Explain why guitar strings of the same length have different pitches.

Because they are made of diffrent materials which produce diffrent tunes when strung

Longer strings produce a lower tone than shorter ones ... Thicker strings produce a lower sound than thinner strings. That is why, even though all the strings on a guitar are the same length, they all sound a different note. String instruments can be plucked, bowed, or in the case of the piano, struck.