The OCC forum is a place where IB teachers can post questions and have discussions about IB subjects and procedures. Over the past couple of days there has been an interesting discussion about wave propagation. Why is it that when a pulse is travelling along a rope it only goes forward and not in both directions? If you lift the sting up into a pulse shape and let it go then a wave goes both ways. If you take a snapshot of a pulse travelling along a string it looks the same as the lifted string but it’s not, one side of the pulse is moving up the other down but when the lifted string is released both sides move down.  Made a simulation of the lifted string to show this, the moving wave you can simulate in the Phet waves in a string applet. One interesting feature of the discussion was the different ways contributors explained the same thing, some focusing on the mathematical equations others on the actual motion. I’m making it sound like there were a lot of contributors in fact there were only 4.