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Homework Help: Normal mode of string in third harmonic

  1. Jan 22, 2009 #1
    1. The problem statement, all variables and given/known data
    A string that is fixed at both ends is vibrating in the third harmonic. the wave has a speed of 186m/s and a frequency of 225Hz. the amplitude of the string at an antinode is 0.0037m.

    How much time does it take the string to go from its largest upward displacement to its largest downward displacement at this point?

    2. Relevant equations

    3. The attempt at a solution
    I think there is a node at x=0 and the third harmonic means there are three antinodes between x=0 and x=L. Therefore the antinodes are located at x=1/6L,1/2L,5/6L and the nodes are located at x=0,2/6L,4/6L,L. I set 0.0037m=y(1/6L,t)=(Aswsin(kx))(sin(omega*t)) and solve for t where
    I found the first antinode to occur at t=1/900s
    However when I solve for t with x=2/6L I cannot calculate t because it requires me to divide by zero because sin(k*2/6L)=0
    what am i doing wrong?
  2. jcsd
  3. Jan 22, 2009 #2
    This is just a thought that might simplify things a lot, the frequency of the wave describes how many times the wave passes a point per second, in this case an antinode. Also, each time the wave passes an antinode it has completed one full cycle from peak to trough and back to peak again.
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