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

Homework Help: Fourier transforms

  1. Mar 1, 2007 #1
    1. The problem statement, all variables and given/known data

    For a visual of what I am talking about, please visit: http://webhost.etc.tuiasi.ro/cin/Downloads/Fourier/Fourier.html
    and scroll down to the "Examples of Fourier Transforms" part

    I am ask to explain why the fourier transform on the rectangle function was similar to the fourier transform on the trangular function.

    2. Relevant equations

    3. The attempt at a solution

    so here what I think, and i'm not totally sure about it. The FT of a rectangular function is sin and rhe FT of the trangular function is a sin^2. The FT are similar because both functions are even, symetric, and always positive. The rectangular function is a constant function, which gives the sin, while the trangular function is a linear function, which gives the sin^2. Maybe a x^2 function with bounds will give a sin^3? not really sure about that. Is my reasoning correct for why the two FTs are similar?
  2. jcsd
  3. Mar 1, 2007 #2
    Can't you calculate the FT of x^2 function? it should be easy..
    define a function bx^2 between -a and a , and see what the FT would be..
  4. Mar 1, 2007 #3
    ok i did it, and it does show that it would be sin^3

    know this, why is it that the higher the power, the larger n is for sin^n?
  5. Mar 1, 2007 #4


    User Avatar
    Science Advisor
    Gold Member

    First notice that the transform of a square pulse is sin(aw)/(aw) which is called sinc(aw). It is not the same as a simple sine.

    To answer your question, here's a different approach--think in terms of convolutions. The convolution of a square pulse with itself is what? (It should be in your book.) Therefore what is the transform of the convolution?

    As for x^2, how would you produce that with a convolution and what is its transform?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook