A tight uniform string with the length of 2.50 m is tied down at both ends.

Question

A tight uniform string with a length of 2.50 m is tied down at both ends and placed under a tension of 100 N. When it vibrates in its second-overtone mode the sound given off has a frequency of 75.0 Hz. What is the mass of the string?

Answer 1

Wikipedia is your friend:

http://en.wikipedia.org/wiki/Vibrating_s...

Note: the second overtone is the third harmonic and therefore at 3 times the frequency of the base note:

http://en.wikipedia.org/wiki/Overtone