inhomogenous wave equation with initial value; PDE












0












$begingroup$



Find the general solution of the inhomogeneous wave equation
$$u_{tt} − c^2u_{xx} = sin t$$
What is the solution to the initial-value problem for this equation with $u(x, 0) = 0$ and $u_t(x, 0) = 1$?




I know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$ and I know how to solve the initial value problem from there. It's the inhomogenous part of this equation that is confusing me.



Thanks in advance!










share|cite|improve this question











$endgroup$












  • $begingroup$
    Consider $u(x,t) = v(x,t) + 2 t - sin(t)$ to obtain a much simpler initial value problem for $v$.
    $endgroup$
    – Christoph
    Jan 30 at 4:15
















0












$begingroup$



Find the general solution of the inhomogeneous wave equation
$$u_{tt} − c^2u_{xx} = sin t$$
What is the solution to the initial-value problem for this equation with $u(x, 0) = 0$ and $u_t(x, 0) = 1$?




I know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$ and I know how to solve the initial value problem from there. It's the inhomogenous part of this equation that is confusing me.



Thanks in advance!










share|cite|improve this question











$endgroup$












  • $begingroup$
    Consider $u(x,t) = v(x,t) + 2 t - sin(t)$ to obtain a much simpler initial value problem for $v$.
    $endgroup$
    – Christoph
    Jan 30 at 4:15














0












0








0





$begingroup$



Find the general solution of the inhomogeneous wave equation
$$u_{tt} − c^2u_{xx} = sin t$$
What is the solution to the initial-value problem for this equation with $u(x, 0) = 0$ and $u_t(x, 0) = 1$?




I know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$ and I know how to solve the initial value problem from there. It's the inhomogenous part of this equation that is confusing me.



Thanks in advance!










share|cite|improve this question











$endgroup$





Find the general solution of the inhomogeneous wave equation
$$u_{tt} − c^2u_{xx} = sin t$$
What is the solution to the initial-value problem for this equation with $u(x, 0) = 0$ and $u_t(x, 0) = 1$?




I know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$ and I know how to solve the initial value problem from there. It's the inhomogenous part of this equation that is confusing me.



Thanks in advance!







pde wave-equation






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share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 30 at 9:26









Dylan

14.2k31127




14.2k31127










asked Jan 29 at 21:33









meff11meff11

466




466












  • $begingroup$
    Consider $u(x,t) = v(x,t) + 2 t - sin(t)$ to obtain a much simpler initial value problem for $v$.
    $endgroup$
    – Christoph
    Jan 30 at 4:15


















  • $begingroup$
    Consider $u(x,t) = v(x,t) + 2 t - sin(t)$ to obtain a much simpler initial value problem for $v$.
    $endgroup$
    – Christoph
    Jan 30 at 4:15
















$begingroup$
Consider $u(x,t) = v(x,t) + 2 t - sin(t)$ to obtain a much simpler initial value problem for $v$.
$endgroup$
– Christoph
Jan 30 at 4:15




$begingroup$
Consider $u(x,t) = v(x,t) + 2 t - sin(t)$ to obtain a much simpler initial value problem for $v$.
$endgroup$
– Christoph
Jan 30 at 4:15










2 Answers
2






active

oldest

votes


















0












$begingroup$

First, observe that a particular solution is $u_p(t) = -sin t$.



Then, find the remaining homogeneous solution $v(x,t)$ such that $u = u_p + v$. This gives the BVP



begin{cases}
v_{tt} - c^2 v_{xx} = 0 \
v(x,0) = u(x,0) - u_p(0) = 0 \
v_t(x,0) = u_t(x,0) - u_p'(0) = 2
end{cases}



d'Alembert's formula gives



$$v(x,t) = frac{1}{2c}int_{x-ct}^{x+ct} 2 ds = frac{1}{c}big[(x+ct) - (x-ct) big] = frac{2ct}{c} = 2t$$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    That particular solution does not agree with the initial condition of du/dt(x,0) = 1. Your particular solution gives -1
    $endgroup$
    – meff11
    Feb 1 at 16:39










  • $begingroup$
    @meff11 It doesn't have to! The particular solution only needs to satisfy the inhomogeneous equation. The initial condition is corrected by the remaining homogeneous solution. See my notes on $v(x,t)$
    $endgroup$
    – Dylan
    Feb 1 at 16:56












  • $begingroup$
    makes sense thanks! also, when I do d'Alembert's formula I keep getting t not 2t. Am I missing something? or could you show your calculations? thanks
    $endgroup$
    – meff11
    Feb 1 at 17:10










  • $begingroup$
    @meff11 No problem. I added the calculation.
    $endgroup$
    – Dylan
    Feb 1 at 17:45



















0












$begingroup$

$$u_{tt} − c^2u_{xx} = sin t$$
You know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$



Then you have to find a particular solution of the inhomogeneous equation (any one). Only by inspection it is easy to see that $u_p=-sin(t)$ is a solution.



If you don't see it, try any fonction of $t$ only, that is on the form $u_p(t)$. So, you solve the ODE $u_p''(t)=sin(t)$ . No need for all solutions. Any one among them is sufficient, for example $u_p=-sin(t)$ .



Thus the general solution of the PDE is :
$$u(x,t)=f(x+ct)+g(x-ct)-sin(t)$$
$u_t=cf'(x+ct)-cg'(x-ct)-cos(t)$



Conditions :



$begin{cases}
u(x,0)=f(x)+g(x)=0quadimpliesquad g(x)=-f(x) \
u-t(x,0)=cf'(x)-cg'(x)-1=1 quadimpliesquad 2cf'(x)=2
end{cases}$



Integrating the second equation leads to $quad f(x)=frac{x}{c}+C quad$ and $quad g(x)=-frac{x}{c}-C$ .



Putting them into the above general solution gives :
$$u(x,t)=(frac{x+ct}{c}+C)+(-frac{x-ct}{c}-C)-sin(t)$$
Finally, after simplification the solution is :
$$u(x,t)=2t-sin(t)$$






share|cite|improve this answer









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    2 Answers
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    2 Answers
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    active

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    0












    $begingroup$

    First, observe that a particular solution is $u_p(t) = -sin t$.



    Then, find the remaining homogeneous solution $v(x,t)$ such that $u = u_p + v$. This gives the BVP



    begin{cases}
    v_{tt} - c^2 v_{xx} = 0 \
    v(x,0) = u(x,0) - u_p(0) = 0 \
    v_t(x,0) = u_t(x,0) - u_p'(0) = 2
    end{cases}



    d'Alembert's formula gives



    $$v(x,t) = frac{1}{2c}int_{x-ct}^{x+ct} 2 ds = frac{1}{c}big[(x+ct) - (x-ct) big] = frac{2ct}{c} = 2t$$






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      That particular solution does not agree with the initial condition of du/dt(x,0) = 1. Your particular solution gives -1
      $endgroup$
      – meff11
      Feb 1 at 16:39










    • $begingroup$
      @meff11 It doesn't have to! The particular solution only needs to satisfy the inhomogeneous equation. The initial condition is corrected by the remaining homogeneous solution. See my notes on $v(x,t)$
      $endgroup$
      – Dylan
      Feb 1 at 16:56












    • $begingroup$
      makes sense thanks! also, when I do d'Alembert's formula I keep getting t not 2t. Am I missing something? or could you show your calculations? thanks
      $endgroup$
      – meff11
      Feb 1 at 17:10










    • $begingroup$
      @meff11 No problem. I added the calculation.
      $endgroup$
      – Dylan
      Feb 1 at 17:45
















    0












    $begingroup$

    First, observe that a particular solution is $u_p(t) = -sin t$.



    Then, find the remaining homogeneous solution $v(x,t)$ such that $u = u_p + v$. This gives the BVP



    begin{cases}
    v_{tt} - c^2 v_{xx} = 0 \
    v(x,0) = u(x,0) - u_p(0) = 0 \
    v_t(x,0) = u_t(x,0) - u_p'(0) = 2
    end{cases}



    d'Alembert's formula gives



    $$v(x,t) = frac{1}{2c}int_{x-ct}^{x+ct} 2 ds = frac{1}{c}big[(x+ct) - (x-ct) big] = frac{2ct}{c} = 2t$$






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      That particular solution does not agree with the initial condition of du/dt(x,0) = 1. Your particular solution gives -1
      $endgroup$
      – meff11
      Feb 1 at 16:39










    • $begingroup$
      @meff11 It doesn't have to! The particular solution only needs to satisfy the inhomogeneous equation. The initial condition is corrected by the remaining homogeneous solution. See my notes on $v(x,t)$
      $endgroup$
      – Dylan
      Feb 1 at 16:56












    • $begingroup$
      makes sense thanks! also, when I do d'Alembert's formula I keep getting t not 2t. Am I missing something? or could you show your calculations? thanks
      $endgroup$
      – meff11
      Feb 1 at 17:10










    • $begingroup$
      @meff11 No problem. I added the calculation.
      $endgroup$
      – Dylan
      Feb 1 at 17:45














    0












    0








    0





    $begingroup$

    First, observe that a particular solution is $u_p(t) = -sin t$.



    Then, find the remaining homogeneous solution $v(x,t)$ such that $u = u_p + v$. This gives the BVP



    begin{cases}
    v_{tt} - c^2 v_{xx} = 0 \
    v(x,0) = u(x,0) - u_p(0) = 0 \
    v_t(x,0) = u_t(x,0) - u_p'(0) = 2
    end{cases}



    d'Alembert's formula gives



    $$v(x,t) = frac{1}{2c}int_{x-ct}^{x+ct} 2 ds = frac{1}{c}big[(x+ct) - (x-ct) big] = frac{2ct}{c} = 2t$$






    share|cite|improve this answer











    $endgroup$



    First, observe that a particular solution is $u_p(t) = -sin t$.



    Then, find the remaining homogeneous solution $v(x,t)$ such that $u = u_p + v$. This gives the BVP



    begin{cases}
    v_{tt} - c^2 v_{xx} = 0 \
    v(x,0) = u(x,0) - u_p(0) = 0 \
    v_t(x,0) = u_t(x,0) - u_p'(0) = 2
    end{cases}



    d'Alembert's formula gives



    $$v(x,t) = frac{1}{2c}int_{x-ct}^{x+ct} 2 ds = frac{1}{c}big[(x+ct) - (x-ct) big] = frac{2ct}{c} = 2t$$







    share|cite|improve this answer














    share|cite|improve this answer



    share|cite|improve this answer








    edited Feb 1 at 17:28

























    answered Jan 30 at 9:29









    DylanDylan

    14.2k31127




    14.2k31127












    • $begingroup$
      That particular solution does not agree with the initial condition of du/dt(x,0) = 1. Your particular solution gives -1
      $endgroup$
      – meff11
      Feb 1 at 16:39










    • $begingroup$
      @meff11 It doesn't have to! The particular solution only needs to satisfy the inhomogeneous equation. The initial condition is corrected by the remaining homogeneous solution. See my notes on $v(x,t)$
      $endgroup$
      – Dylan
      Feb 1 at 16:56












    • $begingroup$
      makes sense thanks! also, when I do d'Alembert's formula I keep getting t not 2t. Am I missing something? or could you show your calculations? thanks
      $endgroup$
      – meff11
      Feb 1 at 17:10










    • $begingroup$
      @meff11 No problem. I added the calculation.
      $endgroup$
      – Dylan
      Feb 1 at 17:45


















    • $begingroup$
      That particular solution does not agree with the initial condition of du/dt(x,0) = 1. Your particular solution gives -1
      $endgroup$
      – meff11
      Feb 1 at 16:39










    • $begingroup$
      @meff11 It doesn't have to! The particular solution only needs to satisfy the inhomogeneous equation. The initial condition is corrected by the remaining homogeneous solution. See my notes on $v(x,t)$
      $endgroup$
      – Dylan
      Feb 1 at 16:56












    • $begingroup$
      makes sense thanks! also, when I do d'Alembert's formula I keep getting t not 2t. Am I missing something? or could you show your calculations? thanks
      $endgroup$
      – meff11
      Feb 1 at 17:10










    • $begingroup$
      @meff11 No problem. I added the calculation.
      $endgroup$
      – Dylan
      Feb 1 at 17:45
















    $begingroup$
    That particular solution does not agree with the initial condition of du/dt(x,0) = 1. Your particular solution gives -1
    $endgroup$
    – meff11
    Feb 1 at 16:39




    $begingroup$
    That particular solution does not agree with the initial condition of du/dt(x,0) = 1. Your particular solution gives -1
    $endgroup$
    – meff11
    Feb 1 at 16:39












    $begingroup$
    @meff11 It doesn't have to! The particular solution only needs to satisfy the inhomogeneous equation. The initial condition is corrected by the remaining homogeneous solution. See my notes on $v(x,t)$
    $endgroup$
    – Dylan
    Feb 1 at 16:56






    $begingroup$
    @meff11 It doesn't have to! The particular solution only needs to satisfy the inhomogeneous equation. The initial condition is corrected by the remaining homogeneous solution. See my notes on $v(x,t)$
    $endgroup$
    – Dylan
    Feb 1 at 16:56














    $begingroup$
    makes sense thanks! also, when I do d'Alembert's formula I keep getting t not 2t. Am I missing something? or could you show your calculations? thanks
    $endgroup$
    – meff11
    Feb 1 at 17:10




    $begingroup$
    makes sense thanks! also, when I do d'Alembert's formula I keep getting t not 2t. Am I missing something? or could you show your calculations? thanks
    $endgroup$
    – meff11
    Feb 1 at 17:10












    $begingroup$
    @meff11 No problem. I added the calculation.
    $endgroup$
    – Dylan
    Feb 1 at 17:45




    $begingroup$
    @meff11 No problem. I added the calculation.
    $endgroup$
    – Dylan
    Feb 1 at 17:45











    0












    $begingroup$

    $$u_{tt} − c^2u_{xx} = sin t$$
    You know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$



    Then you have to find a particular solution of the inhomogeneous equation (any one). Only by inspection it is easy to see that $u_p=-sin(t)$ is a solution.



    If you don't see it, try any fonction of $t$ only, that is on the form $u_p(t)$. So, you solve the ODE $u_p''(t)=sin(t)$ . No need for all solutions. Any one among them is sufficient, for example $u_p=-sin(t)$ .



    Thus the general solution of the PDE is :
    $$u(x,t)=f(x+ct)+g(x-ct)-sin(t)$$
    $u_t=cf'(x+ct)-cg'(x-ct)-cos(t)$



    Conditions :



    $begin{cases}
    u(x,0)=f(x)+g(x)=0quadimpliesquad g(x)=-f(x) \
    u-t(x,0)=cf'(x)-cg'(x)-1=1 quadimpliesquad 2cf'(x)=2
    end{cases}$



    Integrating the second equation leads to $quad f(x)=frac{x}{c}+C quad$ and $quad g(x)=-frac{x}{c}-C$ .



    Putting them into the above general solution gives :
    $$u(x,t)=(frac{x+ct}{c}+C)+(-frac{x-ct}{c}-C)-sin(t)$$
    Finally, after simplification the solution is :
    $$u(x,t)=2t-sin(t)$$






    share|cite|improve this answer









    $endgroup$


















      0












      $begingroup$

      $$u_{tt} − c^2u_{xx} = sin t$$
      You know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$



      Then you have to find a particular solution of the inhomogeneous equation (any one). Only by inspection it is easy to see that $u_p=-sin(t)$ is a solution.



      If you don't see it, try any fonction of $t$ only, that is on the form $u_p(t)$. So, you solve the ODE $u_p''(t)=sin(t)$ . No need for all solutions. Any one among them is sufficient, for example $u_p=-sin(t)$ .



      Thus the general solution of the PDE is :
      $$u(x,t)=f(x+ct)+g(x-ct)-sin(t)$$
      $u_t=cf'(x+ct)-cg'(x-ct)-cos(t)$



      Conditions :



      $begin{cases}
      u(x,0)=f(x)+g(x)=0quadimpliesquad g(x)=-f(x) \
      u-t(x,0)=cf'(x)-cg'(x)-1=1 quadimpliesquad 2cf'(x)=2
      end{cases}$



      Integrating the second equation leads to $quad f(x)=frac{x}{c}+C quad$ and $quad g(x)=-frac{x}{c}-C$ .



      Putting them into the above general solution gives :
      $$u(x,t)=(frac{x+ct}{c}+C)+(-frac{x-ct}{c}-C)-sin(t)$$
      Finally, after simplification the solution is :
      $$u(x,t)=2t-sin(t)$$






      share|cite|improve this answer









      $endgroup$
















        0












        0








        0





        $begingroup$

        $$u_{tt} − c^2u_{xx} = sin t$$
        You know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$



        Then you have to find a particular solution of the inhomogeneous equation (any one). Only by inspection it is easy to see that $u_p=-sin(t)$ is a solution.



        If you don't see it, try any fonction of $t$ only, that is on the form $u_p(t)$. So, you solve the ODE $u_p''(t)=sin(t)$ . No need for all solutions. Any one among them is sufficient, for example $u_p=-sin(t)$ .



        Thus the general solution of the PDE is :
        $$u(x,t)=f(x+ct)+g(x-ct)-sin(t)$$
        $u_t=cf'(x+ct)-cg'(x-ct)-cos(t)$



        Conditions :



        $begin{cases}
        u(x,0)=f(x)+g(x)=0quadimpliesquad g(x)=-f(x) \
        u-t(x,0)=cf'(x)-cg'(x)-1=1 quadimpliesquad 2cf'(x)=2
        end{cases}$



        Integrating the second equation leads to $quad f(x)=frac{x}{c}+C quad$ and $quad g(x)=-frac{x}{c}-C$ .



        Putting them into the above general solution gives :
        $$u(x,t)=(frac{x+ct}{c}+C)+(-frac{x-ct}{c}-C)-sin(t)$$
        Finally, after simplification the solution is :
        $$u(x,t)=2t-sin(t)$$






        share|cite|improve this answer









        $endgroup$



        $$u_{tt} − c^2u_{xx} = sin t$$
        You know that the general solution to the homogenous equation is $u(x,t) = f(x+ct) + g(x-ct)$



        Then you have to find a particular solution of the inhomogeneous equation (any one). Only by inspection it is easy to see that $u_p=-sin(t)$ is a solution.



        If you don't see it, try any fonction of $t$ only, that is on the form $u_p(t)$. So, you solve the ODE $u_p''(t)=sin(t)$ . No need for all solutions. Any one among them is sufficient, for example $u_p=-sin(t)$ .



        Thus the general solution of the PDE is :
        $$u(x,t)=f(x+ct)+g(x-ct)-sin(t)$$
        $u_t=cf'(x+ct)-cg'(x-ct)-cos(t)$



        Conditions :



        $begin{cases}
        u(x,0)=f(x)+g(x)=0quadimpliesquad g(x)=-f(x) \
        u-t(x,0)=cf'(x)-cg'(x)-1=1 quadimpliesquad 2cf'(x)=2
        end{cases}$



        Integrating the second equation leads to $quad f(x)=frac{x}{c}+C quad$ and $quad g(x)=-frac{x}{c}-C$ .



        Putting them into the above general solution gives :
        $$u(x,t)=(frac{x+ct}{c}+C)+(-frac{x-ct}{c}-C)-sin(t)$$
        Finally, after simplification the solution is :
        $$u(x,t)=2t-sin(t)$$







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Feb 2 at 15:53









        JJacquelinJJacquelin

        45.3k21856




        45.3k21856






























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