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Waves-displacement vs movement

  1. Feb 11, 2013 #1
    How can a particle move downwards and be displaced upwards? Sounds paradoxical. I understand the part about movement but not displacement. What's the difference? Isnt displacement like the amplitude so when a particle in a transverse wave move down doesnt it mean its displaced downward?
     
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  3. Feb 11, 2013 #2

    Simon Bridge

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    The same way a skiier can be displaced a long way up a mountain but be moving (skiing) down it.
    You have already met this sort of thing in your work on Newton's laws of motion.

    When you displace a string in the +y direction, and let go, which direction does it move in? What physical quantity describes "movement"?
     
  4. Feb 11, 2013 #3
    I'm sorry i still dont get the skiing part too :( hmm movement the physical qty is distance??
     
  5. Feb 11, 2013 #4

    Simon Bridge

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    for movement, the physical quantity is "momentum" but I'd accept "velocity".
    for position, the physical quantity is "distance", I'd accept "displacement".

    something can have a positive displacement and a negative velocity if it is headed back to the origin.
    have you not covered Newton's laws yet? kinematics? v-t graphs?
     
  6. Feb 11, 2013 #5
    I have covered newton's law and i understand that part. I think i got you now :) so a particle will be displaced downwards irregardless of movement when it is below the eqm position?( in a graph of sinusoidal waves diagram)
     
  7. Feb 11, 2013 #6

    Simon Bridge

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    That's right - in a wave of form ##y(x,t)=A\sin k(x-vt)## each point x will be oscilating about y=0 as ##y(t)=A\sin \omega t## the plot will give you a position-time graph. The velocity time graph is the derivative of this: ##v(t)=\omega A \cos \omega t## ... if you plot them above one another (so the time axis coincides) you'll see the relationship.

    You should know from your Newton's laws and kinematics work that an object can be above the ground (positive height) and be falling (negative velocity). It's not just height that can be positive ... something with a negative horizontal displacement (say, it is to the left of the observer/origin and distances to the right are positive) can have a positive velocity (it is moving left-to-right). This should not be a mystery to you.
     
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