Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B A Question about the Speed of Waves...

  1. Jul 4, 2017 #1
    Okay...I read that speed of a wave for a particular medium is constant, i.e it is independent of its frequency and wavelength...Hence we can conclude for a particular medium, all waves travel with same speed...Is' nt it contradictory with our usual observed phoenomenon..Like, if we take air for example, light travels much faster than sound waves???...Likewise, for a string, waves produced on it by our hand are much slower than sound travelling through the same string???....Any help is appreciated..
     
  2. jcsd
  3. Jul 4, 2017 #2

    berkeman

    User Avatar

    Staff: Mentor

    That is not correct. Please read about Dispersion, and post links to articles that dispute Dispersion.
    Is there a name for the style of your post? I see it sometimes, but haven't seen the name for it yet. Unusual structure (off topic question by me I admit)...
     
  4. Jul 4, 2017 #3

    Nugatory

    User Avatar

    Staff: Mentor

    You have misunderstood what you have read. They're only talking about waves of a given type - for example, light waves and sound waves are different types of waves produced by different mechanisms so can and generally do propagate at different speeds in the same medium.
     
  5. Jul 5, 2017 #4
    Okay then what about normal vibration of string and propagation of sound through string???They also have a speed difference..
     
  6. Jul 5, 2017 #5

    Merlin3189

    User Avatar
    Gold Member

    Longitudinal (sound) and transverse waves (wobbles).
     
  7. Jul 5, 2017 #6
    I see your point...Lets say we take a spring for an example...The normal waves in it ( longitudinal ) will be slower than Sound waves ( longitudinal ) propagating though it...
     
  8. Jul 5, 2017 #7

    berkeman

    User Avatar

    Staff: Mentor

    That is correct.
     
  9. Jul 5, 2017 #8
    Let me rephrase...The normal longitudinal waves will b slower than sound waves, despite being same in nature (longitudinal)...Hence it contradicts with the point that waves of ''same nature'' (or a given wave) have same speed in a given medium, which is actually my question???
     
  10. Jul 5, 2017 #9

    berkeman

    User Avatar

    Staff: Mentor

    I'm not really understanding your point. For your example of spring waves and sound waves in the metal spring, you do realize that there are two different "spring constants" involved, right? Do you know the equation for the speed of sound in a material?

    And what is motivating your overall question?
     
  11. Jul 5, 2017 #10
    My question was that, I read that speed of waves in a given medium is always the same (regardless of wavelength or frequency)...Then a person pointed out that light and sound are different types of waves, so their speed is not same ( even in the same medium)...And thats when I took the example of longitudinal waves on a spring and sound waves through a spring, If being same type of waves means same in nature (longitudinal or transverse), then these are of same nature (If being same in nature is not as I interpretted, then please point out what is means, like what is the fundamental difference between these waves)...If they are of same nature, then their speeds in a given medium should be same....
     
  12. Jul 5, 2017 #11

    berkeman

    User Avatar

    Staff: Mentor

  13. Jul 5, 2017 #12
    I did a bit of reading on this website.
    http://www.physicsclassroom.com/class/waves/Lesson-2/The-Wave-Equation
    The solutions to experiments and the questions at the end all point towards "speed of wave is ONLY dependant upon properties of medium"
     
  14. Jul 5, 2017 #13
    I guessing the problem is my poor understanding of the normal longitudinal waves on spring and sound waves through a spring...Perhaps I will get my answer when someone states the basic difference between the two???Like, I think that speed of normal longitudinal waves on a spring ( different waves of diferent wavelenghts and frequencies) remain same...Likewise, speed of different sound waves of different wavelengths and frequencies propagating through the spring will be same...But What is the basic difference between the two???
     
  15. Jul 5, 2017 #14

    berkeman

    User Avatar

    Staff: Mentor

    https://en.wikipedia.org/wiki/Speed_of_sound

    The speed of wave propagation depends on the "spring constant" or "bulk modulus" of the material (basically how stiff the "spring action" is), and the mass density of the material. Longitudinal waves on a spring or Slinky depend on the spring constant of the spring and the overall mass density of the spiral shape. The speed of sound in the metal of the spring is based on the bulk modulus of the metal (*much" stiffer/stronger than the spring constant) and the linear density of the spring metal wire. Since the bulk modulus is so much higher than the spring constant, the speed of sound is much faster than the mechanical waves traveling on the spring.

    And I don't know if you have done the reading on Dispersion yet, but the speed of wave propagation can vary with frequency in a medium. So even for waves of the same "type" and not too different in frequency, you can still get some differences in the speed of propagation. That is the reason a prism can be used to separate white light into a rainbow of colors... :smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: A Question about the Speed of Waves...
Loading...