What is Laplace transform: Definition and 777 Discussions

In mathematics, the Laplace transform, named after its inventor Pierre-Simon Laplace (), is an integral transform that converts a function of a real variable



t


{\displaystyle t}
(often time) to a function of a complex variable



s


{\displaystyle s}
(complex frequency). The transform has many applications in science and engineering because it is a tool for solving differential equations. In particular, it transforms linear differential equations into algebraic equations and convolution into multiplication.For suitable functions f, the Laplace transform is the integral






L


{
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=



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{\displaystyle {\mathcal {L}}\{f\}(s)=\int _{0}^{\infty }f(t)e^{-st}\,dt.}

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  1. M

    Finding Solutions to DE using Laplace Transform

    Homework Statement y'' -6y' + 9y = f(t) f(t)= 0, 0<t<1 1, 1<t<3 0, t>3 Homework Equations The Attempt at a Solution Turning it to heaviside functions I get: y'' -6y' + 9y = u1 - u3 and I solve. in the answers it should be: y(t) = 2tet + u1(1/9 - 1/9e3(t-1)+...
  2. D

    Laplace Transform of unit step function HELP

    Laplace Transform of unit step function HELP! Homework Statement f(t)= e^t on 0<=t<1 . . . . t on 1<=t<2 . . . . sin(t) on 2<=t<infinity Homework Equations Unit Step Function The Attempt at a Solution Here is my attempt at a solution...
  3. B

    Another capacitor Laplace transform problem

    I'm trying to use the Laplace transform to work out another capacitor problem, the voltage as a function of time on a capacitor that's discharging into another capacitor through a resistor. It's the classic two capacitor problem, but I'd like to actually find an expression for the voltage as a...
  4. B

    Finding the Charging Equation of a Capacitor Using Laplace Transform

    Homework Statement I'm trying to use the Laplace transform to find the charging equation of a capacitor with an initial voltage already on the capacitor.Homework Equations V_i = RC\frac{dVc}{dt} + V_c The initial condition of Vc will be \lambda. The Attempt at a Solution V_i(s) = sRC*V_c(s)...
  5. Saladsamurai

    Laplace Transform Notational Question

    Homework Statement Given: \frac{d^2y}{dt^2} + 12\frac{dy}{dt} + 32y = 32u(t) and that all initial conditions are zero, use the Laplace transform to find y(t) This is an example problem from my text. They start the solution by taking the Laplace of both sides assuing y'(0) = 0 & y(0)...
  6. D

    How Do You Correct the Coefficient in an Inverse Laplace Transform Calculation?

    ok I am given the problem ((s/s^2+4)+s+5))/(s^2+6s+18) I use the expand function of my ti-89 and get ((163/70)*s)/((s+3)^2+9) + ((398/85))/((s+3)^2+9) + ((7/170)*s)/(s^2+4) + (6/85)/(s^2+4) 1st term--------------------2nd term-----------------3rd term-------------4th term ok my...
  7. S

    Differential equation with Laplace transform

    Homework Statement t y'' + (3 t-1) y' + 3 y = 6 e^{-3t} y(0) = 1 y(5) = 2 Homework Equations The Attempt at a Solution I tried applying the Laplace transform to the equation but I was having a little trouble... L [t·y''] = -dL[y'']/ds = s2·Y' + 2·s·Y - y(0) = s2·Y' + 2·s·Y - 1 L...
  8. D

    Inverse laplace transform (easy one)

    I am asked to find inverse laplace of (3s+5)/(s^2-6s+25) I do partial frac and get (3s)/((s-3)^2+16) + (5/(s-3)^2+16) I do the translation and get [(3s)/s^2+16 + 5/(s^2+16)]*e^3t My final answer is e^3t[3cos(4t) + 5/4sin(4t)] but the book says the sin term should be 7/2sin(4t)? I say...
  9. L

    Find if a function is the Laplace transform of a periodic function

    Homework Statement Could \[F(s) = \frac{1}{{s(1 - {e^{ - s}})}}\] be the Laplace transform of some periodic function? Why? If so, find that periodic function The Attempt at a Solution If it was the Laplace transform of some periodic function, the the Laplace transform of the first wave...
  10. L

    Find if a function is bounded through its Laplace transform

    Homework Statement If f(t) transforms into F(s), so that \[ F(s) = \frac{{s + 1}}{{s^2 + as + 1}},a \in \] , prove that if a < 0, the function f(t) isn't bounded, and if a >= 0, it is bounded. Prove that if -2 < a < 2, f(t) oscilates. The Attempt at a Solution I honestly have...
  11. L

    Laplace Transform: Order of transformation

    Homework Statement Find the Laplace transform for \[t\left( {\int\limits_0^{t - a} {f(u)du} } \right)H(t - a)\] proving the properties used. Homework Equations If f(t) transforms into F(s): a) \[{( - t)^{n}f(t) \to {F^{(n)}}(s)\] b) \[\int\limits_0^t {f(u)du} \to...
  12. L

    Laplace transform: function defined by parts

    I have this DE: \[y'' - 4y' + 8y = f(t) = \left\{ \begin{array}{l} t{\rm{ }},t \in [0,2) \\ t + 1{\rm{ }},t \in [2,4) \\ 0{\rm{ }},t \ge 4 \\ \end{array} \right.\] I have problems transforming f(t). I know that when I have a function defined by parts, I...
  13. N

    Laplace Transform of Composition of Functions: Floor(f(t))

    Hello, I am trying to take the Laplace transform of floor(f(t)) in order to solve the differential equation f'(t)=floor(f(t)). I know that http://functions.wolfram.com/IntegerFunctions/Floor/22/04/" gives L(floor(t)) = (e^(-s))/(s(1-e^(-s))) instead -- are these equivalent?) and L(f(t)) =...
  14. N

    Laplace Transform of Composition of Functions

    Hello, I am trying to take the Laplace transform of floor(f(t)) in order to solve the differential equation f'=floor(f(t)). I know that L(floor(t)) = (e^(-s))/(s(1-e^(-s))) and that L(f(t)) = F(t) (of course), but I realized that I have no idea how to take the Laplace transform of a...
  15. S

    How to find the inverse of a Laplace Transform?

    Hi, I want to inverse this laplace transform, -(s^(1/2)), seems that the inverse is in complex plane. Where should i start to find this inverse... Thank you.
  16. W

    How to Simplify Laplace Transform with 3 Terms

    "easy" Laplace Transform...help! Homework Statement Evaluate L{(e^-2t)*t*sinht} Homework Equations translation theorem The Attempt at a Solution Just to clarify: the contents of the bracket is the product of 3 terms: e^-2t (e to the power of -2t) t sinht all...
  17. B

    ODE with Laplace Transform: Solving for Y(s) and Partial Fraction Expansion

    Homework Statement The solution to the ODE y''(t) + 4y(t) = 1 + u(t − 2), y(0) = 0, y'(0) = 0 is given by... The Attempt at a Solution OK well I figured this one is good to solve with Laplace transforms. So I took the Laplace of both sides to obtain (s2 + 4)Y(s) = [e-2s/s] + 1/s, which...
  18. P

    Generalized Laplace transform

    "Generalized" Laplace transform Hello, I'm having trouble proving injectivity of what might be called a "generalized" Laplace transform (not the one by Varma). Let f be a rational function and C be a fixed closed contour in the complex plane, (such that C contains not pole of f): The operator...
  19. H

    What is the Inverse Laplace Transform of (9s+5)/(s2-9) with s>3?

    Homework Statement Find the inverse Laplace Transform of (9s+5)/(s2-9) given s>3 Homework Equations I have a table of Laplace Transforms. it says "sinh at" goes with a/(s2-a2) and "cosh at" goes with s/(s2-a2) The Attempt at a Solution I know how to split it up and find the...
  20. N

    Finding the Laplace Transform of a Piecewise Function

    Homework Statement Find the Laplace Transform of: f(t) = {0, t < 5 t2 - 10t + 31, t\geq5Homework Equations The Attempt at a Solution The answer I got was: F(s) = e-5s (2/s3 + 6/s) Is this correct?
  21. J

    Laplace transform initial value problem

    Homework Statement Use the Laplace transform to solve the following initial value problem: x' = 7 x + 5 y, y'= -2 x + e5t, x(0)=0, y(0)=0 Find the expressions you obtain by taking the Laplace transform of both differential equations and solving for Y(s) and X(s) Homework Equations...
  22. P

    Solving Laplace Transform: L{texp(9t)sin(2t)}

    Homework Statement Find the Laplace transform of L{texp(9t)sin(2t)} Homework Equations (-1)^n F^n (s) The Attempt at a Solution I am not sure how to start it. The exp(9t) term throws me off. Usually I would just do (-1) F'(s) where F(s) is the laplace of sin(2t). But do I take...
  23. J

    Laplace Transform Homework: Proving Informally

    Homework Statement supposed to prove informally that f(x)=Integral 0-->infinity (cosxt/(t^2+1))dt == Pi/2*Exp[-x] Homework Equations The Attempt at a Solution no clue, need some hints to get started
  24. J

    Another Laplace Transform Question

    Homework Statement Find the Laplace transform of f(t). f(t) = -9t + 7 + u5(-7 + 9t + et) Homework Equations The Attempt at a Solution The Laplace transform of -9t is just -9/t2 The Laplace transform of 7 is 7/s But I am not sure how to find it for u5(-7 + 9t + et). In the...
  25. B

    Quick Laplace Transform theory question

    I have some problems where I am to take the Laplace transform of some functions, for example te^{xt} not by using the integral definition but instead by using a table of transforms and the fact that the Laplace transform is a linear operator. However, the instructions say that "A preliminary...
  26. M

    Inverse Laplace Transform of (2s+1)/(s^2+16)

    Homework Statement Determine the inverse Laplace transform of the given function F(s) = (2s+1) / (s^2 + 16 ) Homework Equations the LaPlace transform of different functions The Attempt at a Solution I divide the above equation into 2 fractions one with the 2s in the numerator...
  27. D

    Finding Laplace Transform of F(s): A Struggling Student's Story

    Homework Statement find laplace transform of F(s)= (1/t)*(exp(2t)-1) Homework Equations stated answer: ln(s/(s-2)) The Attempt at a Solution i really don't know where to start with laplace. my teacher has done a terrible job really jusut explaining the simplr process. Should i...
  28. U

    How to tell if F(s) is laplace transform?

    Explain why the functions F(s)=\frac{s^2+4s+17}{s^5+16s^3+2^2+6} G(s)=\frac{2s}{(s^2+4)(\sqrt{1+s^2})} are Laplace transforms. (DO NOT TRY TO INVERT THESE LAPLACE TRANSFORMS) Is it something to do with some basic property of laplace transform?
  29. M

    Is the Laplace Transform F(s) = 1/(1+s^2) Defined for All Real Values of s?

    Homework Statement http://u1.imgupload.co.uk/1257811200/9e4e_image.bmpHomework Equations find aboveThe Attempt at a Solution in fact i had no problem in the progress of doing this question and finally i got F(s)= 1/1+s^2 and i want to ask it is vaild for all value of s since it is s^2? is it...
  30. H

    No Laplace Transform? What can be said about the function?

    Hi, We know that if a function does not have a Fourier transform, then it does not have finite energy. Is there an equivalent intuition associated with a function that does not have a Laplace transform?
  31. A

    Extra negative in in Laplace transform solution

    I have the correct solution of this problem (cos(t)+u_{3\pi}[1-cos(t-3\pi)]). As you can tell from my solution below I am off by a extra negative sign on the cos(t) coefficient. I have had this same issue on several other problems, so I believe I must be missing something on the procedure. Any...
  32. W

    How Do You Solve a Piecewise Laplace Transform When f(t)=t?

    Homework Statement This is my first post, so bear with me. I have seen others who have posted their questions and the problem looked like it was typed in mathcad or something. How do I do that? Ok so I'm trying to figure out how to solve a piecewise Laplace transform when f(t)=t the...
  33. E

    Is f(x) of exponential order for the Laplace Transform?

    Hello, By definition, the forward Laplace transform of a function f(x) is: \mathcal{L}\left\{f(x)\right\}=\int_0^{\infty}\text{e}^{-sx}f(x)\,dx. Can we say the same for the function f\left(\frac{1}{x}\right), i.e...
  34. O

    Differential equation with laplace transform

    Homework Statement Solve: ty''+2y'+y=tJ₂(2√t) with y(0)=y'(0)=0 Homework Equations The Attempt at a Solution Applying the laplace transform i get: L(y)=Y L(2y')=2sY L(ty'')=-2sY-s^2Y Putting this together: -2sY+2sY+Y-s^2(dy/ds)=[e^-(1/s)]/s^3...
  35. D

    Finding the Inverse Laplace Transform of a Rational Function

    Homework Statement hi guys, i need to find the inverse Laplace transform of s/((s+1)^(2) +4) the acual question is an IVP, i have worked trough it and am just about finished but i am stuck on this transform. how would you go about getting this into a form that can be transformed like...
  36. M

    Laplace transform question

    Homework Statement Using Laplace transform to solve 4th order DE with a delta dirac forcing function. Has a tricky denominator, I just need a clue. y^{(4)} - y = \delta (t-2) __ IV's y'''(0)=0 , y''(0)=0 , y'(0)=0 , y(0) = 0 Homework Equations I am asked to convert the solution...
  37. M

    Inverting a Laplace Transform w/non-factorable denominator

    Homework Statement A 2nd order DE with a Delta Dirac forcing function, I have been asked to solve in my DE course. y'' + 2y' + 10y = \delta(t-1) Also given are IV's y'(0)=0, y(0)=0 Homework Equations At this stage in my course I am being asked to only use the method of partial...
  38. J

    Laplace transform of exponential function

    I'm trying to find the laplace transform (if possible) of exp(-2r/a)r^2. --- I did integration by parts to check that the integral of exp(-2r/a)r^2 from 0 to infinity is a^3 / 4. but i cannot get the laplace transform to work out. the answer mathematica online gives would have to have the term...
  39. M

    Using Laplace Transform to Solve Non-Zero Initial Condition PDEs

    Homework Statement Help, I don't know how to do the following question: Using Laplace to solve x' -y =1 2x' +x +y' = (t2-2t+1)e-(t-1) Homework Equations x(1)=0 y(3)=0 The Attempt at a Solution The problem I'm having is the initial conditions aren't at zero, and I'm not sure...
  40. O

    Inverse laplace transform with complex inversion theorm

    Homework Statement Find the inverse laplace transform of In(1+1/s) Homework Equations The Attempt at a Solution Using the complex inversion theorm, and the sum of the residues. The only residue is at s=0. and it is a simple pole of degree one. Therefore lim(s approaches...
  41. M

    Sign problem in laplace transform

    Homework Statement \int_{\pi/2}^{\infty}cos(t)e^{-st}dt Homework Equations \int vdu=uv-\int udv v=cos(t) , dv=-sin(t)dt , du=e^{-st} , u=\frac{-e^{-st}} {s} The Attempt at a Solution \int_{\pi/2}^{\infty}cos(t)e^{-st}dt = \frac{-e^{-st}}{s}cos(t) -\int...
  42. M

    Laplace transform of sin(4t+5)

    Homework Statement get laplace transform of sin(4t+5) Homework Equations using some trig identity i.e. Sin(a+b)=sin(a)cos(b)+cos(a)sin(b) or any other that could simplify it The Attempt at a Solution applying the trig identity =sin(4t)cos(5)+cos(4t)sin(5) taking the transform...
  43. J

    What is the Role of the Hankel Norm in Laplace Transform Convergence?

    I've been trying to understand what the Laplace Transform means via It's definition. Even if you are trying to evaluate a simple exponential function:F(s) = \mathcal{L} \left\{f(t)\right\}=\int_0^{\infty} e^{-st} f(t) \,dt. it will not converge for all S in a Riemann sense. I've been trying...
  44. J

    Inverse Laplace Transform (Proof?)

    According go Wikipedia the inverse Laplace Transform is given by: \mathcal{L}^{-1} \{F(s)\} = f(t) = \frac{1}{2\pi i}\lim_{T\to\infty}\int_{\gamma-iT}^{\gamma+iT}e^{st}F(s)\,ds, How do you probe this? I'm surprised that it doesn't depend on the value of \gama...
  45. J

    Laplace Transform (Numeric Computation?)

    I was exploring the connection between the Laplace transform and the Fourier transform (see "=http://earthcubed.wordpress.com/2009/08/30/using-the-fft-to-calculate-the-laplace-transform/"[/URL])and it occurred to me that the from the definition of the Laplace transform: F(s) = \mathcal{L}...
  46. C

    How Do You Solve Complex Inverse Laplace Transforms?

    My books aren't clear at running through these inverse laplace transforms and this ones got me snookered. I'm trying to perform the laplace transform of: 1/s + 40/(24s^2 + 40s + 40) Factor out 24 becomes 1/s + 1.667/(s^2 +1.667s +1.667) Finding A, B, C A/s + Bs+C/(s^2 +1.667s...
  47. X

    Inverse Laplace transform problem

    Hello! I am trying to solve a PDE for it's Green function, I found myself in a position to know G(is) where "i" is imaginary unit, G(s) is a Laplace transform of g(t) and g(t) is the Green's function I am after. So, I've been snooping around and found Mellin's inverse formula at...
  48. M

    Uniqueness of Laplace Transform

    Hello, I was trying to prove that the Laplace transform is unique and was wondering if anyone could tell me if I've made any errors in my attempt. Here it is: Suppose L(f) = L(g), where L() denotes the Laplace transform. We want to show that f = g. By linearity of the transform, L(f - g) = 0...
  49. S

    Laplace Transform with IVP and completing the square

    Homework Statement Use the Laplace Transform to solve: y"+2y'+2y=t y(0)=y'(0)=1Homework Equations L{y(t)} = Y(s) L{y'(t)} = sY(s)-y(s) L{y"(t)} = s2Y(s)-sy(0)-y'(0) using the laplace transform table: tn = n!/(sn+1) where n=1The Attempt at a Solution Take laplace on both sides: L{y"(t)} +...
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