Fundamental frequency of violin string

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Homework Help Overview

The problem involves a vibrating violin string with an initial length of 330 mm and a fundamental frequency of 659 Hz. The task is to determine the new fundamental frequency when the string is pressed at a point 60 mm from its end, effectively shortening the vibrating length of the string.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the implications of shortening the string and how it affects the fundamental frequency. There is confusion about how to relate the original frequency to the new length of the string and whether the two lengths represent harmonics. Questions arise regarding the role of the original frequency in the calculations and the nature of nodes in the context of the problem.

Discussion Status

The discussion is ongoing, with participants exploring the relationship between the original and new string lengths and their frequencies. Some guidance has been offered regarding the need to use the original frequency to find the new fundamental frequency, but clarity on the mathematical derivation remains elusive.

Contextual Notes

Participants are grappling with the implications of the 60 mm shortening and the assumptions about the string's fixed nodes. There is a noted lack of consensus on how to mathematically derive the new frequency from the original parameters.

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Homework Statement



A vibrating string on a violin is 330 mm long and has a fundamental frequency of 659 Hz. What is its fundamental frequency when the string is pressed against the fingerboard at a point 60 mm from its end?

*The answer is 805 Hz

Homework Equations


f = [tex]\overline{}nv[/tex]/2L
wavelength = v/f

The Attempt at a Solution


I don't understand what to do with the 60mm. It splits the string into 2 unequal parts, which isn't a harmonic I recognize.
 
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I don't understand what to do with the 60mm. It splits the string into 2 unequal parts, which isn't a harmonic I recognize.
That's just telling you how much shorter the string is now. You are ignoring the 60mm part that is pinched off, and considering the length that remains. This is the length you are trying to find the fundamental frequency for.
 
hage567 said:
That's just telling you how much shorter the string is now. You are ignoring the 60mm part that is pinched off, and considering the length that remains. This is the length you are trying to find the fundamental frequency for.

Oh, so there's no node there? So (330-60) mm (=270) is the new length, L, of the string. What do you do with the original frequency they gave you?
By the way, this is a string with a fixed node on both ends, right?
 
Oh, so there's no node there? So (330-60) mm (=270) is the new length, L, of the string.
By the way, this is a string with a fixed node on both ends, right?
Yes, that's correct.
What do you do with the original frequency they gave you?

You will need it to figure out the fundamental frequency of the 270mm length of the string. :smile:

You have the equations you need to solve this. You just need to find a way to relate the first string to the second string.
 
hage567 said:
Yes, that's correct.
You just need to find a way to relate the first string to the second string.

That's what I don't understand--how are the two connected? They're not harmonics. I keep trying to use the formula f = nv/2L, but that doesn't work and it doesn't use the original frequency.
Intuitively, it seems that shortening the length of an already-vibrating string would increase the frequency, but I don't know how to derive this mathematically.
 
Oh ok, I got it--I assumed that the velocity was 343 m/s, but you have to use the original frequency to calculate it.
 

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