Standing waves
Wave speed is equal to the square root of tension divided by the linear density of the string.
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Linear density of the string is equal to the mass divided by the length of the string.
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The fundamental wavelength is equal to two times the length of the string.
Variables
| λ: wavelength (m) | 
Definition of terms
|  Tension (F): (not frequency) in the string (t is used for time in these equations). Units: newtons (N) | 
Fundamental frequency: the frequency when the wavelength is the longest allowed, this gives us the lowest sound that we can get from the system.
In a string, the length of the string is half of the largest wavelength that can create a standing wave, called its fundamental wavelength.