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!

Finding the maximum velocity of a wave on a tight string

  1. Oct 30, 2011 #1
    1. The problem statement, all variables and given/known data
    If the displacement of a tight string is represented by

    y(x,t)= Acos(2∏/λ(x-vt))

    Determine an expression for the velocity vy at which a section of the string travels. What is the maximum value of Vy? When is this maximum value greater than the wave propagation speed v?

    3. The attempt at a solution

    I started by differentiating the equation to get Vy = -A(2∏/λ)vsin((2∏/λ)(x-vt))
    I then said that Vy would reach a maximum when sin((2∏/λ)(x-vt)) = 1 but I don't think this is right. Any help would be appreciated. Thank you.
     
  2. jcsd
  3. Oct 30, 2011 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Why not? It gives a maximum speed of Aω
     
  4. Oct 30, 2011 #3
    This is incorrect:

    y(x,t)= Acos(2∏/λ(x-vt))

    [itex] y(x,t)=Acos(\frac{2 ∏}{λ}(x-vt)) [/itex]

    Then you have differentiated. With respect to t :

    Vy = -A(2∏/λ)vsin((2∏/λ)(x-vt))

    [itex] \frac{d y(x,t)}{dx}= -A(\frac{2∏}{λ}) v sin(\frac{2∏}{λ}(x-vt)) [/itex]

    However, I think that you may have a small sign error. The -v should give you one minus sign, but you also differentiated cos(x) which gives -sin(x)

    So:

    [itex] \frac{d y(x,t)}{dx}= v_{y}= A(\frac{2∏}{λ}) v sin(\frac{2∏}{λ}(x-vt)) [/itex]

    So far so good. Then by stating that the [itex] v_{y}=1 [/itex] you are effectively stating that you believe the maximum value that Vy can be is 1.

    This may be correct in some contexts, but my advise would be to consider how you would find the maximum of a function using differentiation?
     
    Last edited: Oct 30, 2011
  5. Oct 30, 2011 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I don't think OP said that, I thought he said:

    "I then said that Vy would reach a maximum when sin((2∏/λ)(x-vt)) = 1 "


    So: the max v would be A(2∏λ)v wouldn't it?
     
  6. Oct 30, 2011 #5
    My apologies for my mistake you are indeed correct. Please ignore my ramblings.
     
  7. Oct 30, 2011 #6

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    No worries, done that myself.
    Checking via differentiation was good advise though.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Finding the maximum velocity of a wave on a tight string
  1. Wave Velocities (Replies: 2)

  2. Waves on string (Replies: 1)

Loading...