1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Wave power on string question

  1. Oct 8, 2005 #1
    --It is found that a 6.00m segment of a long string contains four complete waves and has a mass of 180g. The string is vibrating sinusoidally with a frequency of 50.0hz and a peak-to-valley distance of 15.0cm. (a) Write the function that describes this wave traveling in the positive x direction. (b) Determine the power being supplied to the string.--

    Ok, I determined that w=2*pi*50hz=314 rad/s. k= (2*pi)/wave length=4.19rad/m (assuming that the 6m long segment with 4 equal length waves would give me a 1.5m wave length)

    so i can answer (a), although having the A=6.00m is starting to bug me since the string is actually a lot longer that the segment experiencing the waves.

    (b), is killin me though. I figure all i have to use is the P=1/2*u*w^2*A^2*v bit, but i can't seem to figure out v.
    v=wave length*freq.

    but if I use what I have determined previously, the first two equations come up with teh same answer, but the third one doesn't. the first 2 are 75m/s, the third is something like .81m/s. This makes me think that either my period, which I determined is .02s, is wrong, or something else is fundamentally wrong. And am I even using the right equation for the power?

    Um, since I don't know how to delete this post, I'll just correct myself here.

    v=sqrt(TENSION!!!!!!!!!...not period/u)

    please ignore this now
    Last edited: Oct 8, 2005
  2. jcsd
  3. Oct 29, 2010 #2
    *I'm just posting this despite the 6 year lapse for anyone else who will use this as future reference*

    from part A, we know that
    v = L/t
    t = 1/f -> four complete waves -> 4/f
    so v = (L * f)/4

    however, for part B, in order to find the power throughout the string, we have to take the velocity from A and divide it by 4. so,
    v = (L * f)/16

    and yes, that was the right power equation.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook