# Beginner Physics - Explaining longitudinal waves

I am having difficulty grasping the concept of a longitudinal wave. My textbook definition "In longitudinal waves, the vibration is backwards and forwards in the direction of motion of the wavefront"

If it vibrates backwards and then forwards, would it not be in the same position it originally was?

Do we assume it vibrates forward at a rate faster than that at which it vibrates backwards?

Further, what am I supposed to 'visualize' when I think of waves, in a physics sense?

What is the purpose of a longitudinal wave?

Sorry if I am asking a lot of questions,

• Read up on: en.wikipedia.org/wiki/Longitudinal_wave It has animations to explain it. If things aren't clear there, maybe as more specific questions about what you don't understand. Feb 1, 2014 at 4:03
• To clear one thing up, the material in the longitudinal wave does keep its original position (except for small vibrations). It is the pattern of disturbance in position that moves. Feb 1, 2014 at 4:28
• @NowIGetToLearnWhatAHeadIs What do you mean by pattern of disturbance? please elaborate Feb 1, 2014 at 4:29
• youtube.com/watch?v=y7qS6SyyrFU Feb 1, 2014 at 7:24

There are two different but connected motions one can speak of when discussing pulse and wave propagation. I suspect you may not have a clear separation of these two ideas.

One type of motion is the disturbance motion or wave motion. When you watch a wave move, this is typically what catches your eye. If you see a wave and you watch it travel, you're watching the disturbance travel. There's not actually a physical object that travels, but it's the disturbance itself that does.

Another type of motion is the particle motion. This is the motion that each individual bit of material is undergoing. Imagine you tie a small bit of string to a slinky. The particle motion is what the bit of string does.

The key idea is that for a longitudinal wave, the particle motion will be "forward and backward" while the motion of the wave will only be "forward".

Take a look at this video of a longitudinal wave. Try to identify the two different types of motion. You'll see the backward and forward motion of the "particles", as well as the forward-only motion of the disturbances.

If you're still unsure about this, that video link ends with a transverse wave. In those waves, the particle motion is actually perpendicular to the wave motion. If the wave is moving to the right, the particles can be moving up and down. This might actually be an easier example for separating the two types of motion.

A traditional longitudinal wave (in water, air) will travel through a medium, matter (molecules). The wave itself is a propagation of energy, So the particles move forward and back and transmit the energy via vibration. So the molecules vibrate forward and back, but remain more or less (in average) in the same place.