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Homework Help: Waves in opposite directions in a wire

  1. Sep 25, 2014 #1
    1. The problem statement, all variables and given/known data
    2 crosswise cosine waves of amplitude 2.5 cm and wavelength 5 cm are travelling in opposite directions in a wire with velocity 6 mm/s. draw graphs of the wire at times t=0, 2 and 4 sec.

    2. Relevant equations
    Waves propagating to the right: ##y=A\cos 2\pi \left( \frac{x}{\lambda}-\frac{t}{T} \right)##
    Waves propagating to the left: ##y=A\cos 2\pi \left( \frac{x}{\lambda}+\frac{t}{T} \right)##

    3. The attempt at a solution
    Isn't it necessary to give the initial conditions at t=0? i mean what was the starting angle of each wave, or is it obvious that they both started with ##\theta=0## but in opposite directions, like in the drawing?

    Attached Files:

  2. jcsd
  3. Sep 25, 2014 #2

    Simon Bridge

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    If you are given no other information than what you wrote above, then you can safely treat the wire as infinitely long ... so you are free to place the origin for time and length anywhere you like: it makes no difference to the physics anyway so you may as well make a choice that provides the least work ;)

    OTOH: if you have other physical information - i.e. the length of the wire, the presence of a wall, etc - then you have to take that into account when you make your choices: don't just assume something - work it out. i.e. at the wall the amplitude is forced to a specific value.

    Note: you have been asked for a graph of the wire, not the waves.
  4. Sep 25, 2014 #3
    I was asked a graph of the shape of the wire, but isn't it the only graph possible? what is a graph of the wire and what is the graph of the waves? aren't they the same thing? i didn't learn anything else but the shape of the waves in a wire
  5. Sep 25, 2014 #4

    Simon Bridge

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    You could do a graph of each wave separately or of the superposition of the two waves.
    You could also graph properties other than the transverse displacement.
    There's lots of graphs you could do - but the wave vs wire thing is a likely confusion.

    A particular wave on a wire may be composed of a large number of components - in this case there are two.
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