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Homework Help: Wave problems

  1. Nov 3, 2005 #1
    Hi I was wondering if someone could tell me if I am heading in the right direction with these Thanks in advance

    Q1 In an experiment on standing waves, a string 90 cm long is
    attached to the prong of an electrically driven tuning fork that oscillates
    perpendicular to the length of the string at a frequency of 60 Hz. The
    mass of the string is 0.044 kg. What tension must the string be under if it
    is to vibrate in four loops?

    I used the formula L=4(λ/2) giving 2λ=L L=1.8m
    V=fλ v=108m/s then using the formula v=sq root(T/μ) giving me a tension of 279.936N Is this correct

    Q2 Two sinusoidal waves with the same amplitude, frequency and
    speed, travel in the same direction along a string and interfere with one
    another to produce a resultant wave with a displacement
    y = (3.0 x 10-3) sin(20x - 4.0t + 0.6) cos(0.6) m. What is (a) the amplitude
    and (b) the wavelength of the waves and (c) the phase difference
    between them?

    Wave is in the form y'(x,t)=[2ym(cos(tetha/2)sin(Kx-wt+1/2(tetha)tetha/2

    answer 1.13mm is this correct?

    k=20 rad/m
    w=4 rad/s


    I know this is wrong please lead me in the right direction
  2. jcsd
  3. Nov 4, 2005 #2


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    Homework Helper

    for Q1: The standing wave has an antinode at the position of the tuning fork and a node at the fixed end. This means that the length of the string will be two and a quarter wavelengths long for four loops. The rest of the approach is correct.
  4. Nov 4, 2005 #3
    Resistivity vs. Resistance

    Hi,could you be so kind to explain me differences between resistance and resistivity?
    I think the resistance is constant for each material.But we use resistivity to differentiate between various materials.
  5. Nov 4, 2005 #4


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    Gold Member

    As far as I understand>>> Certain materials oppose current flow more than others. Even the same material allows current to flow at different rates depending upon shape. This property is called resistance. Resistivity is a physical property of a material to resist or oppose the movement of charge through the material.

    So from that, resistance is not constant for each material.
    [tex]R = p\frac{L}{A}[/tex]

    p = resistivity of conductor
    L = length
    A = area
    Last edited: Nov 4, 2005
  6. Nov 4, 2005 #5


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    Homework Helper

    Superdreamer, you were right about there being a node almost at the tuning fork. If the wave is actually in resonance, without great amounts of damping, the tuning fork oscillation will be less than about 1 mm, while the string wave will have amplitude of about 100mm . (Andrevdh is incorrect)

    your wavelength and frequency look okay.
    the decomposition into 2 waves with same amplitude but a phase shift
    does NOT have a unique solution.
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