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!

Homework Help: Laplace Transform Help

  1. Feb 10, 2013 #1
    1. The problem statement, all variables and given/known data
    Find the laplace transform of:

    v(t) = sin(7t)u(t-3)

    2. Relevant equations

    u(t) is step response

    3. The attempt at a solution

    I know that taken alone the laplace of:

    sin(7t) is 7/(s^2+49)

    u(t-3) is (1/s)*(e^-3s)

    I don't understand how I would figure out the answer to this problem if I have u(t-3). For some reason, seeing the u(t-3) rather than u(t) is really making things hard to understand since I don't know how u(t-3) would be integrated.

    I considered setting t-3 =x which would yield:

    v(t) =sin(7x+21)u(x)

    v(t) = sin(7x)cos(21) + cos(7x)sin(21)

    Laplace = [7*cos(21) + s*sin(21)]/[s^2 + 49]

    but I don't know if I'm that's the right answer.
    Last edited: Feb 10, 2013
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted