MHB Henry's question via email about an Inverse Laplace Transform

AI Thread Summary
The discussion focuses on solving an Inverse Laplace Transform problem involving a complex denominator that does not easily factor. By recognizing a specific form, the expression is transformed through completing the square, allowing for partial fraction decomposition. The coefficients for the partial fractions are determined through a system of equations, leading to a simplified form. The final result of the Inverse Laplace Transform is expressed in terms of hyperbolic functions, specifically involving sine and hyperbolic cosine. The conversation also touches on the confusion regarding the identities of individuals involved in the discussion.
Prove It
Gold Member
MHB
Messages
1,434
Reaction score
20
View attachment 5832

It's not entirely obvious what to do with this question, as the denominator does not easily factorise. However, if we realize that $\displaystyle \begin{align*} s^4 + 40\,000 = \left( s^2 \right) ^2 + 200^2 \end{align*}$ it's possible to do a sneaky completion of the square...

$\displaystyle \begin{align*} \left( s^2 \right) ^2 + 200^2 &= \left( s^2 \right) ^2 + 400\,s^2 + 200^2 - 400\,s^2 \\ &= \left( s^2 + 200 \right) ^2 - 400\,s^2 \\ &= \left( s^2 + 200 \right) ^2 - \left( 20\,s \right) ^2 \\ &= \left( s^2 - 20\,s + 200 \right) \left( s^2 + 20\,s + 200 \right) \end{align*}$

and thus it's now possible to perform partial fractions

$\displaystyle \begin{align*} \frac{A\,s + B}{s^2 - 20\,s + 200 } + \frac{C\,s + D}{s^2 + 20\,s + 200} &\equiv \frac{s^2 + 200}{\left( s^2 - 20\,s + 200 \right) \left( s^2 + 20\,s + 200 \right) } \\ \left( A\,s + B \right) \left( s^2 + 20\,s + 200 \right) + \left( C\,s + D \right) \left( s^2 - 20\,s + 200 \right) &\equiv s^2 + 200 \\ \left( A + C \right) \,s ^3 + \left( 20\,A + B - 20\,C + D \right) \, s^2 + \left( 200\,A + 20\,B + 200\,C - 20\,D \right) \, s + 200\,\left( B + D \right) &\equiv s^2 + 200 \end{align*}$

so it can be seen that $\displaystyle \begin{align*} A + C = 0 , \, \, 20\,A + B - 20\,C + D = 1 , \, \, 200\,A + 20\,B + 200\,C - 20\,D = 0 \textrm{ and } B+ D = 1 \end{align*}$, so solving the system gives

$\displaystyle \begin{align*} \left[ \begin{matrix} 1 & 0 & \phantom{-}1 & \phantom{-}0 & 0 \\ 20 & 1 & -20 & \phantom{-}1 & 1 \\ 200 & 20 & \phantom{-}200 & -20 & 0 \\ 0 & 1 & \phantom{-}0 & \phantom{-}1 & 1 \end{matrix} \right] \end{align*}$

apply R2 - 20R1 to R2 and R3 - 200R1 to R3 and we have

$\displaystyle \begin{align*} \left[ \begin{matrix} 1 & 0 & \phantom{-}1 & \phantom{-}0 & 0 \\ 0 & 1 & -40 & \phantom{-}1 & 1 \\ 0 & 20 & \phantom{-}0 & -20 & 0 \\ 0 & 1 & \phantom{-}0 & \phantom{-}1 & 1 \end{matrix} \right] \end{align*}$

apply R3 - 20R2 to R3 and R4 - R2 to R4 and we have

$\displaystyle \begin{align*} \left[ \begin{matrix} 1 & 0 & \phantom{-}1 & \phantom{-}0 & \phantom{-}0 \\ 0 & 1 & -40 & \phantom{-}1 & \phantom{-}1 \\ 0 & 0 & 800 & -40 & -20 \\ 0 & 0 & \phantom{-}40 & \phantom{-}0 & \phantom{-}0 \end{matrix} \right] \end{align*}$

and thus

$\displaystyle \begin{align*} 40\,C = 0 \implies C = 0 \end{align*}$

$\displaystyle \begin{align*} 800\,C - 40\,D = -20 \implies D = \frac{1}{2} \end{align*}$

$\displaystyle \begin{align*} B - 40\,C + D = 1 \implies B = \frac{1}{2} \end{align*}$

$\displaystyle \begin{align*} A + C = 0 \implies A = 0 \end{align*}$

So the partial fraction decomposition is

$\displaystyle \begin{align*} \frac{1}{2\,\left( s^2 - 20\,s + 200 \right) } + \frac{1}{2\,\left( s^2 + 20\,s + 200 \right) } &\equiv \frac{s^2 + 200}{\left( s^2 - 20\,s + 200 \right) \left( s^2 + 20\,s + 200 \right) } \end{align*}$

So moving on to the Inverse Laplace Transform now...

$\displaystyle \begin{align*} \mathcal{L}^{-1}\,\left\{ \frac{-3\,\left( s^2 + 200 \right) }{ s^2 + 40\,000 } \right\} &= -\frac{3}{2}\,\mathcal{L}^{-1} \,\left\{ \frac{1}{s^2 - 20\,s + 200 } + \frac{1}{s^2 + 20\,s + 200 } \right\} \\ &= -\frac{3}{2}\,\mathcal{L}^{-1}\,\left\{ \frac{1}{s^2 - 20\,s + \left( -10 \right) ^2 - \left( -10 \right) ^2 + 200 } + \frac{1}{s^2 + 20\,s + 10^2 - 10^2 + 200 } \right\} \\ &= -\frac{3}{2}\,\mathcal{L}^{-1} \, \left\{ \frac{1}{ \left( s - 10 \right) ^2 + 100 } + \frac{1}{ \left( s + 10 \right) ^2 + 100 } \right\} \\ &= -\frac{3}{2}\,\mathrm{e}^{10\,t}\,\mathcal{L}^{-1}\,\left\{ \frac{1}{s^2 + 10^2} \right\} - \frac{3}{2}\,\mathrm{e}^{-10\,t}\,\mathcal{L}^{-1}\,\left\{ \frac{1}{s^2 + 10^2} \right\} \\ &= -\frac{3}{2}\,\mathrm{e}^{-10\,t} \,\sin{ \left( 10\,t \right) } - \frac{3}{2}\,\mathrm{e}^{10\,t} \,\sin{ \left( 10\,t \right) } \\ &= -3\sin{ \left( 10\,t \right) } \left[ \frac{1}{2}\,\left( \mathrm{e}^{-10\,t} + \mathrm{e}^{10\,t} \right) \right] \\ &= -3\sin{ \left( 10\,t \right) } \cosh{ \left( 10\,t \right) } \end{align*}$
 

Attachments

  • inv laplace.png
    inv laplace.png
    5.1 KB · Views: 123
Mathematics news on Phys.org
I'm confused. Who is Collin's and Henry and all the others?
 
Joppy said:
I'm confused. Who is Collin's and Henry and all the others?

My students, they email me asking for help, and I direct them here...
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Fermat's Last Theorem has long been one of the most famous mathematical problems, and is now one of the most famous theorems. It simply states that the equation $$ a^n+b^n=c^n $$ has no solutions with positive integers if ##n>2.## It was named after Pierre de Fermat (1607-1665). The problem itself stems from the book Arithmetica by Diophantus of Alexandria. It gained popularity because Fermat noted in his copy "Cubum autem in duos cubos, aut quadratoquadratum in duos quadratoquadratos, et...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Back
Top