Forest UCM Osc Driven
Damped Oscillators driven by an external source
An external force must be supplied to do work on a damped oscillator in an amount that is equal to or greater than the work done by the dissipative force.
An external force (source) is added to the homogeneous differential equation making it inhomogenous
where f(t) represents the external force (source) that depends on time divided by the objects mass.
Differential equations in Operator form
In the previous sections we used the definition
to solve the second order linear differential equation.
Let's take this a step further with the following operator definition
- Linear differential equations have coefficients that can be constant or variable coefficients that can be transformed into constant coefficients.
- is a linear operator
the above is a property of differential calculus where
Solving the Inhomogeneous Diff. Eq.
To solve the Inhomogeneous problem we take advantage of the linear operator such that
Since the solution of the 2nd order differential equation requires exactly two arbitrary constants,
We can formulate the solution by tacking a particular solution onto the homogeneous solution that already has two arbitrary constants.
All we need to do is find the particular solutionthat satisfies
and we will have a complete set of solutions.
Break the equation up into a Homogeneous solution and a Particular Solution
Th Homogenous solution solved the equation
we know from the previous section that the homogenous solution has the form
Case 1 : f(t) is sinusoidal
Consider the case where the driving force is a sinusoidal function
We seek a solution to the particular equation
- is not necessarily at the natural (resonant) frequency
- If a differential equation has the cosine function as a solution then the sine function may also be a solution since the difference between the two is only a phase shift.
It must also be true that
You can construct a complex solution now such that
the constant C is determine by substituting the solution into the equation
The amplitude of the solution is a complex number that may be cast in terms of a real amplitude times the complex exponential such that
where the real amplitude A is given by
What is delta?
Sinceand are not imaginary variables
The phase () must be due to the remaining complex term
To determine the anglewe construct a right triangle in the complex plane where the leg along the real axis is given by and the other leg of the triangle is along the imaginary axis with a length of
- The final complex solution si
Complete Solution for the Driven Damped oscilaltor