Horizontal propagated movement of mechanical wave

[bolzano95]

Homework Statement

A hand induces a transverse wave in a string by periodically moving up and down. This causes the string to move up and down. This movement propagates through the string producing a series of wavefronts which move towards the fixed wall with a velocity v.

How do we physically explain why is this movement propagated horizontally? I was asked this on exam and didn't know. My professor later explained to me that it has to do with forces in a string, but I didn't quite understand him.

Homework Equations

The Attempt at a Solution

I was looking for explanation, but couldn't find it. It seems everyone takes this for granted, but I'm really interested in physical explanation.

 

[kuruman]

Reason it out. If you move your hand holding the string straight up and down, does the entire string move straight up and down in concert with your hand? Why not?