section C (60-90m)

### Estimate the Wavelength

The wave length is the distance between a point on one wave and the same point on the next wave. (eg the distance in metres between consecutive wave crests).

• Estimate the wavelength of the waves in each 30m section you have marked out. Work out your own method to estimate the wavelength and repeat it 5 times to get an average for each section.
• Draw a diagram and write the steps you used to describe your method for estimating the wavelengths.
• Show the data you collected to work out the wavelengths
• Record your final results in a table

### Calculate the Wavelength

To calculate the wavelength you need to measure the velocity and frequency of the waves.

#### Wave Velocity

• Measure the time for a wave to travel 30m

For section A have a timer, with a stopwatch, standing at 0m and a signaller standing on the 30m mark. The signaller will drop their arm when a wave passes them so the timer can start their stopwatch. When the wave reaches the timer they will stop their watch and record the time. Repeat this 5 times for different waves at this section. Now repeat the entire process again for section B (30-60m) and section C (60-90m) sections.

• Calculate the Wave Velocity

Use the formula:  Velocity (m/s) = distance (m)/time (s)  to calculate the average velocity for each section.

Copy table 1 and enter your times and velocity calculations to complete.

Table 1 -  Wave Velocities

Wave Section 1 0-30m Section 2 30-60m Section 3 60-90m
# Distance (m) Time (s) Velocity (m/s) Distance (m) Time (s) Velocity (m/s) Distance (m) Time (s) Velocity (m/s)
1 30 30 30
2 30 30 30
3 30 30 30
4 30 30 30
5 30 30 30

Now calculate your average velocity for each section

#### Wave Frequency

• Measuring the time for 10 waves

At the midpoint of each of the 30m sections time how long it takes 10 waves to go past a point such as a rock outcrop.

• Calculate the Wave Frequency

Use the formula:

Frequency (Hz) = number of waves /time (s)

to calculate the wave frequency for each section.

Enter your calculations in a table (eg table 2 below)

Table 2  - Wave Frequencies near the Beach

 Sample Section 0m-30m Section 30m-60m Section 60m-90m No. of waves Time (s) Frequency (Hz) No. 0f waves Time (s) Frequency (Hz) No. of waves Time (s) Frequency (Hz) 1 10 10 10 2 10 10 10 3 10 10 10 4 10 10 10 5 10 10 10 Average Frequency (Hz) Average Frequency (Hz) Average Frequency (Hz)
• Calculate the Wavelength

Use the formula:

Wavelength (m) = velocity (m/s)/frequency (Hz)

to calculate the average wavelength for each section. Enter into a table (see example table 3 below)  You will need to transfer your average velocity and frequency results from tables 1 and 2.

Table 3  - Wavelengths

 Section 0m-30m Section 30m-60m Section 60m-90m Average Velocity (m/s) Average Frequency (Hz) Average Wavelength (m) Average Velocity (m/s) Average Frequency (Hz) Average Wavelength (m) Average Velocity (m/s) Average Frequency (Hz) Average Wavelength (m) Estimated Wavelegth (m) Average Frequency (Hz) Average Frequency (Hz)

## Activity 2 - Wave Frequencies

Walk up the stairs which go inland and follow the track until you a about halfway to the viewing platform and can clearly see the waves passing the rocks below.

• Carry out the Wave frequency activity (from activity 1)again for waves passing the rocks below your viewing point.
• Move to location 3 which is a lookout platform and carry out the wave frequency activity for the waves passing the rocks below.
• Enter your calculations in a table like table 4 below
• Transfer the your wave frequencies for the 30-60m section from table 2

Table 4 Wave Frequencies at 3 points moving from the beach out to sea.

 Sample Section 30m-60mNear the Beach No. of waves Time (s) Frequency (Hz) No. 0f waves Time (s) Frequency (Hz) No. of waves Time (s) Frequency (Hz) 1 10 10 10 2 10 10 10 3 10 10 10 4 10 10 10 5 10 10 10 Average Frequency (Hz) Average Frequency (Hz) Average Frequency (Hz)

## Activity 3 - Observations

### ONE:  Wave Energy

• Observe some waves as the impact against the rocks
• Observe some waves as they impact onto the sandy beach
• Describe what happens to the water as it impacts against the rocks and compare this with what happens to the water that impacts against the sandy shore. Draw diagrams to enhance your descriptions.

 Diagram and Description of Waves Impacting onto the Rocks Diagram and Description of Waves Impacting onto the Sandy Beach

The waves hitting the rocks and the sandy beach have similar amounts of energy. The energy from the wave is transferred by a force acting over a distance.

• In which situation are the waves hitting with the greatest force? Explain you answer using the idea that force acts over a certain distance.

### TWO:  Waves Breaking

• Observe the waves as the break
• Describe what happens to the waves as they break. Draw a diagram to enhance your description.

 Diagram and Description of Waves Breaking

The velocity of different parts of a wave changes as it move onshore from deeper to shallower water due to friction from the seafloor.

• Using your knowledge of friction and its effect on velocity give an explanation as to why waves break.

## Conclusions

### Analysis

#### Activity 1: Wavelengths

1. With reference to your data, describe what happens to the speed of the waves as they travel from deeper water to the shallow water of the beach.
2. Write a generalisation to describe the trend your data shows about how wave speed varies as a result of the changing depth of the water.
3. Try to explain the pattern you observed in terms of water depth and the amount of friction acting on the waves.
4. With reference to your data, describe how wavelength varies from offshore to onshore.
5. Write a generalisation to describe the trend your data shows about how wavelength varies as the waves travel from offshore to onshore.
6. Try to explain the pattern you observe in relation to the wave speed and wave frequency.

#### Activity 2: Wave Frequencies

1. With reference to your data, describe how the wave frequencies vary across the three points that you sampled.
2. Write a generalisation to describe the trend your data shows about how wave frequency varies at points moving onshore to offshore.
3. Try to explain the pattern you observe.................................................