Problemas Resueltos Trnaformadas De Laplace

Solved Problems: Laplace Transforms

The Laplace Transform is a powerful mathematical tool used extensively in engineering and science to solve linear ordinary differential equations (ODEs). It transforms a function of time into a function of a complex variable, often simplifying the process of solving complex differential equations. This article will delve into several solved problems illustrating the application of Laplace transforms, covering various techniques and scenarios. Understanding these examples will solidify your grasp of the Laplace transform and its practical applications. We will explore the process from defining the problem, applying the Laplace transform, solving in the s-domain, and finally, performing the inverse Laplace transform to obtain the solution in the time domain.

1. Introduction to the Laplace Transform

Before diving into the solved problems, let's briefly revisit the definition of the Laplace transform. Given a function f(t), its Laplace transform, denoted as F(s) or L{f(t)}, is defined as:

F(s) = L{f(t)} = ∫₀^∞ e^(-st) f(t) dt

where 's' is a complex variable. This integral transforms the time-domain function f(t) into the s-domain function F(s). The inverse Laplace transform, denoted as L⁻¹{F(s)}, recovers the original function f(t) from its Laplace transform F(s).

2. Solved Problems: A Step-by-Step Approach

Let's now tackle several solved problems, each showcasing different aspects of applying Laplace transforms.

Problem 1: Solving a First-Order ODE

Problem: Solve the following initial value problem using Laplace transforms:

dy/dt + 2y = e^(-t), y(0) = 1

Solution:

1. Apply the Laplace Transform: Taking the Laplace transform of both sides of the equation, we get:

L{dy/dt} + 2L{y} = L{e^(-t)}

Using the properties of Laplace transforms:

L{dy/dt} = sY(s) - y(0) and L{e^(-t)} = 1/(s+1)

Substituting these and the initial condition y(0) = 1, we get:

sY(s) - 1 + 2Y(s) = 1/(s+1)

2. Solve for Y(s) in the s-domain: Rearranging the equation to solve for Y(s):

Y(s)(s + 2) = 1 + 1/(s+1) = (s+2)/(s+1)

Y(s) = 1/(s+1)

3. Inverse Laplace Transform: Now we need to find the inverse Laplace transform of Y(s) to obtain the solution y(t):

y(t) = L⁻¹{1/(s+1)} = e^(-t)

Therefore, the solution to the initial value problem is y(t) = e^(-t).

Problem 2: Solving a Second-Order ODE

Problem: Solve the following initial value problem:

d²y/dt² + 4dy/dt + 3y = 0, y(0) = 1, y'(0) = 0

Solution:

1. Apply the Laplace Transform: Taking the Laplace transform of the equation:

L{d²y/dt²} + 4L{dy/dt} + 3L{y} = 0

Using the properties of Laplace transforms:

L{d²y/dt²} = s²Y(s) - sy(0) - y'(0) and L{dy/dt} = sY(s) - y(0)

Substituting and using the initial conditions:

s²Y(s) - s - 0 + 4(sY(s) - 1) + 3Y(s) = 0

2. Solve for Y(s):

Y(s)(s² + 4s + 3) = s + 4

Y(s) = (s + 4) / (s² + 4s + 3) = (s + 4) / [(s + 1)(s + 3)]

3. Partial Fraction Decomposition: To find the inverse Laplace transform, we perform partial fraction decomposition:

(s + 4) / [(s + 1)(s + 3)] = A/(s + 1) + B/(s + 3)

Solving for A and B, we find A = 3/2 and B = -1/2. Therefore:

Y(s) = (3/2)/(s + 1) - (1/2)/(s + 3)

4. Inverse Laplace Transform:

y(t) = L⁻¹{Y(s)} = (3/2)e^(-t) - (1/2)e^(-3t)

Therefore, the solution is y(t) = (3/2)e^(-t) - (1/2)e^(-3t).

Problem 3: Laplace Transform of a Unit Step Function

Problem: Find the Laplace transform of the unit step function u(t-a), where a > 0.

Solution: The unit step function u(t-a) is defined as:

u(t-a) = 0, t < a u(t-a) = 1, t ≥ a

The Laplace transform is:

L{u(t-a)} = ∫ₐ^∞ e^(-st) dt = [-e^(-st)/s]ₐ^∞ = e^(-as)/s

Therefore, the Laplace transform of the unit step function u(t-a) is e^(-as)/s.

Problem 4: Solving an ODE with a Unit Step Function

Problem: Solve the initial value problem:

d²y/dt² + y = u(t - π), y(0) = 0, y'(0) = 0

Solution:

1. Apply the Laplace Transform:

s²Y(s) - sy(0) - y'(0) + Y(s) = e^(-πs)/s

2. Solve for Y(s):

Y(s)(s² + 1) = e^(-πs)/s

Y(s) = e^(-πs) / [s(s² + 1)]

3. Partial Fraction Decomposition:

e^(-πs) / [s(s² + 1)] = e^(-πs) [A/s + (Bs + C)/(s² + 1)]

Solving for A, B, and C yields A = 1, B = -1, and C = 0. Thus:

Y(s) = e^(-πs) [1/s - s/(s² + 1)]

4. Inverse Laplace Transform: Using the time-shifting property of the inverse Laplace transform:

y(t) = u(t - π) [1 - cos(t - π)] = u(t - π) [1 + cos(t)]

Therefore, the solution is y(t) = u(t - π) [1 + cos(t)].

Problem 5: Involving Dirac Delta Function

Problem: Solve the initial value problem:

d²y/dt² + 2dy/dt + y = δ(t - 1), y(0) = 0, y'(0) = 0, where δ(t-1) is the Dirac delta function.

Solution:

1. Laplace Transform:

s²Y(s) - sy(0) - y'(0) + 2[sY(s) - y(0)] + Y(s) = e^(-s)

2. Solve for Y(s):

Y(s)(s² + 2s + 1) = e^(-s)

Y(s) = e^(-s) / (s + 1)²

3. Inverse Laplace Transform: Using the time-shifting property and the inverse Laplace transform of 1/(s+1)²:

y(t) = u(t - 1)(t - 1)e^(-(t-1))

Therefore, the solution is y(t) = u(t - 1)(t - 1)e^(-(t-1)).

3. Explanation of Techniques and Properties Used

Throughout the solved problems, several key techniques and properties of Laplace transforms were employed:

• Linearity: The Laplace transform is a linear operator, meaning L{af(t) + bg(t)} = aL{f(t)} + bL{g(t)}, where 'a' and 'b' are constants.
• Derivative Property: The Laplace transform of derivatives simplifies the solution process of differential equations. For example, L{dy/dt} = sY(s) - y(0).
• Partial Fraction Decomposition: This technique is crucial for inverting complex Laplace transforms. It involves breaking down a rational function into simpler fractions that are easily inverted.
• Time-Shifting Property: This property simplifies dealing with functions involving the unit step function or the Dirac delta function. For a function f(t), L{f(t-a)u(t-a)} = e^(-as)F(s).
• Inverse Laplace Transform: This step converts the solution from the s-domain back to the time domain, giving the solution in terms of the original variable, t. Various techniques are used for this, including tables of Laplace transforms and partial fraction decomposition.

4. Frequently Asked Questions (FAQ)

• Q: What are the limitations of the Laplace transform? A: The Laplace transform is primarily suited for linear ODEs with constant coefficients. Nonlinear equations or those with variable coefficients may not be easily solved using this method.

• Q: How do I choose the appropriate method for inverse Laplace transform? A: The choice depends on the complexity of the Laplace transform obtained. Partial fraction decomposition is commonly used for rational functions. Tables of Laplace transforms are helpful for recognizing standard forms.

• Q: Are there alternative methods to solve ODEs besides Laplace transforms? A: Yes, other methods include the characteristic equation method, variation of parameters, and numerical methods.

• Q: What software can assist with Laplace transforms? A: Several mathematical software packages, such as Matlab, Maple, and Mathematica, include functions for computing Laplace transforms and their inverses.

5. Conclusion

The Laplace transform is an invaluable tool for solving linear ordinary differential equations. Mastering its application, as demonstrated through these solved problems, will significantly enhance your ability to analyze and solve a wide range of engineering and scientific problems involving dynamic systems. Remember to practice regularly to solidify your understanding and build confidence in applying this powerful technique. Through consistent practice and a thorough understanding of the underlying principles, you can effectively utilize Laplace transforms to tackle increasingly complex problems. The key is to break down the problem systematically, applying the appropriate properties and techniques at each stage. Remember to always check your solution for reasonableness and accuracy.