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Homework Help: Laplace transform question

  1. Jun 14, 2008 #1
    1. The problem statement, all variables and given/known data

    This has to do with the unit step function. The question is;

    Sketch the function g(t) = 1 for 0<t<1 and is 0 for t>1.

    Express g(t) in terms of the unit step function and hence or otherwise show that

    L(g(t)) = 1/s^2 - e^-s (1/s^2 + 1/2)

    2. Relevant equations

    3. The attempt at a solution

    I sketched the graph (see attachment below). Im guessing the unit step function is t(u(t) - u(t-1)).

    I tried one way of getting that answer like this t( u(t-1) u(t+1) - u(t-1) ). Obviously it didnt work out. I think I need to get u(t) in the form u(t-a) so I can use the table of Laplace transforms and just read out of it. How do I manipulate this kind of functions?


    Attached Files:

  2. jcsd
  3. Jun 15, 2008 #2


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    Homework Helper

    This is from my notes:

    http://img508.imageshack.us/img508/1289/unitstepdk5.jpg [Broken]

    In this case, the function should be u(t) - u(t-1).

    The Laplace transform you should be using is [tex]L[f(t-a)u(t-a)] = e^{-as}F(s) \ \mbox{where} \ F(s) = L[f(t)][/tex]

    Note that u(t-a) = f(t-a)u(t-a) where f(t) = 1. f is a constant function.

    So you only need to find the laplace transform of 1 and add in the e^(-as) factor in front of it.
    Last edited by a moderator: May 3, 2017
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