Why does f(x-vt) represent a wave along +x?

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Discussion Overview

The discussion centers around the mathematical representation of waves, specifically why the function f(x-vt) is associated with a wave propagating in the positive x direction. Participants explore the implications of this representation in the context of wave behavior, including transverse and longitudinal waves.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the representation of f(x-vt) as a wave moving in the +x direction, suggesting that f(x+vt) would intuitively represent a wave in that direction instead.
  • Another participant proposes that if coordinates are set such that x=0 when t=0, then the equation x=vt shows that f(0) moves along the x-axis at speed v.
  • A different viewpoint suggests that f(x-vt) represents a pattern that propagates in the +x direction, with the center of the feature moving at speed v, as indicated by the relationship x=vt.
  • One participant notes that the statement about f(x-vt) representing a transverse wave does not apply universally, indicating that only certain functions fit this description.
  • Another participant questions what the restrictions are for the function to represent a wave, mentioning continuity as a potential factor.
  • A participant provides an example of a constant function, f(x-vt) = k, as a simple case.
  • One participant shares their understanding that f(x-vt) indicates the function's value at position x based on its value at the origin t seconds ago, explaining the negative sign in the time variable.
  • Another participant clarifies that the term "transverse" is not a requirement of the f(x-vt) form, as it can apply to various types of waves, including longitudinal waves.
  • A suggestion is made to use graphing software to visualize the function f(x) = cos(x - vt) to see the wave's movement over time, which one participant finds helpful.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding the representation of waves with f(x-vt). There is no consensus on the interpretation of the function, and multiple competing views remain regarding its implications and the types of waves it can represent.

Contextual Notes

Participants mention potential restrictions on the function's applicability, such as continuity, but do not reach a definitive conclusion on what those restrictions entail.

binrdow
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I just can't understand how this makes sense. Why does f(x-vt) represent a transverse wave along +x? Where v is the velocity, t is time, and x is position along the x axis. It seems to be exactly the opposite to what I would think makes sense, with f(x+vt) representing a wave along +x and f(x-vt) representing a wave along -x. But this isn't the case! I've been trying to wrap my head around it, not matter how I look at it.
Can anyone explain this concept in a way that makes sense?

Thanks! :smile:
 
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Let's for simplicity set our coordinates so that x= 0 when t= 0. If the object is moving at speed v, then at any future time, t, x= vt. So x-vt= 0. That is, f(0) "moves" along the x-axis at speed v.
 
Think of the function f(x-vt) as being a pattern of some kind. The pattern will propagate in the +x direction at speed v, because at x=0 and t=0, the pattern is f(0) (think of this as the center of the feature that is propagating), but at any other time t and x=vt, the pattern will still be f(0). So the center of the feature is always found at some x=vt, so is moving at speed v in the +x direction, because that's just what x=vt means.
 
Oh! Okay, so you just have to look at the equation for the position x first as x=vt. Thanks much! :biggrin:
 
Why does f(x-vt) represent a transverse wave along +x?

Just remember that this statement does not apply to every f(x-vt) only suitable ones, although there are a great many such.
 
Studiot said:
Just remember that this statement does not apply to every f(x-vt) only suitable ones, although there are a great many such.

Which would be those restrictions? continuity?
 
The simplest example would be

f(x-vt) = k
 
binrdow said:
I just can't understand how this makes sense. Why does f(x-vt) represent a transverse wave along +x? Where v is the velocity, t is time, and x is position along the x axis. It seems to be exactly the opposite to what I would think makes sense, with f(x+vt) representing a wave along +x and f(x-vt) representing a wave along -x. But this isn't the case! I've been trying to wrap my head around it, not matter how I look at it.
Can anyone explain this concept in a way that makes sense?

Thanks! :smile:

I had this problem too and eventually came to this conclusion (a nice arm waving one).
f(x-vt) tells you what the function will be at x in terms of what it was, at the origin, t seconds ago (i.e. t is the time it took to propagate to x from the origin) - hence the negative t sign.
Confused more? - Sorry if you are but it helped me, once.
 
TheBlackNinja said:
Which would be those restrictions? continuity?
I believe the issue was the idea that the wave is "transverse", which is not a requirement of the f(x-vt) form. Anything that propagates at v has the form f(x-vt), including longitudinal and water waves.
 
  • #10
sophiecentaur said:
Confused more? - Sorry if you are but it helped me, once.

Actually, this helps allot! Thanks! :)
 
  • #11
If you have mathematica or a scientific calculator, you can plot the functions f(x) = cos(x - vt), let v equal some random number, and plot it as you vary t between different graphs. You'll actually SEE the graph moving to the right as you increase t.

I think it's a fantastically convincing argument for seeing how this works.
 

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