What physics does this BVP represent?

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Discussion Overview

The discussion revolves around the boundary value problem (BVP) represented by the equation $$u''(x) = -\lambda u(x) : u(0)=u(1) = 0$$. Participants explore the physical implications of this equation, its relation to oscillatory systems, and the nature of boundary conditions required to solve it. The conversation touches on theoretical aspects, practical applications, and the distinction between boundary value problems and initial value problems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that the BVP represents an oscillating system similar to a one-dimensional spring, relating it to Hooke's law.
  • Others argue that the boundary conditions leave the amplitude of the oscillation arbitrary, raising questions about how to determine it.
  • A participant suggests that specifying the function u and its first derivative at a given value of x could serve as a physical boundary condition, but notes that this would convert the problem into an initial value problem (IVP).
  • There is a discussion about the distinction between boundary value problems and initial value problems, with examples provided to illustrate the differences.
  • Some participants express uncertainty about whether the BVP models a physical system, questioning the appropriateness of the boundary conditions in the context of a mass-spring system.
  • A later reply introduces the idea of design constraint boundary conditions from an engineering perspective, suggesting that the problem may be framed as either a BVP or IVP depending on the available information.
  • One participant provides a hypothetical scenario involving a mass damper for a building, illustrating practical applications of the concepts discussed.

Areas of Agreement / Disagreement

Participants generally agree on the oscillatory nature of the system described by the BVP, but there is no consensus on the appropriateness of the boundary conditions or whether the problem should be treated as a BVP or IVP. Multiple competing views remain regarding the physical interpretation and implications of the problem.

Contextual Notes

Participants express uncertainty about the implications of the boundary conditions and their relation to physical systems. The discussion highlights the dependence on definitions and the context in which the problem is framed.

Who May Find This Useful

This discussion may be of interest to students and professionals in physics, engineering, and applied mathematics, particularly those exploring boundary value problems and their applications in oscillatory systems.

member 428835
Hi PF!

What physics does this BVP represent:

$$u''(x) = -\lambda u(x) : u(0)=u(1) = 0$$

Also, I know a general solution is ##u = \sum_{n=1} c_n \sin(n\pi x)##. There is no way of determining ##c_n##; does that mean this problem is ill-posed? I ask about the physics because I'm wondering if there is another equation (depending on what physics this is) that could close the problem.
 
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It is an oscillator, but the boundary conditions leave the amplitude arbitrary.
 
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Thanks for the responses! So what is a physical BC to determine the amplitude?
 
One physical BC is to specify the function u and its first derivative u' at a given value of x. For example,
u(0) = A, u'(0) = 0 will do the job.
 
Chandra Prayaga said:
One physical BC is to specify the function u and its first derivative u' at a given value of x. For example,
u(0) = A, u'(0) = 0 will do the job.
But then it's an IVP and not a BVP.
 
Here's the distinction:

Boundary value problems are similar to initial value problems. A boundary value problem has conditions specified at the extremes ("boundaries") of the independent variable in the equation whereas an initial value problem has all of the conditions specified at the same value of the independent variable (and that value is at the lower boundary of the domain, thus the term "initial" value).

For example, if the independent variable is time over the domain [0,1], a boundary value problem would specify values for {\displaystyle y(t)}
397de1edef5bf2ee15c020f325d7d781a3aa7f50
at both {\displaystyle t=0}
43469ec032d858feae5aa87029e22eaaf0109e9c
and {\displaystyle t=1}
970dea4a5f5ec5355c4cdd62f6396fbc8b1baaa1
, whereas an initial value problem would specify a value of {\displaystyle y(t)}
397de1edef5bf2ee15c020f325d7d781a3aa7f50
and {\displaystyle y'(t)}
ac415aa71b96af9b4e78aea31eff4ba122383095
at time {\displaystyle t=0}
43469ec032d858feae5aa87029e22eaaf0109e9c
.

Finding the temperature at all points of an iron bar with one end kept at absolute zero and the other end at the freezing point of water would be a boundary value problem.

from the wikipedia article:

https://en.wikipedia.org/wiki/Boundary_value_problem
 
jedishrfu said:
Here's the distinction:
from the wikipedia article:

https://en.wikipedia.org/wiki/Boundary_value_problem
Right, so we agree? Also, the oscillating system has time as the independent variable, so initial conditions are used. With boundary conditions, we have ##x## (space) as the independent variable. Do you know if this models any physical system?
 
Yes, your equations model an ideal simple spring sliding back and forth (ie no friction) as long as it stay within the limits and acts according to Hooke's law otherwise all bets are off and the system will either become chaotic or damper out.
 
  • #10
jedishrfu said:
Yes, your equations model an ideal simple spring sliding back and forth (ie no friction) as long as it stay within the limits and acts according to Hooke's law otherwise all bets are off and the system will either become chaotic or damper out.
But then the boundary conditions I impose, those are actually two different moments in time, right? Isn't there a bending beam that behaves similarly, or am I remembering something wrong?

I understand that a mass-spring system is modeled by the ODE, but the BC don't make sense to me in that context (seems an IVP is more appropriate).
 
  • #11
What if an engineer came to you with design constraint boundary conditions for the spring thing?

You may be trying to pidgeon hole this problem too much. Some problem can be described as IVP and at other times as BVP depending on what info you have available to solve it.
 
  • #12
jedishrfu said:
What if an engineer came to you with design constraint boundary conditions for the spring thing?
Can you elaborate please?
 
  • #13
I need a mass damper for a building with the following constraints. I don't want it to sway more than 10 feet in any direction. I want the mass damper to respond to changes as small as X...

https://en.wikipedia.org/wiki/Tuned_mass_damper

This is a made-up example I'm sure the engineers at PF will laugh and provide a much better example.
 
  • #14
Thanks!
 

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