When the particles of a medium move back and forth along the same direction as the wave motion, it is called a longitudinal wave.

Here's why:

* Longitudinal waves involve the particles of the medium vibrating parallel to the direction the wave travels. Think of a spring: when you compress one end, the compression travels down the spring as individual coils move back and forth in the same direction the compression is moving.

* Transverse waves, on the other hand, have particles that vibrate perpendicular to the direction of wave motion. Imagine a rope tied to a wall. If you shake it up and down, the wave will travel horizontally along the rope, but the rope itself moves up and down.

Examples of longitudinal waves:

* Sound waves: Sound travels through air as compressions and rarefactions, which are changes in pressure.

* Seismic P-waves: These waves are the first to arrive during an earthquake and cause the ground to move back and forth.

Let me know if you have any more questions!