Question

explain the to two ways by which the pitch of the sound generated by a streched string or wire can be changed

Answer 1

If the parameters of the string or wire—the length, tension and mass—are correct, the sound made from plucking the string will be a musical note that is pleasing to the ear. But if they are slightly different, the sound may not be musical and just be a sound.

Harmonics in string

In most cases, the string will vibrate at the fundamental frequency or 1st harmonic. But if you pull the string harder, it can be made to vibrate with a shorter wavelength and higher frequency or the 2nd harmonic, 3rd harmonic or even higher.