Laplace transform Definition and 779 Threads

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


{
f
}
(
s
)
=



0





f
(
t
)

e


s
t



d
t
.


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

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  1. M

    Is My Proof of Laplace Transform Uniqueness Correct?

    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...
  2. 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)} +...
  3. S

    Solving Laplace Transform: y"+2y'+2y=t, y(0)=y'(0)=1

    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)} +...
  4. C

    Laplace Transform of t*e^(it): Real Part

    Homework Statement t*e^(it) how do we take the laplace transform of this . would it be 1/((s-i)^2) then how would we get the real part of that .
  5. K

    Why must X(s) occur in conjugate reciprocal pairs for a real x(t)?

    Hello, if i said that x(t) is real, why X(s) must occur in conjugate reciprocal pairs ?
  6. Mapes

    Laplace transform of a Taylor series expansion

    I'm reading a paper on tissue cell rheology ("Viscoelasticity of the human red blood cell") that models the creep compliance of the cell (in the s-domain) as J(s) = \frac{1}{As+Bs^{a+1}} where 0\leq a\leq 1. Since there's no closed-form inverse Laplace transform for this expression, they...
  7. D

    Diff Eq problem, Laplace Transform

    Homework Statement Find the Laplace Transform of the given function H(t-1)t^2 I'm not sure how to add (t-1) to the t^2 term to solve the problem Any help would be greatly appreciated.
  8. E

    Laplace transform heavyside function

    Homework Statement find the transform of sin (wt) u(t - b) I have to get it into the form for the time shift but I do not know how to deal with the unit function help appreciated
  9. M

    Laplace Transform solution help

    Homework Statement Find the solution of the given initial value problem: y''+4y=upi(t)-u3pi(t) y(0)=7, y'(0)=5 The Attempt at a Solution So I found the L{} of the above equation: s2Y-s*f(0)-f'(0)+4Y = (e-pi*s)/s-(e-3pi*s)/s Combining and substituting the numbers I get...
  10. N

    Complex Analysis: Inverse Laplace Transform

    Homework Statement Hi all. I have found the Laplace transform of the following piecewise function: f(x) = \left\{ {\begin{array}{*{20}c} {0\,\,\,\,{\rm{for}}\,\,\,\,x < 0} \\ {x\,\,\,\,{\rm{for}}\,\,x \in (0;1)} \\ {0\,\,\,\,{\rm{for}}\,\,\,\,x > 1} \\ \end{array}} \right. I...
  11. D

    Inverse Laplace Transform of s2-5/s3+4s2+3s

    Homework Statement s2-5 / s3+4s2+3s Homework Equations find the inverse laplace transform The Attempt at a Solution for the denominator, it can be factored out to s(s+3)(s+1) or one could complete the square and thus the denominator would be s(s+2)2-1. neither of this help in...
  12. K

    Solving Differential Equations with Laplace Transforms

    I am reading a text about Laplace transform in solving differential equations. Seems that this method is so powerful. To practice how it works, I makeup a very simple problem \frac{dy}{dt} = e^{wt}y This equation could be solved by variable separation and then intergrate LHS and RHS. But I...
  13. Y

    Proof of L-1{F(s)} = L-1{F(z-2)} = g(t) * e-2t

    to calculate the inverse laplace transform of a function F(s), s+2 was replaced with z for convenience. the inverse laplace transform of z was found--let's denote the function g(t). now, how do i prove that L-1{F(s)} = L-1{F(z-2)} = g(t) * e-2t i am attempting to prove by rewriting...
  14. mbisCool

    Laplace Transform f(t)=tcos(t)

    Homework Statement I need to find the laplace transform of f(t)=tcos(t). Homework Equations \int e^-^s^ttcos(t)dt The Attempt at a Solution I just need help on how to integrate this. I can find the answer easily using the f(t)=tcos(kt) general formula but I wish to find it directly.
  15. F

    How to Solve a PDE using Laplace Transform?

    Homework Statement Use the Laplace Transform to solve the PDE for u(x,t) with x>0 and t>0: x(du/dx) + du/dt = xt with IC: u(x,0) = 0 and BC: u(0,t) = 0 Homework Equations The Attempt at a Solution After taking LT of the PDE wrt t, the PDE becomes x(dU/dx) + sU = x/(s2)...
  16. X

    Reducing final answer of laplace transform

    Homework Statement The problem is not getting the answer to the laplace transform but instead reducing my answer so i dnt lose marks. If i work out the laplace transform of: L(t^3 * sinh(4t)) to be 3!/(2(s- 4)^4)- 3!/(s(s+ 4)^4) then how do i add these to get a single fraction? Its doing...
  17. tony873004

    How Do You Calculate the Inverse Laplace Transform of \( \frac{1}{(s+2)^3} \)?

    L-1(1/(s+2)3) I don't see this one in the table. How do I solve the inverse Laplace transform? I know from class notes that the answer is (1/2) t2e-2t But I don't know to get it. Thanks!
  18. R

    Laplace Transform of y''(t)-y(t)=0 w/ Initial Conditions

    I need to find the laplace transform of y''(t)-y(t)=0 with initial conditions y(0)=1 and y'(0)=3 My attempt: Ly''(t)=(s^2)F(s)-s(1)-(3) Ly'(t)=sF(s)-1 so (s^2)F(s)-s(1)-(3)-sF(s)-1=0 I need to isolate F(s) so F(s)=-(2/s^2) Is this correct?
  19. R

    Laplace Transform of S+4: Explanation & Examples

    http://users.on.net/~rohanlal/laplacetrans5.jpg In the third line, what is S+4 the laplace transform of?
  20. N

    Help with Inverse Laplace Transform

    Hello, I know normally giving solutions is frowned upon, but I have lost my colleagues data sheets and desperately need this transform to continue my work. I am looking for the Inverse Laplace Transform of the Transfer Function: H(s) = 1 / (s^2 + 20s + 5K) where K is an as yet unknown...
  21. B

    Laplace Transform Problem - Peacewise Functions

    Okay so this problem has been giving me nightmares ever since I laid eyes on it. The problem: y'' - y = f(t), y(0) = 1, y'(0) = 0 f(t) = 1, 0 (less than equal to) t (less than) pi/2 f(t) = sin(t), t (greater than equal to) pi/2 Any help would...
  22. S

    Laplace Transform: Cos(3t), Cos(3), Cosh, Sinh, Partial Fractions

    1) If Laplace transform...Cos(3t) = s/(s^2+9) but how about laplace transform ...Cos(3) = ? 2) Cos(0) =1 but how about Cosh(0) =? ; Cosh(1) =? Sinh(0) = ?; Sinh(1) = ? 3) How to do the partial fractions for ... (s+13)/(s^2 +2s+10) = ?? Thx for help =)
  23. D

    Laplace transform of Diff. Eq containing sine Function

    Homework Statement Find the Laplace transform of the pendulum equation. Homework Equations The pendulum equation: \frac{d^{2}\theta}{dt^{2}} + \alpha \frac{d \theta}{dt} + g \sin(\theta) = 0 s = \sigma + i \omega The Attempt at a Solution Taking the laplace transform I get...
  24. S

    Solving Laplace Transform of (1-3cost)/t^2

    Homework Statement Find the laplace transform of the following: (1-3cost)/t^2 Homework Equations The Attempt at a Solution I know many methods to find the laplace transform like the first and second shift theorems, the differentiation one, convolution, but for this particular...
  25. P

    How Do I Find the Inverse Laplace Transform for These Functions?

    I'm having some trouble getting the inverse Laplace to the following problems...I need some help F(s)=24/s^5 F(s)= 4/[((s-2)^2)+25 F(s)= s/(s-1)(s+1)
  26. E

    Laplace Transform of cos(kt) using Power Series expansion

    Homework Statement The problem just states to find the Laplace Transform of cos(kt) from its power series expansion, instead of using the formula for the transform of a periodic function.Homework Equations Equation for Laplace transform of a function f(t) ->\int(e^{-st}f(t))dt Power Series...
  27. M

    Solve Laplace Transform for sqrt(t/pi)cos(5t)

    Homework Statement (1 pt) Given that L(cos(5t)/(pi*t)^.5)=exp(-5/s)/sqrt(s) find the Laplace transform of . sqrt(t/pi)cos(5t) Homework Equations I honestly have no idea. There is the integral of f(t)exp(-st) though The Attempt at a Solution Ive tried many reasonings...
  28. Y

    Laplace Transform Homework: Finding f(t)

    Homework Statement I'm a little a bit confused about the following exercise because of the two segments of the function. How can we find the Laplace transform of this function f(t) = \begin {cases} t , 0\le t < 4 \\ 5 , t\ge 4\end {cases} Homework Equations The Attempt at a...
  29. B

    Help show that Laplace transform exists

    Homework Statement Show that f(t)=e^(5t) sin(t) satisfies the condition for the LaPlace transform to exist I can solve the Laplace and get 2/((s-5)^2 + 4) How do I show that the conditions exist? If it is solvable using the table, shouldn't that be enough?
  30. JJBladester

    Find f(t) given an inverse Laplace transform

    Homework Statement Find f(t). Homework Equations L^{-1}\left\{\frac{s}{s^{2}+4s+5}\right\} The Attempt at a Solution I tried completing the square to get to the solution and I ended up with: L^{-1}\left\{\frac{s}{s^{2}+4s+5}\right\} =...
  31. JJBladester

    Laplace transform of sin(2t)cos(2t)

    Homework Statement Find the Laplace transform of f(t) = sin(2t)cos(2t) using a trig identity. Homework Equations N/A. The Attempt at a Solution I know the double-angle formula sin(2t) = 2sin(t)cos(t) but that's not helping much. Can you give me some advice on how to...
  32. P

    Inverse Laplace Transform of \frac{1}{\sqrt{s+1}}: How to Calculate

    Homework Statement Where to begin when trying to calculate the inverse Laplace transform of \hat{f}(s)=\frac{1}{\sqrt{s+1}}? I know it's tabulated, but I'd like to calculate it without resorting to a tabulated result. Thanks
  33. M

    Solving Laplace Transform Homework: y''+y = f(t)

    Homework Statement y''+y = f(t) y(0) = 0; y'(0)=1 f(t) = 1, 0<=t<pi/2 0, pi/2<=t The Attempt at a Solution so far, i have (s^2+1)*L{y} = \frac{s-e^(-pi/2s)}{s} +1 what is next ?
  34. N

    Laplace Transform of 2nd Order ODE

    Homework Statement This is a problem from my book that I'm very close to finding the solution to, but I'm a little off. I have a feeling it's some small error I'm just overlooking because I'm so hungry/sleep-deprived. Anyway, the question asks you to find the Laplace transform of the given...
  35. K

    Frustrated with Inverse Laplace Transform: Help Needed!

    Homework Statement This isn't a homework, I'm just trying to recap for a mid-term. Anyways, it's about inverse Laplace transformation and this crap is starting to piss me off! How the heck are you supposed to go from \frac{ \frac{-U}{s}}{R+sL+ \frac{1}{sC}} to - \frac{2 \sqrt{10}}{...
  36. tony873004

    How to Perform Inverse Laplace Transform for 1/(s+a)^n?

    How do I do the inverse of this? 1/(s+a)n is not in the table. \begin{array}{l} Y\left( s \right) = \frac{1}{{\left( {s + 4} \right)^4 }} \\ y\left( t \right) = L^{ - 1} \left[ {Y\left( s \right)} \right] \\ \end{array}
  37. tony873004

    Solving Laplace Transform: Finding \frac{1}{{\left( {s + 4} \right)^2 }}

    From the class notes: \begin{array}{l} y'' + 8y' + 16y = te^{ - 4t} ,\,\,\,\,\,y\left( 0 \right) = y'\left( 0 \right) = 0 \\ \\ L\left[ {y''} \right] + 8L\left[ {y'} \right] + 16L\left[ y \right] = \frac{1}{{\left( {s + 4} \right)^2 }} \\ \end{array} How did he get...
  38. M

    What is the Laplace Transform of sinh(bt)?

    Homework Statement L{sinh(bt)} Homework Equations sinh(bt)=(e^bt-e^-bt) The Attempt at a Solution so the answer says it's b/(s^2-b^2) but get get that far.. here's what i have ... 1/2L{e^bt}+1/2L{e^-bt} = 1/2[e^((s-b)t)-e^((s+b)t)]
  39. P

    Laplace transform using the basic integral

    The question: Find the Laplace transform of f(t)=te^2^t I have got: Though, this will not become f(s) = 1/(s+2)^2 Anyone got an idea about what I am doing wrong?
  40. P

    Laplace transform Heaviside step function

    Homework Statement What is the Laplace transform of f(t) = 0 for 0 < t < 2 and f(t) = (4-t) for 2 < t < 3 and f(t) = 1 for 3 < t < 4 and f(t) = (5-t) for 4 < t < 5 and f(t) = 0 for t > 5? Homework Equations The Attempt at a Solution f(t) = H(t-2)(4-t) - H(t-3)(4-t) + H(t-3) - H(t-4) +...
  41. C

    Laplace transform of step function

    this isn't homework, this is just general knowledge and i can't figure it out.. please help, thx Find the Laplace transform of the given function: f(t) = { 0 t<2, (t-2)^2 t>=2 I tried working it out and this is where i get stuck f(t) = (t-2)^2 * u(t-2) I am not sure if I got the...
  42. Kleanthis

    Solving Initial Value Problem with Laplace Transform

    Hello from Greece!Gongrats for your forum. I was wondering if anyone could give me a hand with this initial value problem. It s to be solved via Laplace transform. y''(x)-xy'(x)+y(x)=5 , y(0)=5 and y'(0)=3 Applying the transform to the given equation I end up to ...
  43. H

    Laplace transform as a dual space

    Hello, I'm trying to find some information concerning Laplace transforms. Are they "just" an integral transformation, or do they have some algebraic meaning similar to Fourier transforms (the "plane wave" basis vectors)? Thanks!
  44. TheFerruccio

    Inverse Laplace Transform for Negative a^2?

    There are lots of tables out there for finding the inverse laplace transform of: \frac{1}{(s+b)^{2}+a^{2}} or \frac{s}{(s+b)^{2}+a^{2}} but what if a^{2} is negative? I don't know what useful formula I should split it up into.
  45. A

    Laplace Transform of an equation with more than one variable

    Hey guys, I'm really struggling with an equation that I have to use for a piece of coursework. I think I'm missing something really basic but I can't seem to get past it and wondered if somebody else could help. I want to know if it's possible to find the Laplace transform of the following...
  46. A

    Meaning of Sigma in Laplace transform

    Hello, I've got some questions for the hardcore analytical mathematicians and electronic engineers. The context: A gain relation in a circuit of RCL and dependent sources ends up in an H(s) which is a quotient of polynomials in s. Number of poles is the number of energy storing...
  47. rootX

    Inverse Laplace transform of ex(-s)/(s+3)^3

    matlab: >> ilaplace(exp(-1*s)/(s+3)^3) ans = 1/2*heaviside(t-1)*(t-1)^2*exp(-3*t+3) But I think there should be an additional exp(3) multiplied by all. =exp(-1*s)/(s+3)^3 =exp(-1*(s+3)+3)/(s+3)^3 Taking out exp(-3t). In above step I did (s+3)+3 because exp(-3t) says all...
  48. L

    Laplace Transform Practice: Finding Inverse and Basic Forms

    Homework Statement Find the laplace transform of t sin(t) and t cos(t), and the inverse transform of \frac{1}{(1+s^2)^2} 2. The attempt at a solution I found the two laplace forms: \frac{2s}{(s^2+1)^2} and \frac{s^2-1}{(s^2+1)^2} I guess I'm supposed to use the two laplace...
  49. J

    Fick's Second Law: Laplace Transform to solve PDE in Spherical Coords

    Fick's second law in general form: \frac{\partial C}{\partial t} = D\nabla^2 C In spherical form: \frac{\partial C}{\partial t} = D\frac{1}{r^2}\frac{\partial}{\partial r}\left( r^2\frac{\partial C}{\partial r} \right) (Assume all changes in phi and theta to be zero, so we are only concerned...
  50. R

    What Is the Inverse Laplace Transform of Y(s)/(s+1)?

    Homework Statement What is the inverse Laplace transform of Y(s)/(s+1) ? Homework Equations The Attempt at a Solution I think it is e^(-t) * y(t). Am i right?
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