### Author Topic: Web Bonus Problem to Week 7 (#1)  (Read 3426 times)

#### Victor Ivrii

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##### Web Bonus Problem to Week 7 (#1)
« on: October 26, 2015, 08:59:39 AM »
« Last Edit: October 26, 2015, 09:01:14 AM by Victor Ivrii »

#### Emily Deibert

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##### Re: Web Bonus Problem to Week 7 (#1)
« Reply #1 on: October 30, 2015, 05:32:29 PM »
We start by assuming a solution:

U(x,t) = X(x)T(x)

Then plugging this into the wave equation:

X(x)T''(t) - c^2T(t)X''(x) = 0

Dividing through by $U(x,t)$ gives:

\frac{T''(t)}{T(t)} = c^2\frac{X''(x)}{X(x)} = \lambda

In this case we will first consider $\lambda = \omega{}^2 > 0$. Then this gives us two ODEs. Solving for $T(t)$:

\frac{T''(t)}{c^2T(t)} = \omega{}^2 \rightarrow T''(t) - \omega{}^2c^2T(t) = 0

The characteristic equation is:

r^2 = \omega{}^2 = 0 \rightarrow r^2 = \omega{}^2 \rightarrow r = \pm \omega

We solve this to get the solution for $T(t)$:

T(t) = Ae^{c\omega{}t} + Be^{-c\omega{}t}

Likewise, we will have the following equation for $X(x)$:

\frac{X''(x)}{X(x)} = \omega{}^2

We can solve this to show that:

X(x) = Ce^{\omega{}x} + De^{-\omega{}x}

I will continue the rest of the problem soon, or anyone else can contribute if they want to.

#### Victor Ivrii

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##### Re: Web Bonus Problem to Week 7 (#1)
« Reply #2 on: November 01, 2015, 09:56:46 AM »
Hint: your $\lambda<0$ so $\lambda=-\omega^2$.

Hint: Look at boundary condition $(u_x+\alpha u_t)|_{x=0}=0$ which would imply that $T(t)$ is "some" exponent so $T=e^{\pm i\omega t}$. Investigate each sign.

#### Xi Yue Wang

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##### Re: Web Bonus Problem to Week 7 (#1)
« Reply #3 on: December 05, 2015, 07:47:37 PM »
So, let $$\frac{X''(x)}{X(x)} = -\lambda,\ \frac{T''(t)}{T(t)} = -c^2\lambda$$
Then we have,$$X''(x) + \lambda X(x) = 0,\ T''(t) + c^2\lambda T(t) = 0$$
And let $-\lambda = \omega^2$, We get $$X(x) = A\cos(\omega x) +B\sin(\omega x),\ T(t) = C\cos(c\omega t) + D\sin(c\omega t)$$
Given conditions that, $u_x(0,t) = 0,\ (u_x + i\alpha u_t)(l,t) = 0$
We have $$X'(0)T(t) = 0\\X'(0) = 0 = \omega B \\B = 0$$
Hence, $$X(x) = A\cos(\omega x)$$
$$X'(l)T(t) + i\alpha X(l)T'(t) = 0\\ -\omega A\sin(\omega l ) T(t) + i\alpha A\cos(\omega l)T'(t) = 0\\T'(t) = \frac{\omega\sin(\omega l)}{i\alpha\cos(\omega l)}T(t)\\ T'(t) = \frac{\omega}{i\alpha}\tan(\omega l)T(t)\\T'(t) = \frac{-i\omega}{\alpha}\tan(\omega l)T(t)$$
Solve this ODE, we get $$T(t) =Ke^{\frac{-i\omega t}{\alpha}\tan(\omega l)}$$