Investigating Energy Transfer in Ocean Waves

This is a description of a series of lessons I use to introduce students to the interaction of water and energy in the motion of water waves. The culminating activity is to go to beach and complete an investigation of waves that each group wrote after their initial investigations into the behavior of waves.

Ocean waves are a complex interaction of sand, water, wind, and other factors. To begin an investigation of wave interactions, it is better to start with a more simplistic situation.

One way that waves can be easily observed is by sending wave pulses along a long spring. If two people sit on opposite ends of the spring with the spring stretched out between them they can work through the following investigations to see what happens.

• What is the difference between a pulse and a wave?
• What is the difference between a longitudinal and transverse pulse?
• How do you change the frequency of a wave?
• How do you change the wavelength of a wave?
• How do you change the speed of a wave?
• How do you change the amplitude of a wave?
• What happens when pulses overlap?
• Why do pulses die?
• What happens when a wave comes to the end of a spring that someone is holding?
• What happens when a wave comes to the end of a spring that is free to move?

Each of the pulses travel the way they do because of the elastic forces between the coils of the springs. See the model of the forces involved in this interaction for a visual of this situation.

While students are completing their investigations, it is important for them to record their observations in as clear a detail as possible. I introduced two methods of recording those observations, a wavefront diagram and a ray diagram. A wavefront diagram illustrates the shape of the front of the wave as it travels. A ray is a vector that comes out perpendicular to the wavefront in the direction that the wave is traveling.

As each investigation is completed, students are asked to observe: wavelength, amplitude, wave direction, period, and frequency.

With the equipment students are asked to investigation how waves bounce back from an obstacle. After this investigation, the interaction is named as reflection.

Second, students are asked to investigate the behavior of waves as they travel from deep water to shallow water by putting a plastic rectangle under the water. After the observations, the interaction is named as refraction.

Third, students are asked to investigate what happens when a wave goes through an opening in a barrier. After this investigation, the interaction is named as diffraction.

To prepare for designing their own investigations, students are next asked to read about some active research about ocean waves. The reading I selected includes apparatus that measure quantitatively the movement and/or energy of waves from Woods Hole Oceanographic Institute.

http://www.whoi.edu/page.do?pid=14939&tid=282&cid=...

After reading the passage, we discuss as a whole class what devices were used in the investigations and how each one actually measures something about a wave. This is the beginning of the brainstorming session for students designing their own investigations in a scale model beach in the classroom.

The task given to students is: Use your mini “beach” design an experiment to quantitatively investigate some effect of the wave energy on your beach, then complete your investigation. Based on your results, construct a beach protection feature and repeat your measurements to test the effect of your feature. Explain how you designed your beach protection feature, and write a conclusion that cites evidence from the first and second sets of measurements, meaning with and without your beach protection feature.

Post Investigation Question: How can you modify this experiment to collect data on the beach? In your response write a specific procedure for data collection, a list of equipment and materials that will be required, and prepare an empty data table for use during your experiment.

Once students had completed their small scale investigation, they designed an investigation to be completed at the beach. The requirements were that the investigation had to be quantitative, and that the group had worked out a way to analyze the data in advance.

Some ideas that students investigated were:

How does beach erosion compare on sections with and without a jetty?

How does wave motion move a beach ball? (Beach ball was tethered on a string.)

How does wave motion move a canoe? (Investigations were completed parallel to the beach and perpendicular to the beach each at different initial positions relative to the beach.)

How does the amplitude of waves change over time?

How does the slope of the sand change as you move away from shore?

Students used their foot length to pace out distances, plastic rulers to measure sand depth over time, cell phones to measure time, cell phones to record photos and videos, photo analysis and video analysis of wave motion using the computers when we returned to the classroom, mapping of landmarks and relative motion during the canoe investigations, and other techniques as well.

The imagination is the only limiting factor when it comes to getting students out to investigate their natural world! Next trip I hope to bring along some equipment to study the wave action under the waves.