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Why Does A Wave Move As It Does?

  1. Mar 9, 2006 #1
    Why Does A Wave Move As It Does!?

    My friend, being a highly religious and faith filled person, has come up with several question for me to answer without having to revert to the idea that divine intervention of some sort is involved. On of them is...why does a wave move like it does? What influences a wave to follow the up and down motion, as opposed to moving in a straight line. I must say i couldn't answer the question. And being a pesimist when it comes to religious matters, i know there must be a logical explanation. So...anyone?

    Thanks!!
     
  2. jcsd
  3. Mar 9, 2006 #2

    russ_watters

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    There are several different types of waves and some do move in a straight line. Sound waves are a good example. The way they move is simply that one molecule moves forward into the next, the force(energy transfer) of the impact stopping the first molecule and setting the next one in motion, etc., etc., etc.

    Your friend's objection to science appears to be based on ignorance of it. In fact - it appears that ignorance itself is the explanation for the belief: "I don't know, therefore it must be God." Horrible way to approach knowledge.

    That said, there are some "why" questions that are inherrently unanswerable. But even that doesn't require God to fill in the blanks. The anthropic principle works just as well.
     
  4. Mar 9, 2006 #3
    Yes but what makes this energy transfer between serial molecules so predictable and precise? Why don't the molecules move in random directions, why is the force applied from the previous molecule so accurate everytime?
     
  5. Mar 9, 2006 #4
    You can only fundamentally break the world down so far until you arrive at unanswerable (at the moment) questions. To quickly attribute such things to a higher power of human creation is more of an arbitrary human explanation than any based even remotely on science. Even if a higher power was confirmed one would question where it came from and how it works, eventually arriving back to the question of "why"...right back where you started.
     
  6. Mar 9, 2006 #5

    russ_watters

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    The law of conservation of momentum/Newton's laws. If you push something forward, how is it going to move in any other direction but forward? Whatever the orientation of the collision, the next molecule will travel exactly in the direction of the force applied, just like in a game of billiards.

    http://www.glenbrook.k12.il.us/gbssci/phys/Class/momentum/u4l2b.html
    http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

    Now that said, in the real world, most collisions are not exactly in-line every time, so waves (you need to differentiate between the motion of the individual particles and the motion of the waves) do not just travel in a straight line, they spread out like the waves on the surface of a pond when a pebble is thrown in. That's why you can hear yourself talk - some of the sound is scattered around to where it gets to your ears. But regardless, those waves are still longitudinal, not not transverse as your original question asked. To understand the difference, look here:
    http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html
     
    Last edited: Mar 9, 2006
  7. Mar 9, 2006 #6

    Claude Bile

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    There will be some element of randomness, the thermal energy of the gas for example will induce a random walk type motion for the gas molecules. It must be understood, however, that a sound wave involves the movement of an extremely large number of molecules, while we cannot reasonably predict the motion of each molecule, we can predict the motion of the collective group accurately using statistical methods.

    Claude.
     
  8. Mar 9, 2006 #7
    Russ - those animations are pretty good. I understand the longitudinal wave, but why does the transverse wave propagate? It doesn't look like any force is applied anywhere.
     
  9. Mar 9, 2006 #8

    russ_watters

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    The red line on the left could represent a speaker. It literally pushes the molecules next to it to the right. And that's how a speaker works - the cone-shaped part (called the driver) moves back and forth (for low-freq sounds, you can actually see the motion), pushing on the air next to it. Though the waves are different, it works similar to if you push your hand back and forth across the surface of a pool, pushing a wave in front of your hand.
     
  10. Mar 9, 2006 #9
    Right, but what makes a transverse waves move if the molecules do not contact eachother?
     
  11. Mar 9, 2006 #10

    The way I like to think about it is, Imagine all the particles are the same charge, for theory's sake lets say positive. It is well known that like charges repel eachother, yet they stay in formation inside the nucleus of an atom do to a very interesting and complex set of forces. When you breath, you inhale some of these particles. When you exhale, or speak, the particles are sent back out into this abundance of like charged particles. the ones you are exhaling are not in unison like the ones all around you, so when they approach one another they move away from each other, which in turn approaches other ones, et cetera... so they never touch, the particles are constanlty being dispalced and replaced in other areas, but never touching.
     
  12. Mar 9, 2006 #11

    jtbell

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    In a gas, you cannot have a sound-like transverse wave because, as you said, the molecules would not touch each other and would not be able to influence each other.

    In order to have a transverse wave, you need a rigid or semi-rigid medium in which the molecules are "stuck" to each other somehow, so that when one moves sidewise, it pulls its neighbor sidewise.

    Consider a string stretched horizontally. When one bit of the string moves sideways, it causes the next bit to move sideways also. Otherwise the string would fall apart. There's a bit of a time delay, so the whole string doesn't move up and down in unison.
     
  13. Mar 9, 2006 #12
    Thanks for the great explanation! All someone had to do was say transverse waves are only in solids (and liquids) :rolleyes:
     
  14. Mar 9, 2006 #13

    russ_watters

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    Oops - I switched the names in my post with the big explanation and was still thinking about the wrong one in my last post. Fixed it now....

    Anyway, I'd like to clarify the previous responses (which were pretty good) that in a longitudinal wave, molecules still touch (when they impact), they just aren't connected to each other like a guitar string in a transverse wave.
     
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