GEOL/METR 309: Investigating Land, Sea and Air Interactions Fall 2005, SFSU Exploration Activity: Wave Motions (Thursday, Oct. 13) Dr. Dave Dempsey Dr. Lisa White (Dept. of Geosciences)

(This activity can be accessed at:
http://funnel.sfsu.edu/courses/gm309/F05/labs/waves.html)

#### Introduction

Waves are patterns of disturbance that travel or "propagate" through a medium. There is energy associated with wave disturbances, so they are capable of doing damage or useful work.

Simple, regular wave patterns can be characterized by their wavelength, wave height (amplitude), wave velocity or speed, wave period, and wave frequency. Examples of waves in nature are numerous: sound waves (which travel through air or water); water waves; earthquake waves (which travel through rock); electromagnetic radiation such as visible light, radio waves, microwaves, and X-rays (all of which travel through space); etc.

There is a common confusion between (1) the motion of a wave disturbance; and (2) the motion of "particles" of the medium through which the wave disturbance travels. It is tempting to think of a moving wave crest as a material object in its own right, but this is not generally correct. Rather, waves are a traveling pattern of displacement or disturbance in a medium, while the "particles" in the medium itself might oscillate as a wave passes through but ultimately experience little or no net change in position at all. (Breaking waves in water are an exception.) In this activity you will explore this distinction with the help of a computer animation of deep water wave motions in idealized "water".

#### Objective

• distinguish between the motion of a wave disturbance in deep water and the motion of "particles" of the water through which the wave travels

#### Instructions and Questions

You can carry out the steps below either on your own or together with someone else. In class, be prepared for interruptions by the instructor to get and give feedback.

1. Go to the following WWWeb site, created by Dr. Dan Russell of Kettering University's Dept. of Applied Physics:

"Longitudinal and Transverse Wave Motion"
(http://www.gmi.edu/~drussell/Demos/waves/wavemotion.html)

Here you will find animations of four different types of waves:

1. Longitudinal waves. (Examples include sound waves in air or water, and the type of earthquake waves called P-waves. This kind of wave tends to be relatively fast-moving, and the more dense the medium through which they travel the faster they move.)

2. Transverse waves. (An example is the type of earthquake wave called S-waves or "shear" waves. These are slower than P-waves and therefore arrive later. The perspective could be either a "map view" looking down on the wave from above, or a cross-section view looking at the wave from the side.)

3. Water waves. (The ones shown here are characteristic of waves in deep water. Waves in shallow water "feel" the bottom and behave a little differently in some respects, though the basic idea is the same until the shallow water waves begin to break.)

4. Raleigh surface waves. (An example is the type of earthquake wave called a surface wave; it's the kind of undulating wave that some observers can see coming across the earth's surface immediately after an earthquake. These are slower than P-waves and arrive later.)

2. Take a closer look at the water waves (the third type above). Pick out a wave "crest" and follow it. In which direction is the wave disturbance moving?

3. Focus on one "particle" of the medium and describe its motion. (One of the two blue dots is easiest to follow.) How does it's motion compare to the wave motion? How do the particle motions vary with depth?

4. Focus on two particles of the medium that are next to each other, and describe their motions relative to each other. Can you think of how the motions of adjacent particles relative to each other might relate to the motion of the wave disturbance in each case?

(For example, why does the water surface go up and down? What motions within the water are required for this to happen?

It might help to imagine a narrow column extending from the water surface down to the bottom, with water able to move in and out of the column through the sides. How would (net) motions of water in and out of the column affect the up/down motion of the water surface?)