Can D'Alembert's Solution Solve the 1-Dimensional Wave Equation?

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Discussion Overview

The discussion revolves around solving the 1-dimensional wave equation using D'Alembert's solution. Participants explore the formulation of the solution, the interpretation of initial conditions, and the verification of proposed solutions.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant, Dan, presents a formula for D'Alembert's solution and asks about the role of the initial conditions and the function g(s) in the integral.
  • Another participant critiques Dan's formulation, suggesting that the general D'Alembert solution involves two functions F and G, rather than the specific form Dan provided.
  • This critique includes a clarification that the initial conditions should be interpreted as φ(x,0) = f(x) and φ_t(x,0) = g(x), and that these lead to a system of equations for F and G.
  • Dan later confirms the initial conditions as specific functions, indicating a need to solve the equations derived from the initial conditions.
  • A subsequent post proposes a solution of the form φ(x,t) = e^(-(x-ct)^2) and questions its validity.
  • Another participant suggests checking the proposed solution by calculating the second derivatives and verifying the initial conditions.

Areas of Agreement / Disagreement

Participants express differing views on the formulation of D'Alembert's solution and the interpretation of initial conditions. There is no consensus on the correctness of the proposed solution, as it remains to be verified against the wave equation.

Contextual Notes

Participants highlight potential confusion regarding the definitions and roles of functions in the context of the wave equation. The discussion reflects varying interpretations of the initial conditions and the specific form of the solution.

Who May Find This Useful

This discussion may be useful for students and practitioners interested in wave equations, mathematical physics, and the application of D'Alembert's solution in solving differential equations.

DanielO_o
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I have to solve the 1-dimensional wave equation using D'Alembert's solution.

I know the solution has the forum 1/2[f(x-ct) + f(x+ct)] + 1/2C*integral(g(s)ds)

and i have two intial conditions f(x) and g(x)... do i just plug in (x-ct) where i see x etc...?

also what is g(s) in the integral?

Thanks

Dan
 
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DanielO_o said:
I have to solve the 1-dimensional wave equation using D'Alembert's solution.

I know the solution has the forum 1/2[f(x-ct) + f(x+ct)] + 1/2C*integral(g(s)ds)

and i have two intial conditions f(x) and g(x)... do i just plug in (x-ct) where i see x etc...?

also what is g(s) in the integral?

Thanks

Dan

You've written this very confusingly! You have f used in at least two different ways and g used in two different ways.

What you are giving as the "D'Alembert solution" is too specialized.

The general D'Alembert solution to the wave equation is [itex]\phi (x,t)= F(x+ ct)+ G(x- ct)[/itex] where F and G can be any two twice differentiable functions of a single variable. Then [itex]\phi_t(x,t)= cF'(x+ct)- cG'(x- ct)[/itex]. If one initial condition is [itex]\phi_t(x,0)= 0[/itex], then it follows that [itex]F'(x)= G'(x)[/itex] so F and G differ by a constant- in that case, [itex]\phi (x,t)= (1/2)(F(x+ct)+ F(x-ct))+ constant[/itex].

But that is not the case here. (It is also confusing to say just "initial values f(x) and g(x)" since functions are not "conditions". I assume you mean [itex]\phi (x,0)= f(x)[/itex] and [itex]\phi_t(x,0)= g(x)[/itex].)

Then you have [itex]\phi (x,0)= F(x)+ G(x)= f(x)[/itex] and [itex]\phi_t(x,0)= cF'(x)- cG'(x)= g(x)[/itex]. Solve those two equations for F and G.
 
Yes, sorry to be confusing but your assumptions were right...

The initial conditions are #(x,0) = e^(-x^2) and #'(x,0) = 2cxe^(-x^2)...

I wasn't sure how to use these to get to the solution but it seems like i need to solve those two equations.

Thanks!
 
I have got the answer #(x,t) = e^-((x-ct)^2)

Am i right?
 
Well, it's easy to check, isn't it? Calculate #xx and #tt and see if it satisfies the differential equation. Is #(x,0)= e^(-x^2)?
Is #_t(x,0)= -2cxe^(-x^2)?
 

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