1. A wave whose particles vibrate perpendicularly to the direction it is traveling is called transverse wave. Here is my question, in the mechanical waves, I am sort of confused with 'wave' and 'particles in the medium'? Which one is really vibrating? It is said that sine waves describe particles vibrating with simple harmonic motion. I don't know what's the relationship between wave and particles. For example, a pebble is dropped into a pond, and particles in the medium, water molecules move in vertical circles as waves pass. What's the relations betweeen water molecules and waves? 2. Is that the difference between 'pulse wave' and 'periodic wave' is one consists of only one pulse, the other one consists of many pulses, just like a sine wave? Thanks for helping me.