Let u(x,t) is a function, which describes vibration of string. So we can rewrite task in the next form:
∂ 2 u ∂ t 2 = b 2 ∂ 2 u ∂ x 2 \dfrac{\partial^2 u}{\partial t^2}=b^2\dfrac{\partial^2 u}{\partial x^2} ∂ t 2 ∂ 2 u = b 2 ∂ x 2 ∂ 2 u
u ( 0 , t ) = u ( 1 , t ) = 0 u(0,t)=u(1,t)=0 u ( 0 , t ) = u ( 1 , t ) = 0
u ( x , 0 ) = f ( x ) = a ( x − x 2 ) u(x,0)=f(x)=a(x-x^2) u ( x , 0 ) = f ( x ) = a ( x − x 2 )
∂ u ∂ t ∣ t = 0 = 0 \dfrac{\partial u}{\partial t}|\begin{matrix} \\ t=0\end{matrix}=0 ∂ t ∂ u ∣ t = 0 = 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) u ( x , t ) = X ( x ) ∗ T ( t )
Then,
T ′ ′ X = b 2 T X ′ ′ T''X=b^2TX'' T ′′ X = b 2 T X ′′
T ′ ′ b 2 T = X ′ ′ X = c o n s t = − λ 2 \dfrac{T''}{b^2T}=\dfrac{X''}{X}=const=-\lambda^2 b 2 T T ′′ = X X ′′ = co n s t = − λ 2
So we have two tasks:
1 ) X ′ ′ + λ 2 X = 0 1) X''+\lambda^2X=0 1 ) X ′′ + λ 2 X = 0
2 ) T ′ ′ + ( b λ ) 2 T = 0 2)T''+(b\lambda)^2T=0 2 ) T ′′ + ( bλ ) 2 T = 0
For the task 1) we have 2 boundary conditions:
u ( 0 , t ) = X ( 0 ) T ( t ) = 0 = > X ( 0 ) = 0 u(0,t)=X(0)T(t)=0=>X(0)=0 u ( 0 , t ) = X ( 0 ) T ( t ) = 0 => X ( 0 ) = 0
u ( 1 , t ) = X ( 1 ) T ( t ) = 0 = > X ( 1 ) = 0 u(1,t)=X(1)T(t)=0=>X(1)=0 u ( 1 , t ) = X ( 1 ) T ( t ) = 0 => X ( 1 ) = 0
The first task solution is:
X ( x ) = A c o s ( λ x ) + B s i n ( λ x ) X(x)=Acos(\lambda x)+Bsin(\lambda x) X ( x ) = A cos ( λ x ) + B s in ( λ x )
From the first boundary condition we have that A=0. The second give us:
X ( 1 ) = B s i n ( λ ) = 0 = > λ = π n = > X ( x ) = B s i n ( π n x ) X(1)=Bsin(\lambda)=0=>\lambda=\pi n=>X(x)=Bsin(\pi n x) X ( 1 ) = B s in ( λ ) = 0 => λ = πn => X ( x ) = B s in ( πn x )
But we know that:
∣ ∣ X ( x ) ∣ ∣ = 1 = > ||X(x)||=1=> ∣∣ X ( x ) ∣∣ = 1 =>
1 ∫ 0 ∣ X ( x ) ∣ 2 d x = 1 ∫ 0 B 2 s i n 2 ( π n x ) d x = B 2 ∗ 1 2 = 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 1 ∫ 0 ∣ X ( x ) ∣ 2 d x = 1 ∫ 0 B 2 s i n 2 ( πn x ) d x = B 2 ∗ 2 1 = 1
B = 2 B=\sqrt 2 B = 2
For the task 2) we have next solution:
T ( t ) = C c o s ( λ b t ) + D s i n ( λ b t ) = C c o s ( π b n t ) + D s i n ( π b n t ) T(t)=Ccos(\lambda bt) + Dsin(\lambda bt)=Ccos(\pi bn t)+Dsin(\pi b n t) T ( t ) = C cos ( λb t ) + Ds in ( λb t ) = C cos ( πbn t ) + Ds in ( πbn t )
The result of the main task is:
u ( x , t ) = 2 ∑ i = 1 ∞ ( C c o s ( π n b t ) + D s i n ( π n b t ) ) s i n ( π n x ) u(x,t)=\sqrt 2\displaystyle\sum_{i=1}^{\infty} (Ccos(\pi n b t) +Dsin(\pi nbt)) sin(\pi n x) u ( x , t ) = 2 i = 1 ∑ ∞ ( C cos ( πnb t ) + Ds in ( πnb t )) s in ( πn x )
But we also have two initial conditions. The first one:
u ( x , 0 ) = 2 ∑ i = 1 ∞ C s i n ( π n x ) = f ( x ) u(x,0)=\sqrt 2 \displaystyle\sum_{i=1}^{\infty} C sin(\pi nx)=f(x) u ( x , 0 ) = 2 i = 1 ∑ ∞ C s in ( πn x ) = f ( x )
Multiply both side by sin ( π k x ) \sin(\pi kx) sin ( πk x ) :
f ( x ) s i n ( π k x ) = 2 ∑ i = 1 ∞ C s i n ( π n x ) s i n ( π k x ) f(x)sin(\pi kx)=\sqrt 2\displaystyle\sum_{i=1}^{\infty} C sin(\pi nx)sin(\pi kx) f ( x ) s in ( πk x ) = 2 i = 1 ∑ ∞ C s in ( πn x ) s in ( πk x )
Now integrate both sides with respect to x:
∫ 0 1 f ( x ) s i n ( π k x ) d x = 2 ∫ 0 1 C s i n ( π n x ) s i n ( π k x ) d x \displaystyle\int_{0}^{1} f(x) sin(\pi kx)dx=\sqrt 2\displaystyle\int_{0}^{1} C sin(\pi nx)sin(\pi kx)dx ∫ 0 1 f ( x ) s in ( πk x ) d x = 2 ∫ 0 1 C s in ( πn x ) s in ( πk x ) d x
Because trigonomical function are ortogonal, the right side give us:
{ 0 if k = / n 1 / 2 if k = n \begin{cases} 0 &\text {if } k {=}\mathllap{/\,} n \\ 1/2 &\text {if } k = n \end{cases} { 0 1/2 if k = / n if k = n
So
C = 2 ∫ 0 1 f ( x ) s i n ( π n x ) d x C=\sqrt 2 \displaystyle\int_{0}^{1} f(x) sin(\pi nx)dx\\ C = 2 ∫ 0 1 f ( x ) s in ( πn x ) d x
C = 2 a ∫ 0 1 ( x − x 2 ) s i n ( π n x ) d x = − 2 a ( π n ) 3 c o s ( π n x ) ∣ 1 0 = − 2 a ( π n ) 3 ( c o s ( π n ) − 1 ) = 4 a ( π ( 2 n − 1 ) ) 3 C=\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} C = 2 a ∫ 0 1 ( x − x 2 ) s in ( πn x ) d x = − ( πn ) 3 2 a cos ( πn x ) ∣ 1 0 = − ( πn ) 3 2 a ( cos ( πn ) − 1 ) = ( π ( 2 n − 1 ) ) 3 4 a
The second initial condition:
∂ u ∂ t ∣ t = 0 = 2 π n a ∑ i = 1 ∞ D s i n ( π n x ) = 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 ∂ t ∂ u ∣ t = 0 = 2 πna i = 1 ∑ ∞ Ds in ( πn x ) = 0 => D = 0
So the answer is:
u ( x , t ) = 4 a 2 π 3 ∑ i = 1 ∞ 1 ( 2 n − 1 ) 3 c o s ( π ( 2 n − 1 ) b t ) s i n ( π ( 2 n − 1 ) 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) u ( x , t ) = π 3 4 a 2 i = 1 ∑ ∞ ( 2 n − 1 ) 3 1 cos ( π ( 2 n − 1 ) b t ) s in ( π ( 2 n − 1 ) x )
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