# What physics does this BVP represent?

Gold Member
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.

jedishrfu
Mentor
• joshmccraney and fresh_42
Chandra Prayaga
It is an oscillator, but the boundary conditions leave the amplitude arbitrary.

• joshmccraney
Gold Member
Thanks for the responses! So what is a physical BC to determine the amplitude?

Chandra Prayaga
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.

Gold Member
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.

jedishrfu
Mentor
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)} at both {\displaystyle t=0} and {\displaystyle t=1} , whereas an initial value problem would specify a value of {\displaystyle y(t)} and {\displaystyle y'(t)} at time {\displaystyle t=0} .

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

Gold Member
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?

jedishrfu
Mentor
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.

Gold Member
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).

jedishrfu
Mentor
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.

Gold Member
What if an engineer came to you with design constraint boundary conditions for the spring thing?

jedishrfu
Mentor
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.

Gold Member
Thanks!