A tuning fork produces a sound of musical note middle C. It moves back and forth 256 time each second. what is the frequency of the tuning fork?
256 Hz. If you want to get confused read this site about "scientific pitch notation" and 261.6 Hz for middle C.
http://en.wikipedia.org/wiki/C_%28musical_note%29
To find the frequency of the tuning fork, we need to know the number of vibrations it completes in one second. In this case, the tuning fork moves back and forth 256 times each second.
So, the frequency of the tuning fork is 256 vibrations per second, or 256 Hertz (Hz).
To summarize:
Frequency of the tuning fork = 256 vibrations per second = 256 Hz