Longitudinal Wave 2


With a longitudinal wave the motion of the medium, often called the disturbance, is moving parallel to the motion of the wave. The motion of the wave is called its propagation. See notes below.

 

In the animation that precedes this one we imagined sending a wave pulse down a spring. That animation showed only one tiny section of the spring. Here, in this animation, imagine the several yellow boxes to each be a tiny section along the length of the spring.

Also, here we imagine a continuous group of pulses being sent down the spring. One would need to shake the spring horizontally to do this, at first compressing the spring and then pulling back and stretching the spring, then compressing again, and then stretching again, and so on. This would be difficult to do manually. However, we could imagine a machine performing this motion.

You should be able to visualize the wave shape moving from the left to the right. This wave shape will probably be best seen by following the spacing between the yellow boxes. Follow the location where the boxes are stretched apart or follow the location where they are compressed closely. There is a tendency to set your mind on the motion of only one box. Resist that tendency and see the stretches, called rarefactions, and the compressions between the boxes as they move along.

Each small section of spring moves left and right with horizontal simple harmonic motion. Each horizontal simple harmonic motion is slightly shifted from its neighbors. The combined horizontal simple harmonic motion of each small section generates the horizontal wave motion.

The wave moves from the left to the right horizontally, and the medium also moves left and right.

This is the central concept to the definition of a longitudinal wave.

A longitudinal wave is a wave with the motion of the medium being parallel to the motion of the wave. More carefully, we would say that the direction of the velocity of the medium is parallel to the direction of the velocity of the wave.

Often, when everyone in the conversation understands the details, we simply say that a longitudinal wave is a wave with the disturbance parallel to the propagation.



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