Question #109632
Find the deflection of vibration of string of unit length having fixed ends with initial velocity zero and initial deflection f(x)=a (x-x2)
1
Expert's answer
2020-04-15T13:35:47-0400

Let u(x,t) is a function, which describes vibration of string. So we can rewrite task in the next form:

2ut2=b22ux2\dfrac{\partial^2 u}{\partial t^2}=b^2\dfrac{\partial^2 u}{\partial x^2}

u(0,t)=u(1,t)=0u(0,t)=u(1,t)=0

u(x,0)=f(x)=a(xx2)u(x,0)=f(x)=a(x-x^2)

utt=0=0\dfrac{\partial u}{\partial t}|\begin{matrix} \\ t=0\end{matrix}=0

To solve this task I use a Fourier method.

Let:

u(x,t)=X(x)T(t)u(x,t)=X(x)*T(t)

Then,

TX=b2TXT''X=b^2TX''

Tb2T=XX=const=λ2\dfrac{T''}{b^2T}=\dfrac{X''}{X}=const=-\lambda^2

So we have two tasks:

1)X+λ2X=01) X''+\lambda^2X=0

2)T+(bλ)2T=02)T''+(b\lambda)^2T=0

For the task 1) we have 2 boundary conditions:

u(0,t)=X(0)T(t)=0=>X(0)=0u(0,t)=X(0)T(t)=0=>X(0)=0

u(1,t)=X(1)T(t)=0=>X(1)=0u(1,t)=X(1)T(t)=0=>X(1)=0

The first task solution is:

X(x)=Acos(λx)+Bsin(λx)X(x)=Acos(\lambda x)+Bsin(\lambda x)

From the first boundary condition we have that A=0. The second give us:

X(1)=Bsin(λ)=0=>λ=πn=>X(x)=Bsin(πnx)X(1)=Bsin(\lambda)=0=>\lambda=\pi n=>X(x)=Bsin(\pi n x)

But we know that:

X(x)=1=>||X(x)||=1=>

10X(x)2dx=10B2sin2(πnx)dx=B212=1\begin{matrix} 1\\ \int \\ 0 \end{matrix} |X(x)|^2 dx=\begin{matrix} 1\\ \int \\ 0 \end{matrix} B^2sin^2(\pi nx)dx= B^2 * \dfrac{1}{2}=1

B=2B=\sqrt 2

For the task 2) we have next solution:

T(t)=Ccos(λbt)+Dsin(λbt)=Ccos(πbnt)+Dsin(πbnt)T(t)=Ccos(\lambda bt) + Dsin(\lambda bt)=Ccos(\pi bn t)+Dsin(\pi b n t)

The result of the main task is:

u(x,t)=2i=1(Ccos(πnbt)+Dsin(πnbt))sin(πnx)u(x,t)=\sqrt 2\displaystyle\sum_{i=1}^{\infty} (Ccos(\pi n b t) +Dsin(\pi nbt)) sin(\pi n x)

But we also have two initial conditions. The first one:

u(x,0)=2i=1Csin(πnx)=f(x)u(x,0)=\sqrt 2 \displaystyle\sum_{i=1}^{\infty} C sin(\pi nx)=f(x)

Multiply both side by sin(πkx)\sin(\pi kx):

f(x)sin(πkx)=2i=1Csin(πnx)sin(πkx)f(x)sin(\pi kx)=\sqrt 2\displaystyle\sum_{i=1}^{\infty} C sin(\pi nx)sin(\pi kx)

Now integrate both sides with respect to x:

01f(x)sin(πkx)dx=201Csin(πnx)sin(πkx)dx\displaystyle\int_{0}^{1} f(x) sin(\pi kx)dx=\sqrt 2\displaystyle\int_{0}^{1} C sin(\pi nx)sin(\pi kx)dx

Because trigonomical function are ortogonal, the right side give us:

{0if k=/n1/2if k=n\begin{cases} 0 &\text {if } k {=}\mathllap{/\,} n \\ 1/2 &\text {if } k = n \end{cases}

So

C=201f(x)sin(πnx)dxC=\sqrt 2 \displaystyle\int_{0}^{1} f(x) sin(\pi nx)dx\\

C=2a01(xx2)sin(πnx)dx=2a(πn)3cos(πnx)10=2a(πn)3(cos(πn)1)=4a(π(2n1))3C=\sqrt 2 a \displaystyle\int_{0}^{1} (x-x^2) sin(\pi nx)dx = -\dfrac{2a}{(\pi n)^3}cos(\pi nx)|\begin{matrix} 1 \\ 0 \end{matrix} =-\dfrac{2a}{(\pi n)^3}(cos(\pi n)-1)=\dfrac{4a}{(\pi (2n-1))^3}

The second initial condition:

utt=0=2πnai=1Dsin(πnx)=0=>D=0\dfrac{\partial u}{\partial t}|\begin{matrix} \\ t=0\end{matrix} = \sqrt 2 \pi na\displaystyle\sum_{i=1}^{\infty} Dsin(\pi n x)=0=>D=0

So the answer is:

u(x,t)=4a2π3i=11(2n1)3cos(π(2n1)bt)sin(π(2n1)x)u(x,t)=\dfrac{4a\sqrt 2}{\pi ^3}\displaystyle\sum_{i=1}^{\infty} \dfrac{1}{(2n-1)^3}cos(\pi (2n-1) b t) sin(\pi (2n-1) x)


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