- #1
Marcus95
- 50
- 2
Homework Statement
We can treat the following coupled system of differential equations as an eigenvalue
problem:
## 2 \frac{dy_1}{dt} = 2f_1 - 3y_1 + y_2 ##
## 2\frac{dy_2}{dt} = 2f_2 + y_1 -3y_2 ##
## \frac{dy_3}{dt} = f_3 - 4y_3 ##
where f1, f2 and f3 is a set of time-dependent sources, and y1, y2 and y3 is a set of
time-dependent responses.
(a) If these equations are written using matrix notation,
## \frac{d\vec{y}}{dt} + K \vec{y} = \vec{f} ##
what are the elements of K? Find the eigenvalues and eigenvectors of K.
(b) In the case when the system is not excited, f = 0, find all of the solutions having
the form
##\vec{y}=\vec{y}_0 e^{-\gamma t} ##
where ##\gamma## > 0 is a decay constant.
(c) If f is held constant at f0, the response vector y has the steady state value y0 (that
is, with ##\frac{d\vec{y}}{dt} = 0 ##). Write down y0 in terms of f0, and find y0 in the case where f0 = (1, 1, 1)T .
(d) Assume that y starts in the steady state solution y0 given in (c) with f0 = (1, 1, 1)T . Now suppose the source function abruptly falls to zero, f0 = (0, 0, 0)T , so that the response vector y moves away from y0. Writing y as a linear combination of the allowed solutions found in (b), derive an expression for the subsequent time evolution
of the system.
Homework Equations
Eigenvalue equation: ## det(M - \lambda I ) = 0 ##
The Attempt at a Solution
Part a) was relatively straight forward, by just rearanging and observing I find:
K = ( c1 c2 c3) with c1 =(3/2, -1/2, 0)T, c2= (-1/2, 3/2, 0)T and c3=(0, 0, 4)T.
I also found the eigenvalues to be λ = 1, 2, 4 with eigenvectors (1, 1, 0) , (1, -1, 0) and (0, 0, 1).
However, after this I am completely stuck. I have no idea how to apply this to the differential equations in part b). I can solve the last equation: ##y_3 = y_{03} e^{-4t}##, but the first two equations are coupled and I am not sure how to deak with it (with or without matrix notation).
Thank you for any help! :)