# Homework Help: Inverse Laplace of a Mass-Spring System

1. Jun 18, 2010

### happycamper

1. The problem statement, all variables and given/known data

Given a transfer function in the Laplace Domain

Detemine an expression for x(t), given f(t) is a sinusodial input with frequency omega = root(k2/m2) and amplitude of 1 N (initial conditions equal 0)

2. Relevant equations
[URL]http://latex.codecogs.com/gif.latex?X_1/F=(m_2&space;s^2&plus;k_2)/(m_1&space;m_2&space;s^4&plus;k_2&space;(m_1&plus;m_2)s^2&space;)[/URL]

Inverse laplace 1/s^2 = t.u(t)
Inverse laplace (omega/s^2+omega^2) = sin(omega.t) . u(t)

3. The attempt at a solution

I divided the transfer function by m2 to obtain omega^2. I then brought the F over to the LHS as a sin function in the laplace domain (omega/s^2+omega^2). I have obtained the following equation

[URL]http://latex.codecogs.com/gif.latex?X_1=(1/s^2)&space;.w/((s^2&space;m_1&plus;w^2&space;((m_1&plus;m_2)/m_2&space;))[/URL]

What is the next step? I am given inverse laplace transforms for 1/s^2 and omega/s^2+omega^2

Last edited by a moderator: Apr 25, 2017
2. Jun 18, 2010

### gabbagabbahey

First, I think you have a small error in your equation. I get

$$X_1(s)=\frac{\omega}{s^2[m_1s^2+(m_1+m_2)\omega^2]}$$

since $m_1m_2s^4+(m_1+m_2)k_2s^2=m_2s^2[m_1s^2+(m_1+m_2)\omega^2]$

Second, use partial fraction decomposition. Say

$$\frac{\omega}{s^2[m_1s^2+(m_1+m_2)\omega^2]}=\frac{A}{s}+\frac{B}{s^2}+\frac{Cs+D}{m_1s^2+(m_1+m_2)\omega^2}$$

and find the constants $A$, $B$, $C$ and $D$.

Last edited by a moderator: Apr 25, 2017