Graph of stationary wave after moving for half period

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Homework Help Overview

The discussion revolves around a stationary wave traveling along a stretched string, with a specific focus on its behavior after a time interval of half its period. Participants are examining the implications of this time interval on the wave's displacement and characteristics.

Discussion Character

  • Conceptual clarification, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the nature of the wave, questioning whether it is stationary or progressive. There are discussions about the expected displacement after half a period and the implications of starting from a non-zero displacement. Some participants suggest that the mark scheme may be incorrect based on the question's phrasing.

Discussion Status

The discussion is active, with participants raising questions about the definitions and assumptions in the problem statement. Some have pointed out potential inconsistencies in the question and the mark scheme, while others are seeking clarification on the expected behavior of the wave after the specified time interval.

Contextual Notes

There are references to discrepancies in the problem statement and the mark scheme, indicating that the original question may have been altered from previous versions without proper updates to the accompanying materials. This has led to confusion regarding the time intervals and wave characteristics being discussed.

songoku
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Homework Statement


A stationary wave travels along a stretched string. Part of this wave at a particular
instant is shown.

https://www.google.co.id/imgres?imgurl=http://1.bp.blogspot.com/-0ltZhcjZgMk/VFjPCek0qDI/AAAAAAAAFpQ/v5tEMYIVsrQ/s1600/stationary-wave-june2011p23q6c.png&imgrefurl=http://physics-ref.blogspot.com/2014/11/physics-9702-doubts-help-page-7.html&docid=o0LHAI0i1F88PM&tbnid=cX0R4qmI0GkYfM:&w=917&h=272&bih=667&biw=1366&ved=0ahUKEwjrpYGWv7rMAhWFFqYKHXJEAB8QMwhOKCkwKQ&iact=mrc&uact=8

The period of vibration of this wave is 20 ms. Sketch the stationary wave 10 ms after the instant shown.

Homework Equations


stationary wave

The Attempt at a Solution


I think the wave will be reversed after moving half period. Let say the equation of the wave is sin, then after moving for 10 ms the equation will be - sin. But the answer says that the graph will be horizontal line. I don't understand how it can be horizontal line only (I assume the answer means that all the amplitude is zero)

Thanks
 
Last edited:
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2. Relevant equation
stationary wave
... but the problem statement says:
A transverse progressive wave travels along a stretched string.
... which is it: progressive or stationary?
 
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Simon Bridge said:
... but the problem statement says: ... which is it: progressive or stationary?
Sorry my bad. Should be stationary wave. I have edited the question
 
Yeah I don't see how you can start at non-zero displacement and end up with zero displacement after half a period.
You can go from max to zero in a quarter of a period.

Note: you are still writing: "A stationary wave travels along a stretched string"
If it travels, it is not stationary.
 
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Simon Bridge said:
Yeah I don't see how you can start at non-zero displacement and end up with zero displacement after half a period.
You can go from max to zero in a quarter of a period.

Note: you are still writing: "A stationary wave travels along a stretched string"
If it travels, it is not stationary.
Ah my bad again. But if the question is progressive wave, will it still be reversed and not zero after moving half period?
 
This question comes from CIE specimen paper 2 from 2015 Q7b(iv) - the mark scheme is wrong according to the question. Either the question should say:

The period of vibration of this wave is 20ms. On Fig. 7.2, sketch the stationary wave 5ms after the instant shown in Fig. 7.2.​

- in which Fig 7.2. looks like the image above OR the mark scheme should describe the wave as being 180o out of phase.

The problem with specimen papers is that they frequently cut and paste from previous years and then when they alter them a little - they don't update the mark scheme to reflect that. In this case the original question came from June 2011 paper 23 question 6 and the original text read like this:

The period of vibration of this wave is τ. On Fig. 6.2, sketch the stationary wave 0.25 τ after the instant shown in Fig. 6.2.​

Clearly 0.25 τ is a quarter of a period or 5ms later not 10ms.

Check it out here:

http://www.sciwebhop.net/sci_web/science/asp/struc/default_ks5.asp?q=1472
 
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Ian Lindley said:
This question comes from CIE specimen paper 2 from 2015 Q7b(iv) - the mark scheme is wrong according to the question. Either the question should say:

The period of vibration of this wave is 20ms. On Fig. 7.2, sketch the stationary wave 5ms after the instant shown in Fig. 7.2.​

- in which Fig 7.2. looks like the image above OR the mark scheme should describe the wave as being 180o out of phase.

The problem with specimen papers is that they frequently cut and paste from previous years and then when they alter them a little - they don't update the mark scheme to reflect that. In this case the original question came from June 2011 paper 23 question 6 and the original text read like this:

The period of vibration of this wave is τ. On Fig. 6.2, sketch the stationary wave 0.25 τ after the instant shown in Fig. 6.2.​

Clearly 0.25 τ is a quarter of a period or 5ms later not 10ms.

Check it out here:

http://www.sciwebhop.net/sci_web/science/asp/struc/default_ks5.asp?q=1472

Thanks a lot for your help and thanks to you too Simon
 

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