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Homework Help: Shape of waves

  1. May 3, 2014 #1
    a wave pulse on a string has the dimension shown in figure (see attachment) the wave speed is v=1 cm/s. point o is a free end, the shape of wave at time t=3s is?
    someone please explain why would the answer to this question differernt if the point O was a fixed end?
    how does it make a difference if the string is fixed at both the ends or at a single only??

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    Last edited by a moderator: May 3, 2014
  2. jcsd
  3. May 3, 2014 #2


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    Staff: Mentor

    The reflection you get from a fixed end of the string is different from the reflection you get if the end of the string is open (like if it is a frictionless ring on a vertical rod, so it is free to move up and down without resistance.

    Can you tell us what is different about the two reflections (first when the string end is fixed, and second when it is free to move)?
  4. May 3, 2014 #3
    i am sorry sir... but i didnt get your point.
    please explain..
    Last edited: May 3, 2014
  5. May 3, 2014 #4
    when the end is fixed the wave flips upside down upon reflection. That doesn't happen when the end es free to move in which case the wave doesn't flip. In that question you're asked to draw the shape of the string halfway through reflection which means half of the wave is still coming in while the other half has already reflected and is moving back out. depending on the kind of reflection (flip or no-flip) you will obviously get a different shape when you combine the incoming half with the outgoing half.
  6. May 3, 2014 #5
    you mean the final amplitude after reflection will become 2cm?
  7. May 3, 2014 #6
    yes, you figured it out
  8. May 3, 2014 #7
    What do you think would happen if the end was fixed?
  9. May 4, 2014 #8
    i assume the amplitude would remain same...and the wave would flip to the opposite direction.. am i right?
  10. May 4, 2014 #9
    Yes, the wave flips. What happens when you add half a wave going in with the other half coming out considering that the reflected part has flipped upside-down?
  11. May 4, 2014 #10
    the wave would become stationary?
  12. May 4, 2014 #11
    Stationary would mean the string stops moving. That doesn't happen since the wave will continue its motion. We're talking about the shape of the wave, not its movement.
  13. May 4, 2014 #12
    would it become a straight line?
  14. May 4, 2014 #13
    That's right. You're better at physics than you seem to believe.
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