# Wave length of a transverse wave

1) A transverse wave of frequency 25 Hz propa-
gates down a string. Two points 30 cm apart
are out of phase by (5*pi)/3
What is the wave length of the wave? An-
swer in units of cm.

Im kinda lost here, Im unsure how wave length will relate to the given info. Im sure the phase is part of the relation but Im unsure of how. The best Ive been able to figure so far would be converting the frequency to a period, then using v= lambda/T.

I suppose phase is key to solving this problem, but maybe Im unsure of exactly how phase relates?

2) Given two equations Y1 = A / (Bx - Ct)^2 Y2= -A / (Bx + Ct - E)^2
At what time will the two waves exactly cancel everywhere? At what point do the waves always cancel?

Ive been reading in my book about this problem and Im unsure exactly what the case is for two waves to cancel each other out. I would assume they have to be exactly out of phase with each other, so that the peak of one corresponds with the valley of another. Although Im unsure how to meet this condition in the above equations.

Thanks for any help!

berkeman
Mentor
-1- There are 2*pi radians in a cycle, right? What part of that is 5*pi/3?

-2- Weird question -- I don't get the equation forms either. Y1 and Y2 don't seem like waves, and how can there be a "time" where the waves cancel everywhere? What's everywhere? All x? All t? Are you sure that you've copied that question correctly? Are there other examples of "waves" in your text that use this equation form? Are there maybe some complex exponentials missing from the equations or something?

berkeman said:
-1- There are 2*pi radians in a cycle, right? What part of that is 5*pi/3?

-2- Weird question -- I don't get the equation forms either. Y1 and Y2 don't seem like waves, and how can there be a "time" where the waves cancel everywhere? What's everywhere? All x? All t? Are you sure that you've copied that question correctly? Are there other examples of "waves" in your text that use this equation form? Are there maybe some complex exponentials missing from the equations or something?

For #2, here is the full question:

http://img137.imageshack.us/img137/8669/q229ni.gif [Broken]

As for the first problem, their cycles are out of phase (5pi/3)/2pi = .83333, right? so if the points are 30cm apart, the wavelength is .8333 *30cm?

Last edited by a moderator:
The two waves cancel each other out at any time t when y1+y2=0

daveb said:
The two waves cancel each other out at any time t when y1+y2=0

Thanks, after that it was easy to solve.

Iam still however lost on the first problem. So is it .83333 of a cycle out of phase, but Im still unsure as how to relate this to wavelength?

ness9660 said:
Thanks, after that it was easy to solve.

Iam still however lost on the first problem. So is it .83333 of a cycle out of phase, but Im still unsure as how to relate this to wavelength?

Actually I just figured it out, thanks for the help!