How do sound vibrations create music?

Science and music

Today, a member of class 7, brought his harp into school for science to help us understand how sound vibrations work on musical instruments. 
He said "The harp is the only instrument which uses the length of the string to control  
the pitch and duration of the note that you are playing."  
He then played the well-known French song 'Frere Jaques' on his beautiful sounding harp. When he was done, he kindly let the other members of the class have a go at strumming the harp. 
You could see the vibrations but they were so fast that they were just blurs.

By Emma and Felix.