- #1
yungman
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This is not homework, I need someone to verify (9) and (10) below whether I am correct because I don't have the answer. I need to find the solution of the following:
Question
Find the solution of the equation for damping vibration string stretched from x=0 to x=L where
[tex]\frac{\partial ^2 u}{\partial t^2} \;+\; 2k\frac{\partial u}{\partial t} \;=\; c^2\frac{\partial ^2 u}{\partial x^2} \;\;\;\;\; (1)[/tex]
With boundary and initial condition given:
[tex] u(0,t) = u(L,t)= 0 \;\;\; (2) [/tex]
[tex] u(x,0) = f(x)\; ,\; \frac{\partial u}{\partial t}(x,0) = g(x) \;\;\;\;(3)[/tex]
We assume [tex]u(x,t)=X_{(x)}T_{(t)} \;\Rightarrow\; X''+\mu^2X=0 \;\;and\;\; T'' \;+\; 2kT' \;+\; (\mu c)^2 T \;=\; 0 [/tex]
Attempted steps:
[tex]1)\;\; X''+\mu^2X=0 \Rightarrow \mu=\mu_n = \frac{n\pi}{L} \;\;\;\Rightarrow\;\;\; X \;=\; X_n \;=\; sin(\frac{n\pi}{L})x \;\;\;\;n=1,2,3...(4)[/tex]
[tex]2)\;\; T'' \;+\; 2kT' \;+\; (\frac{n\pi}{L}c)T \;=\; 0\;\; \;\;(5)[/tex] Using constant coeficients ODE, [tex]m=\frac{-2k^+_-\sqrt{4k^2-4(\frac{n\pi}{L}c)^2}}{2}[/tex]
Three cases of [tex]k^2-(\frac{n\pi}{L}c)^2 \;\;\;\;Let\;\lambda_n =\sqrt{|k^2-(\frac{n\pi}{L}c)^2|} [/tex]
[tex]Case \;1\;\;\;\;k^2-(\frac{n\pi}{L}c)^2\;< 0 \Rightarrow\; n > (\frac{kL}{\pi c}) \;\;\;\Rightarrow\;\; \; T= e^{-kt}[c_n cos(\lambda_n t) \;+\; d_n sin(\lambda_n t)]\;\;(6)[/tex]
[tex]Case \;2\;\;\;k^2-(\frac{n\pi}{L}c)^2\;= 0 \Rightarrow\; n = (\frac{kL}{\pi c}) \;\;\;\Rightarrow\;\; \; T= h_{n=\frac{kL}{\pi c}}e^{-kt} \;+\; j_{n=\frac{kL}{\pi c}}te^{-kt} \;\;\;\;(7)[/tex]
[tex]Case \;3\;\;\;\;k^2-(\frac{n\pi}{L}c)^2\;> 0 \Rightarrow\; n < (\frac{kL}{\pi c}) \;\;\;\Rightarrow\;\; \; T= e^{-kt}[a_n cosh(\lambda_n t) \;+\; b_n sinh(\lambda_n t)] \;\;\;(8)[/tex]
For n=1,2,3...
Solution of wave equation:
3)For [tex](\frac{kL}{\pi c}) [/tex] is not a possitive integer:
[tex]\Rightarrow u(x,t) = e^{-kt} \sum_{n=1} ^{n < (\frac{kL}{\pi c})} \; sin(\frac{n\pi}{L})x[a_n cosh(\lambda_n t) \;+\; b_n sinh(\lambda_n t)] \;+\; e^{-kt} \sum_{(\frac{kL}{\pi c})<n} ^{\infty} \; sin(\frac{n\pi}{L})x[c_n cos(\lambda_n t) \;+\; d_n sin(\lambda_n t)] \;\;\;(9)[/tex]
4)For [tex] (\frac{kL}{\pi c}) [/tex] is a possitive integer:
We add the term where[tex] n= (\frac{kL}{\pi c}) \;\Rightarrow\; sin(\frac{k}{c}x)[ h_{n=\frac{kL}{\pi c}}e^{-kt} \;+\; j_{n=\frac{kL}{\pi c}}te^{-kt} ] [/tex] to (9) above.
[tex]\Rightarrow u(x,t) = e^{-kt} \sum_{n=1} ^{n < (\frac{kL}{\pi c})} \; sin(\frac{n\pi}{L})x[a_n cosh(\lambda_n t) \;+\; b_n sinh(\lambda_n t)] \;+\; sin(\frac{k}{c}x)[ h_{n=\frac{kL}{\pi c}}e^{-kt} \;+\; j_{n=\frac{kL}{\pi c}}te^{-kt} ] \;[/tex]
[tex].\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;+\; e^{-kt} \sum_{(\frac{kL}{\pi c})<n} ^{\infty} \; sin(\frac{n\pi}{L})x[c_n cos(\lambda_n t) \;+\; d_n sin(\lambda_n t)] \;\;\;(10)[/tex]
My question:
1) k is the attenuation constant?
2) Bottom line the solution depend on [tex] (\frac{kL}{\pi c}) [/tex] and solution can be combination of all three cases shown above?
3) Is it true solution require separation constant [tex]-\mu ^2\;<0\;?[/tex] Because if not, we only get trivial solution?
Question
Find the solution of the equation for damping vibration string stretched from x=0 to x=L where
[tex]\frac{\partial ^2 u}{\partial t^2} \;+\; 2k\frac{\partial u}{\partial t} \;=\; c^2\frac{\partial ^2 u}{\partial x^2} \;\;\;\;\; (1)[/tex]
With boundary and initial condition given:
[tex] u(0,t) = u(L,t)= 0 \;\;\; (2) [/tex]
[tex] u(x,0) = f(x)\; ,\; \frac{\partial u}{\partial t}(x,0) = g(x) \;\;\;\;(3)[/tex]
We assume [tex]u(x,t)=X_{(x)}T_{(t)} \;\Rightarrow\; X''+\mu^2X=0 \;\;and\;\; T'' \;+\; 2kT' \;+\; (\mu c)^2 T \;=\; 0 [/tex]
Attempted steps:
[tex]1)\;\; X''+\mu^2X=0 \Rightarrow \mu=\mu_n = \frac{n\pi}{L} \;\;\;\Rightarrow\;\;\; X \;=\; X_n \;=\; sin(\frac{n\pi}{L})x \;\;\;\;n=1,2,3...(4)[/tex]
[tex]2)\;\; T'' \;+\; 2kT' \;+\; (\frac{n\pi}{L}c)T \;=\; 0\;\; \;\;(5)[/tex] Using constant coeficients ODE, [tex]m=\frac{-2k^+_-\sqrt{4k^2-4(\frac{n\pi}{L}c)^2}}{2}[/tex]
Three cases of [tex]k^2-(\frac{n\pi}{L}c)^2 \;\;\;\;Let\;\lambda_n =\sqrt{|k^2-(\frac{n\pi}{L}c)^2|} [/tex]
[tex]Case \;1\;\;\;\;k^2-(\frac{n\pi}{L}c)^2\;< 0 \Rightarrow\; n > (\frac{kL}{\pi c}) \;\;\;\Rightarrow\;\; \; T= e^{-kt}[c_n cos(\lambda_n t) \;+\; d_n sin(\lambda_n t)]\;\;(6)[/tex]
[tex]Case \;2\;\;\;k^2-(\frac{n\pi}{L}c)^2\;= 0 \Rightarrow\; n = (\frac{kL}{\pi c}) \;\;\;\Rightarrow\;\; \; T= h_{n=\frac{kL}{\pi c}}e^{-kt} \;+\; j_{n=\frac{kL}{\pi c}}te^{-kt} \;\;\;\;(7)[/tex]
[tex]Case \;3\;\;\;\;k^2-(\frac{n\pi}{L}c)^2\;> 0 \Rightarrow\; n < (\frac{kL}{\pi c}) \;\;\;\Rightarrow\;\; \; T= e^{-kt}[a_n cosh(\lambda_n t) \;+\; b_n sinh(\lambda_n t)] \;\;\;(8)[/tex]
For n=1,2,3...
Solution of wave equation:
3)For [tex](\frac{kL}{\pi c}) [/tex] is not a possitive integer:
[tex]\Rightarrow u(x,t) = e^{-kt} \sum_{n=1} ^{n < (\frac{kL}{\pi c})} \; sin(\frac{n\pi}{L})x[a_n cosh(\lambda_n t) \;+\; b_n sinh(\lambda_n t)] \;+\; e^{-kt} \sum_{(\frac{kL}{\pi c})<n} ^{\infty} \; sin(\frac{n\pi}{L})x[c_n cos(\lambda_n t) \;+\; d_n sin(\lambda_n t)] \;\;\;(9)[/tex]
4)For [tex] (\frac{kL}{\pi c}) [/tex] is a possitive integer:
We add the term where[tex] n= (\frac{kL}{\pi c}) \;\Rightarrow\; sin(\frac{k}{c}x)[ h_{n=\frac{kL}{\pi c}}e^{-kt} \;+\; j_{n=\frac{kL}{\pi c}}te^{-kt} ] [/tex] to (9) above.
[tex]\Rightarrow u(x,t) = e^{-kt} \sum_{n=1} ^{n < (\frac{kL}{\pi c})} \; sin(\frac{n\pi}{L})x[a_n cosh(\lambda_n t) \;+\; b_n sinh(\lambda_n t)] \;+\; sin(\frac{k}{c}x)[ h_{n=\frac{kL}{\pi c}}e^{-kt} \;+\; j_{n=\frac{kL}{\pi c}}te^{-kt} ] \;[/tex]
[tex].\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;+\; e^{-kt} \sum_{(\frac{kL}{\pi c})<n} ^{\infty} \; sin(\frac{n\pi}{L})x[c_n cos(\lambda_n t) \;+\; d_n sin(\lambda_n t)] \;\;\;(10)[/tex]
My question:
1) k is the attenuation constant?
2) Bottom line the solution depend on [tex] (\frac{kL}{\pi c}) [/tex] and solution can be combination of all three cases shown above?
3) Is it true solution require separation constant [tex]-\mu ^2\;<0\;?[/tex] Because if not, we only get trivial solution?
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