- #1

jellicorse

- 40

- 0

## Homework Statement

Following a worked example in my book, I have been trying to get a solution for the equation

[itex]\frac{d^2u}{dt^2} + \frac{k}{m}u = Fcos\omega t[/itex]

The book says that at resonance, i.e. when [itex]\omega_0[/itex] (the natural frequency) = [itex]\omega[/itex] (the forcing frequency), the term [itex]F cos\omega t[/itex] is a solution to the homogenous equation and the solution to the differential equation above is[tex]Acos\omega_0t +Bsin\omega_0t+\frac{F}{2m\omega_o^2}tsin\omega_0t[/tex]

## Homework Equations

## The Attempt at a Solution

__To get the full solution:__

**Complementary Function:**[tex]Acos\omega_0t+Bsin\omega_0t[/tex], where [tex]\omega_0=\sqrt{\frac{k}{m}}[/tex]

__To get the Particular Integral:__Assume [itex]u= Ctcos\omega t +Dtsin\omega t[/itex]

Then [tex]\frac{du}{dt}= C cos\omega t - Ct\omega sin \omega t + D sin\omega t +Dt \omega cos \omega t[/tex]

[tex]\frac{du}{dt}= (Dt\omega +C ) cos\omega t + (D-Ct\omega) sin\omega t[/tex]

And [tex]\frac{d^2u}{dt^2} = -C \omega sin \omega t -C\omega sin \omega t - C \omega^2 t cos \omega t + D\omega cos \omega t + D\omega cos \omega t - D \omega^2 t sin \omega t[/tex]

[tex]\frac{d^2u}{dt^"}=(2D\omega -c \omega^2t)cos\omega t +(-2C\omega -D\omega^2t)sin \omega t [/tex]

__Back substituting these into the original differential equation:__

[tex](2D\omega -C\omega^2t + \frac{k Ct}{m}) cos\omega t + (\frac{KDt}{m}-2C\omega -D\omega^2t) sin \omega t = \frac{F}{m} cos\omega t[/tex]

Equating coefficients:

Equating coefficients:

[tex](2D\omega -C\omega^2t + \frac{k Ct}{m}) =\frac{F}{m}[/tex]

and [tex](\frac{kDt}{m} -2 C \omega - D\omega^2 t) = 0[/tex]

After this, I have tried solving for D and C but it seems to end up in a pretty intractable mess.

I know somehow D should be equal to [itex] \frac{F}{2m\omega_0^2} [/itex] (and I assume since we are talking about a situation in which [itex]\omega_0=\omega[/itex], D= [tex] \frac{F}{2m\omega_0^2} = \frac{F}{2m\omega} [/tex]) but can not see how to get there. Can anyone tell me if I have been going in the right direction so far?

Last edited by a moderator: