1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Standing waves and reflection

  1. Dec 5, 2014 #1
    I don't know from where this type of image corresponds to standing waves?
    http://www.stmary.ws/highschool/physics/home/notes/waves/img7A.gif [Broken]
    As at any point of time either original wave would be present or it's reflection as when the original wave would reach and hit wall then only it would reflect from wall so only one thing either original wave or it's reflection would be present at any given time not both as in picture given above.I think it can be like first wave's reflection and the second original or new wave hence we get this type of pattern as in image ,right?
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Dec 5, 2014 #2

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I have just finished with a thread that you launched earlier. I suggest that you look at the Wikipedia and Hyperphysics pages about standing waves if you want a good explanation. Some self driven research will be useful for you.
    BTW, that diagram is not a good one because it seems to suggest that the 'string' has only three positions (or possibly two). The string moves up and down over its whole length. as a result of the interference between the leftwards and rightwards moving waves so the area between the upper and lower set of curves should be filled in.
     
  4. Dec 5, 2014 #3
    can someone explain in this video at time 10:14 the loose stand doesn't change the direction of propagation of wave so reflected wave from this loose stand would look like?just pause this video at 10:14 have a look at snapshot,does reflected wave from this loose stand would be on top of the wave present at 10:14 snapshot?
     
  5. Dec 5, 2014 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I don't understand your question but he explains it all correctly. Look at it again very carefully.

    The loose stand does change the direction of propagation because no wave exits to the right.
     
  6. Dec 5, 2014 #5
    My question-I was asking in my original post that which two wave superimpose in order to form standing wave ?possible answers
    (1)the wave which is going towards right and the wave which arises by it's own reflection,these two wave superimpose to form standing wave.OR
    (2)first wave's reflection from left and the second new wave from right (as waves are continueously generated) ,these two wave superimpose hence we get stationary waves
    which one is correct i think second one..
     
  7. Dec 5, 2014 #6
  8. Dec 6, 2014 #7
    standing waves in demonstration as in this video
    look like it is never totally horizontal i.e with no crests and no trough just a string tied at two ends like this

    upload_2014-12-6_15-50-2.png
    But it is actually present for a small instant as in image below at t2 and t4 second but it is so rapid that we are not able to see it
    4_1.gif
    RIGHT?
     
    Last edited: Dec 6, 2014
  9. Dec 6, 2014 #8
    The nodes remain horizontal at zero and yes at some point in time the string is flat if the wave is balanced.
     
  10. Dec 6, 2014 #9
    it is present for a small instant as in last image of my previous post at t2 and t4 second .right?
     
  11. Dec 6, 2014 #10
    Yes, as it shows the superposition to the right, the total between the crests and troughs are zero across the string at all times.
     
  12. Dec 6, 2014 #11
    then why we are not able to see it? as in the video i have mentioned i was not able to see that the string is flat at some instants.is this because the motion is so rapid ?i.e because of high frequency of the wave?
     
  13. Dec 6, 2014 #12
    It is the point at which the string is moving the fastest, making it the most difficult to see with your eyes. The string slows down to change direction at the crest and then speeds up as it passes the zero line slowing again as it approaches the trough.
     
  14. Dec 6, 2014 #13
    hmmm...........it means when the string looks still , it is fastest .right?
     
  15. Dec 6, 2014 #14
    If that were true it would look more like a straight line, it is slowest at crest or trough so that is why you mainly see the wave amplitude.
     
  16. Dec 6, 2014 #15
    how can it be slowest at some point in time and fastest at the other ,as it's frequency remain same?
     
  17. Dec 6, 2014 #16
    The frequency is another way to say wavelength. The string is just demonstrating the wavelength. The nodes are points at 1/2 wavelength in the standing wave. The dots in the wave video from post #3 show the velocity as distance between the dots. See how they are closer together at the top and bottom of the wave then in between the dots get farther apart?
     
  18. Dec 6, 2014 #17
    ya i saw that dots are closer together at the top and bottom of the wave then in between the dots get farther apart but why this is so?
    i did not understand this line of your post.
     
  19. Dec 6, 2014 #18
    Assuming the time between each dot is constant, the distance is greater therefore the velocity is greater.
     
  20. Dec 6, 2014 #19
    the time between each dot means?
     
  21. Dec 6, 2014 #20
    One interval of the integral on the graph from left to right. A standing wave is a 1 dimensional occurrence being shown in 2 dimensions, to the right above the string is + and below is -, and the wave is just 1 dot moving up and down. Lights strobing from red to blue to red is 1 wave where the EM frequency changes but the frequency of wave we are looking at doesn't even have a dimension where the light does to reach your eyes.
     
  22. Dec 6, 2014 #21
    how this post answer my question ' the time between each dot means'?
     
  23. Dec 6, 2014 #22
    One line distance on a graph of time. When you slice the wave into equal fractions of a second you see the dot slow up and speed back down when your time interval is constant. The string of balls shows this configuration. The string as in the standing wave from post #1 is a solid line showing the peak of the waves and crests more prominently, by showing peak amplitude for a bit longer duration.
     
  24. Dec 7, 2014 #23

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    You should qualify that statement. Frequency times wavelength is wave speed, so for a non dispersive medium (only) the wavelength will be inversely proportional to frequency.

    Also, something that has not been brought up yet is the fact at a real standing wave (when the string is vibrating at resonance) is best thought of as many many waves (not just 'a wave' going left and 'a wave' going right) -it is the result of all the sections of the original launched wave, going up and back, endlessly because the returning wave is also reflected by the source end. This is not often mentioned in popular 'explanations'. The fact is that all standing waves will dissipate some energy (Friction / radiated sound / resistance / radiated EM waves etc). Without some steady loss of energy, the wave will just grow and grow, building up each time the initial wave does a lap and extra energy is added. The final amplitude reached as a standing wave builds up is when the rate of energy put in is equal to the rate of energy loss. When the source end has 'just the right' resistive impedance, the returning energy will be dissipated so the peaks of the standing wave will be least, Then you will actually, no longer have actual resonance and there will be a small standing wave for all frequencies (because of only one far end reflection). Most demos of standing waves will involve a loosely coupled input signal and it may take several hundreds of cycles before the amplitude builds up to a maximum.
    Videos of standing waves on strings will always tend to miss showing the string when it is horizontal because it is at its maximum speed at that time. Each section of the string performs Simple Harmonic Motion; with amplitude which depends on the position on the string. There is much less blurring at the maximum extremities of the motion so the camera catches that part most. (That's the same for all SHM depiction).
     
  25. Dec 7, 2014 #24
    thanks sophiecentaur and jerromyjon.
     
  26. Dec 7, 2014 #25

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I think you would be well advised to get up to date with SHM (oscillations) before you launch into how waves behave. Waves are a significant step further on from simple Oscillations.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook