Wave Equation: A = A cos (kx - ωt), Meaning & Significance

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Discussion Overview

The discussion revolves around the wave equation represented as A = A cos(kx - ωt) and its alternative form A = A sin(ωt - kx). Participants explore the implications of the sign changes in the arguments of the cosine and sine functions, the direction of wave propagation, and the nature of wave solutions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the argument of the sine function changes by a minus sign when comparing it to the cosine function in the wave equation.
  • Another participant states that the equation kx - ωt indicates that the wave propagates in the positive x direction over time.
  • A participant clarifies that the expression provided is not a wave equation but rather solutions to a wave equation, noting that cosine is an even function, allowing for the argument to be rearranged.
  • There is a discussion about the general solution to the wave equation, which includes both cosine and sine terms, and how initial or boundary conditions affect the coefficients.
  • Participants express curiosity about the implications of choosing sine over cosine in the wave equation and how it relates to the signs and values of coefficients in the general solution.

Areas of Agreement / Disagreement

Participants express varying interpretations of the wave equation and its forms, with no consensus reached on the implications of the sign changes or the preference for sine versus cosine representations.

Contextual Notes

Some assumptions about wave propagation direction and the nature of wave solutions are not fully explored, and the discussion does not resolve the implications of using sine versus cosine in the context of boundary conditions.

quietrain
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a wave equation is given as A = A cos (kx - ωt)

so why if someone describes the wave equation to be A = A sin (ωt - kx) , the argument of the sin function changes by a minus sign?

and is there a meaning to it?

also i still don't really understand why the minus sign in the first equation signifies wave moving forward whereas a + sign signifies wave moving backwards ...

thanks a lot for the help!
 
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kx - ωt = kx' - ωt' if both x' > x and t' > t. => The same wave value A cos(kx - ωt) propagates with time in positive x direction.
 
You don't have a wave equation there, you have solutions to a wave equation.

As well, since cos(-z) = cos(z), i.e. cos is an even function you can write the cos solution as A cos (ωt - kx) if you wish.

The general solution to the wave equation is
Acos(wt - kx) + Bsin(wt -kx)
where A and B are determined by the initial or boundary conditions
If you wrote the general solution in terms of (kx - wt) then the sign of the factor multiplying the sin function would change to accoomdate this.
 
oh...

so cos (kx - ωt) = cos (ωt- kx ) because it is an even function.

so what's the difference if we choose to write it in sin instead?
 
quietrain said:
...so what's the difference if we choose to write it in sin instead?
You can write in either way. The initial/boundary conditions will determine the signs and values of coefficients A and B in the general solution Acos(wt - kx) + Bsin(wt -kx).
 
oh i see thanks
 

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