# Need help,baffled!

1. Nov 20, 2008

### noobie!

hi,im a newbie over here,my physics quite poor so i really need help by understanding it,i jz wanted to ask could any1 please explain to me what is in phase,&out of phase?;im totally baffled.:blushingAnd what's frequencies of the normal mode in standing waves in a string fixed in both ends?i hope any1 cud help me with this..tq!!!!

2. Nov 20, 2008

### Nick89

If you have two waves, let's assume of the same frequency, you can 'define' a phase difference between them. The phase difference simply tells you how much one wave is 'ahead of' the other wave, and is often given in degrees (0 to 360) or radians (0 to 2pi).

If you look at the following image:

Clearly, the waves have the same frequency, but one wave is slightly ahead of the other: there is a phase difference between the two.

In general, there are a few phase differences of special interest: a phase difference of 0 radians (or 0 degrees) means the waves are exactly "in phase", one is not ahead of the other. A phase difference of pi (or 180 degrees) is often called exactly "out of phase", one wave is exactly half a wavelength ahead of the other wave, which means that the peaks of one wave fall together with the troughs ('dips') of the other wave.

When the two waves are for example sound waves (it is equally valid for all other kind of waves) and when they exist in the same place, then the waves will interfere. A phase difference of 0 will then cause the interference wave to be the sum of both waves. With a phase difference of pi (180 degrees), the peaks and troughs will cancel eachother out and the resulting wave is zero everywhere: there is no longer a wave.

3. Nov 20, 2008

### noobie!

THANKS a lot man..now i can really understand it completely,thx for ur help,one more thg im nt clear abt it was the standing waves in an open end due to displacement of air and pressure variation in the air!!!???i wud appreciate it very much if i understand it...tq!

4. Nov 20, 2008

### Nick89

I assume you are talking about sound waves in an open ended tube? Then yes, they are caused by pressure variation in the air; as are all sound waves...

5. Nov 20, 2008

### noobie!

oh ya sound waves..but what i dun really understand is the pattern of the waves..why does the waves due to displacement of air differ frm pressure variation in the air???