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# Longitudinal Waves

Media Type:
Video

Running Time: 0m 12s
Size: 1.1 MB

or

### Collection Developed by:

Animations and essays were created and written by Dr. Michael Gallis, the Pennsylvania State University.

Teachers who reviewed the collection were Patrick Callahan, Tara Clopper, and David R. McCachren. Instructional designer was Rucha Modak. Multimedia producer was Betsy Hutton. Director of Educational Services, Penn State Public Broadcasting is Dr. Babs Bengtson.

### Collection Funded by:

This physics collection was funded by Penn State University Outreach's Thematic Initiative Fund.

A wave is some sort of physical disturbance that carries energy from one place to another without carrying matter. Sound and light are two very common waves we encounter that have some similarities and some important differences.

One important aspect of a type of wave is how the disturbance moves relative to the direction the wave is travelling. In this silent animation a hand compresses a slinky to start a compression or longitudinal wave, a type of wave where the disturbance lines up with the direction the wave travels.

Background Essay

A wave is some sort of physical disturbance that carries energy from one place to another without carrying matter. While the word wave often conjures up memories of water waves on a pond, in the ocean or at the pool, there are many other kinds of waves that we come across in everyday life. Sound and light are two very common waves we encounter that have some similarities and some important differences.

One important aspect of a type of wave is how the disturbance moves relative to the direction the wave is traveling. Imagine a long line of people waiting patiently in the cafeteria, and someone toward the rear of the line stumbles and bumps the person in front of them. The bumped person bumps into the person in front of them, and so on. The disturbance (the bumping of people) travels on up the line and the front back motion of each individual in the line is along the direction the disturbance travels. Furthermore, none of the bumpers get to change their position in line as the disturbance propagates to the front of the line. We can contrast our traveling bump wave with the type of wave we often see in the stands at sporting events, where people move up and down (the disturbance) but the wave travels sideways across the stands.

A wave where the disturbance is lined up with the direction the wave will travel is called a longitudinal wave or a compression wave. A good example of a compression wave is sound. Compressions waves can occur whenever there is material that will spring back when compressed, which includes solids, liquids and gasses. As a result, sound can travel though just about any kind of matter.

Earthquakes produce vibrations which create waves that can travel through the earth. Geologists call the compression waves which travel through the earth P-waves or pressure waves. Because the speed of the P-wave depends upon the materials the wave travels through, monitoring the vibrations created around the globe by an earthquake helps scientists determine the nature of the interior of the earth. Earthquakes in a sense allow scientists to take a sonogram of the interior of the earth!

Discussion Questions

• We often talk about “the speed of sound”. Do you think loud sound travels faster than a softer sound?
• Imagine you hold one end of a slinky or a coiled spring and a friend holds the other end to maintain some tension. You move your hand quickly towards your friend to create a disturbance which moves towards him along the slinky. Would the pulse travel down the slinky any faster if you move your hand through a greater distance? Would the pulse travel down the slinky any faster if you move your hand more quickly?
• Imagine you hold one end of a slinky or a coiled spring and a friend holds the other end to maintain some tension. You move your hand quickly towards your friend to create a disturbance which moves towards him along the slinky. Suppose the coil is made of a heavier metal. Would the pulse travel down the coil any faster or slower? Now suppose the coil is made with a stiffer spring. How do you think the stiffness of the spring would affect the speed of the pulse? Now consider a longitudinal wave traveling though water and air. Would they travel at the same rate?

Standards

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