Solving DE with Laplace Transforms and g(t) Function

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Homework Help Overview

The discussion revolves around solving a differential equation of the form y'' + 4y = g(t) using Laplace transforms, with initial conditions y(0) = 2 and y'(0) = 2. The function g(t) is described as a piecewise function, and participants are examining the implications of this setup.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are discussing the application of Laplace transforms to the given differential equation and questioning the treatment of the piecewise function g(t). Some express uncertainty about specific steps in their calculations, particularly regarding the handling of the exponential term and the definition of the unit step function.

Discussion Status

There is an ongoing exploration of the problem with various interpretations of the piecewise function and its implications for the Laplace transform. Some participants have offered insights into the definition of the unit step function and suggested alternative approaches to the integral calculations involved.

Contextual Notes

Participants have noted potential typos and clarifications regarding the notation used for the unit step function, indicating a need for precision in the mathematical expressions being discussed.

s3a
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Homework Statement


Solve the differential equation y'' + 4y = g(t); y(0) = 2, y'(0) = 2 using Laplace transforms.

The g(t) function is a piecewise function and is attached as g.jpg.


Homework Equations


Laplace transforms of regular and unit step/Heaviside functions.


The Attempt at a Solution


My work is attached as MyWork.jpg. I suspect the part with the e to be the culprit but I'm not specifically sure as to what I did wrong and would appreciate it if someone could point it out to me.

Thanks in advance!
 

Attachments

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s3a said:

Homework Statement


Solve the differential equation y'' + 4y = g(t); y(0) = 2, y'(0) = 2 using Laplace transforms.

The g(t) function is a piecewise function and is attached as g.jpg.


Homework Equations


Laplace transforms of regular and unit step/Heaviside functions.


The Attempt at a Solution


My work is attached as MyWork.jpg. I suspect the part with the e to be the culprit but I'm not specifically sure as to what I did wrong and would appreciate it if someone could point it out to me.

Thanks in advance!

I didn't check all your steps, but it looks to me you haven't taken into account the f(t) = t part. Your formula for that is$$
f(t) = t(1-u(8\pi))+8\pi u(t-8\pi)$$
 


What does u(8π) mean?
 


s3a said:

Homework Statement


Solve the differential equation y'' + 4y = g(t); y(0) = 2, y'(0) = 2 using Laplace transforms.

The g(t) function is a piecewise function and is attached as g.jpg.

Homework Equations


Laplace transforms of regular and unit step/Heaviside functions.

The Attempt at a Solution


My work is attached as MyWork.jpg. I suspect the part with the e to be the culprit but I'm not specifically sure as to what I did wrong and would appreciate it if someone could point it out to me.

Thanks in advance!

Why don't you just use the definition, rather than applying formulas that can be misused (as you did)? For [itex]g(t) = \min(t,8 \pi)[/itex] we have
[tex]L[g](s) = \int_0^\infty e^{-st} g(t) \, dt = \int_0^{8 \pi} e^{-st} t \, dt <br /> + \int_{8 \pi}^\infty e^{-st} 8 \pi \, dt,[/tex] and just do both integrals.

RGV
 
Last edited:


s3a said:
What does u(8π) mean?

Sorry, that was a typo; that expression should be ##u(t-8\pi)## but the time for me to edit and correct it has expired.
 


u(t-8∏) is a unit step function. You can think of it as energy going into your system after 8∏ time has elapsed. The function is 0 <= 8∏ and 1 after 8∏.
 

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