# Standing wave, phase and antiphase

• maxelcat
In summary, the conversation discusses the phase differences between points P, Q, and R in a standing wave. The points P and R are pi radians out of phase, reaching their max/min at the same time. Antiphase is a phase difference that is an odd-integer multiple of pi (180 degrees). The term "out of phase" is often used to describe any phase difference that is different from being in phase. The question at hand is whether points P and Q are in phase, antiphase, or neither. After further discussion and clarification, it is determined that P and Q are in phase.

#### maxelcat

Homework Statement
The diagram shows a stationary wave on a string at one instant in time. box
P, Q and R are three points on the string.
Relevant Equations
none that I think are needed...

I think I understand that points P and R are pi radians out of phase - reaching their max/min at the same time.
But are P and Q in anti phase?

What is antiphase exactly - is it when they are 180deg out of phase - or is it when they are anything other than totally in phase? I seem to find conflicting answers

Thanks

maxelcat said:
View attachment 320750I think I understand that points P and R are pi radians out of phase - reaching their max/min at the same time.
Agreed.

maxelcat said:
But are P and Q in anti phase?

What is antiphase exactly - is it when they are 180deg out of phase - or is it when they are anything other than totally in phase? I seem to find conflicting answers
Watch the animation carefully:
https://www.cyberphysics.co.uk/graphics/animations/standing-wave.gif

Antiphase is a phase difference which is an exact odd-integer multiple of π (180º), i.e. π rad, 3π rad, 5π rad, etc.

Edit. The term 'out of phase' is often used to mean any phase difference different to 'in phase'. It is not the same as 'anti phase'.

So do you now think P and Q are in phase, antiphase, or neither?

Last edited:
Your homework statement is incomplete. There is no question there.

My eyes aren't what they used to be but I think the phase difference between P and R isn't 180 degrees. I see R is at the "bottom" of the wave whereas P isn't at the "top"

Gordianus said:
My eyes aren't what they used to be but I think the phase difference between P and R isn't 180 degrees. I see R is at the "bottom" of the wave whereas P isn't at the "top"
Remember that the question is about a stationary (standing) wave, not a progressive wave. Watching the animation in Post #2 should clarify the issue.

Edit: P and R are 180º out of phase, but with different amplitudes.

nasu said:
Your homework statement is incomplete. There is no question there.
actually it is there because the question is to identify which is antiphase

Steve4Physics said:
Agreed.Watch the animation carefully:
https://www.cyberphysics.co.uk/graphics/animations/standing-wave.gif

Antiphase is a phase difference which is an exact odd-integer multiple of π (180º), i.e. π rad, 3π rad, 5π rad, etc.

Edit. The term 'out of phase' is often used to mean any phase difference different to 'in phase'. It is not the same as 'anti phase'.

So do you now think P and Q are in phase, antiphase, or neither?
no I think P and Q are in phase (both reaching their max at the same time - thanks for the explanation - very helpful

Steve4Physics
maxelcat said:
actually it is there because the question is to identify which is antiphase
There is no question in the OP, under "Homework statement". You start by saying what you think without saying what is the actual question and how is that relevant.