Physics/A/String vibration
“Among vibrating bodies there are none that occupy a more prominent position than Stretched Strings. From the earliest times they have been employed for musical purposes ... . To the mathematician they must always possess a peculiar interest as a battle-field on which were fought out the controversies of D’Alembert, Euler, Bernoulli, and Lagrange relating to the nature of the solutions of partial differential equations. To the student of Acoustics they are doubly important." --"Lord Rayleigh:[32, Vol. I, Chap. VI].[1]
openstax.org/books/university-physics-volume-1/pages/5-5-newtons-third-law
Intro edit
This discussion was motivated by the fact that the flow of energy associated with Poynting vector does not seem physical. This is not the only case of a conservation law seems to leave unanswered questions about the details. Here we consider the conservation of energy in a stretched string. In this case, there is a unique answer to the question, "where is the potential energy". What is odd is how difficult that question is to answer. Rowland published what seems to be acceptable but approximate solutions for some special cases.
DEBUG edit
This discussion focuses on making a subset of those approximate solutions more accessible to novice lovers of physics.
It is worth pointing out that stax exchange seems to be wrong on this topic: https://physics.stackexchange.com/questions/414521/does-a-vibrating-string-produce-changes-in-tension-in-the-tring
Three tricks: edit
XXX
First derivative revisited edit
Most readers have probably seen a definition of the derivative that can be found at:
Second derivative edit
w:Finite difference coefficient
Let "1" be a small parameter edit
- Let
Calculation of force in Figure 1 edit
introducing kappa as spring constant edit
Used https://www.symbolab.com/
3 edit
-
shownbelow
(?)
algebra for 3 edit
Extended content
|
---|
x1: equals minus x2: x3: x4: y1: equals minus y2: y3: y4 |
Constructing the wave equation edit
Here we adopt the convention that unit of length. We label masses with the variable that represents each mass by an integer . To obtain a wave equation we focus on the three consecutive integers, . When the string is at equilibrium (i.e., zero wave amplitude), we can also use non-integral values of to form a coordinate system that labels points in space between the masses, as shown in the top of Figure 2.
For non-zero wave amplitude, each mass can move away from its equilibrium point by in the x direction and in the y-direction (Rowland, et al, use the symbol to describe motion in the other transverse direction.)
Defining to be the vector associated with this displacement, we have,
We denote
The convention used by OpenStax Physics[2] is that refers to the force on object A by object B. To keep the the notation in Figure 2 compact, we define the displacement vector from A to B as:
<math></math>
Wave equation edit
Leave as exercise for the readers to verify the Table (with both compact and PDE forms). And also to realte X to x.
Equation for kappas and a edit
Define => =>
UNDER CONSTRUCTION: Allowing ℓ≠1 edit
The dimensional analysis conventions introduced in OpenStax University Physics permit us to show that () and () are equivalent to results obtained in reference <sub><big><big>
DEBUG3 edit
From from reference RowlandEJP:
Symbolic computation probably renders this exercise unnecessary, but one way to "guess" the wave equation when is to use dimensional analysis, though if you want certainty it might be better to repeat all the steps with the extra term included. Using
Wave-wave interactions edit
Product rule edit
From Phasor_algebra:
<math></math>
problem edit
Variables edit
* position with unit vectors
* coordinate variable parallel to string
Equilibrium
* spring constant
*relaxed spring length
* equilibrium spring length (no wave present)
* equilibrium tension in string
* linear mass density at equilibrium
*
* transverse wave speed
*longitudinal wave speed
Perturbation
* x-deviation from equilibrium (longitudinal)
* y-deviation from equilibrium (transverse)
* energy density (kinetic+potential) kappa is mine. I don't like their e or k_e.
Avoid
* zeta is used for third dimension (polarized waves)
*
*
Footnotes edit
- Need to look at: P. M. Morse and K. U. Ingard, Theoretical Acoustics ~McGraw–Hill, New York, 1968 referenced in Rowland 1999.
Appendix edit
elsewhere
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