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Homework Help: Quick Tension problem

  1. Apr 17, 2006 #1
    I've been working on this problem and I got every part right except part F which asks for tension.

    A sinusoidal wave is traveling on a string with speed 110 cm/s. The displacement of the particles of the string at x = 23 cm is found to vary with time according to the equation
    y = (4.9 cm) sin[1.5 - (7.2 s-1)t].
    The linear density of the string is 2.2 g/cm. What are (a) the frequency and (b) the wavelength of the wave? If the wave equation is of the form
    y(x,t) = ym sin(kx - (omega)t),
    what are (c)ym, (d)k, and (e)Omega? (f) What is the tension in the string?

    a)f=1.15 Hz
    b)wavelength = 95.7cm
    d)k=6.52 rad/m
    e)omega=7.2 rad/s

    f) my answers so far were 0.00266 N and 0.0000266 N and both were wrong (they seem too low). I tried the equation V = Sqrt(T/u) where T is tension, and u is linear density but I must be making a mistake somewhere.
  2. jcsd
  3. Apr 17, 2006 #2

    Doc Al

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    Staff: Mentor

    Check your units. Convert everything into standard units--meters, kilograms--before plugging into the equation.
  4. Apr 17, 2006 #3
    Yeah, after I triple checked my units, I realized they were wrong again. Got the correct answer of 0.266 newtons. Thanks for the reply
  5. Apr 17, 2006 #4


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    Science Advisor
    Homework Helper
    Gold Member

    Seems to be just a problem of units. To get the speed in m/s, you must put T in Newtons and u in kg/m. Notice that u = .22 kg/m (my guess is that you left u in g/m instead of kg/m)
    Try this and you will get a much larger tension than what you wrote above.
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