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B Confusion with Waves

  1. May 1, 2016 #1
    Hi everyone.
    I'm currently studying waves in physics at the moment but I'm super confused and hoping someone could help me clear up some things. Firstly I'll post what I think it correct (I know it's wrong) and hopefully someone could pick up exactly where I am getting confused.

    Waves are a periodic motion which flows from one place to another along a medium or empty space.
    Waves have two categories:
    1) Mechanical (requires a medium)
    2) Electromagnetic (does not require a medium)

    1) Mechanical waves have two subcategories depending on what direction the particles are displaced reference to the direction of energy transport.
    i) Transverse Waves: Particles are displaced perpindicular to the direction of energy of transport.
    ii) Longitudinal Waves: Particles are displaced parallel to the direction of energy of transport.

    I can understand transverse and longitudinal wave examples. We got introduced to nodes & antinodes and I get the definitions of each and the difference between their respective displacements but this is the area where I'm getting confused. Do all transverse waves have antinodes and nodes?
    I know that longitudinal waves have crests and troughs (compressions and rarefactions) so does that mean transverse waves have nodes and antinodes?

    Now I'm also getting confused with progressive and standing waves.
    From what I've read, I've heard that these are types of wave PATTERNS and not waves themselves. If so how would you know if you're going to experience a standing wave or a progressive wave in an object?
    We are currently doing pipes (Open & Closed) and Strings. But I'm confused why an open pipe column will have standing waves when it's open on both ends. Shouldn't the wave just continue from one end of the pipe to the other in an uninterrupted fashion?
    Also, Is a progressive wave also known as a travelling wave?

    Sorry for the essay, but I really can't wrap my head around it.
    Thanks in advance,
    Ash
     
  2. jcsd
  3. May 1, 2016 #2

    Simon Bridge

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    Nodes and antinodes are characteristics of standing waves only.
    The antinode is where the value changes lots and the node is where it remains the same.

    "Progressive" means "travelling".
    To work out if you have a standing or progressive wave, you need to measure the displacement (say) wrt time for at least two places and look at the relationship.
    i.e. if you have two toy boats tethered to two pylons at the warf ... and you see one boat bob up then down and right after that the next boat does the same - a ravelling wave probably just passed by.... however if the boats go up and down in unison, just to different max heights, then they are on a standing wave.

    That's one solution: where the air just blows in one end and out the other ... but, in that case, there were no oscillations to start with.

    You can blow across the end of a pipe and get a noise even though both ends are open.
    A flute is also a tube open at each end - there is a reed a short way inside that makes the oscillations.
    Basically - the wave reflects off the sudden change in pressure at the ends of the pipe... some of the wave does escape.

    Yes. And a standing wave is also known as a stationary wave.
     
  4. May 2, 2016 #3
    Thanks for the reply!
    You've cleared up 99% of my confusion, I just got 1 more question.

    If you pluck a guitar string, it has it's fundamental frequency, with respective harmonics following it.
    As a guitar string has nodes and anti-nodes, what is the relationship between harmonics and standing waves?
    Is there a relationship?
    I understand that there would be more nodes and antinodes when you increase up in harmonics (e.g., 3rd harmonic, 4th harmonic, 5th harmonic etc). But is a harmonic a standing wave or shouldn't you make a correlation between the two?

    Cheers
     
  5. May 2, 2016 #4

    Simon Bridge

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    Harmonics and standing waves are the same thing.
     
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