# Ok, need a little help in a couple of areas (2nd order w/forcing terms)

1. Nov 4, 2004

### amb123

1) For one question, the forcing term is 8cos2x - 4sinx. I am trying to solve by the method of undetermined coefficients. The solution to the homogeneous equation is c1cosx + c2sinx so for the particular solution I was using :
Acos2x + Bsin2x + Cxcosx + Dxsinx, where I added the x's into the last two terms because without those they would be solutions of the homogenous equation. Where am I going wrong here? I end up able to solve for A and B, but not for C and D. I have tried using convolutions but the constants become messy, and there are many many terms to deal with.

2) Another question is an eq of motion w/o damping with a forcing term of sinwt (w=omega). the question is "for what value of omega do oscillations increase unbounded?". I am not sure exactly what I am looking for here. My book mentions briefly the consequences of something being underdamped, and the frequency matching and making bridges collapse, etc, but nothing like this question. Where do I begin to look for the guidance to solve this type of question?

Any/all guidance is appreciated.
thx.
-A

2. Nov 5, 2004

### ReyChiquito

it would help if you post the ode+(boundary conditios) as your description is rather vague.

3. Nov 5, 2004

### amb123

First question is x'' + 16x = sinwt which describes a mass-spring system under s sinusoidal driving force. the question is for what value of w do the oscillations of this system increase w/o bound. I just don't know exactly what i'm supposed to do, any hints on where to begin or what i'm looking for here?

another question is y'' + y = 8cos2x - 4sinx where y(pi/2) = -1 and y'(pi/2) = 0
I have tried this all different ways and keep having problems. I was just shown a new method earlier today and i'm going to try figure that one out this weekend, i'd be interested to find out if there is a method by which this is best solved and it works out well.

I'm also getting a nasty answer for y'' - 2y - 8y = 2e^-2x - e^-x where y(0)=y'(0) = 0. I get a really nasty answer using convolutions.

I'm most concerned with the first problem, though, because I can't figure out what i'm really trying to do with it.

thx for any help.
-A

4. Nov 6, 2004

### arildno

"First question is x'' + 16x = sinwt which describes a mass-spring system under s sinusoidal driving force. the question is for what value of w do the oscillations of this system increase w/o bound. I just don't know exactly what i'm supposed to do, any hints on where to begin or what i'm looking for here?"
Hint:
When the forcing term hits upon the natural frequency of the system, you'll get resonance.

"another question is y'' + y = 8cos2x - 4sinx where y(pi/2) = -1 and y'(pi/2) = 0
I have tried this all different ways and keep having problems. I was just shown a new method earlier today and i'm going to try figure that one out this weekend, i'd be interested to find out if there is a method by which this is best solved and it works out well."
Find the solution of the homogenous system.
Add to that particular solutions of your differential equation, use suitable trigonometric trial functions here.

5. Nov 8, 2004

### amb123

If we are just looking for w/w0 to equal near 1, then if we know that the value of omega is sqrt(k/m) and we have k & m, then find that w0 = 4, then we just want w from the forcing term to also be four in order for this to resonate, correct? Am I way off base here? Is this much more difficult than that?

I am working on another word problem then i'm going to redo the other one from scratch using method of undet coeffs.

Thanks!
Angela.

6. Nov 9, 2004

### arildno

You're right, if you hit the natural frequency (given by k&m) with your external force, the system experiences resonance.

7. Nov 9, 2004

### amb123

Wow, I thought it was going to be a much more complicated problem than that! Thanks!

Ok, another one I have a problem with is the one that says "The steady state solution of the diff eq x'' + x' + 2x = cos(t) can be written x(t) = sqrt(1/2)*cos(t + sigma) and asks for sigma where -pi < sigma <+ pi. I haven't a clue on this one, any suggestions? I get yc = e^-t/2[C1cos((sqrt(7)/2)*t + C2sin((sqrt(7)/2)*t] but i'm not sure how this helps.

And, I just worked out this problem : y'' + y = 8cos(2x) - 4sin(x) where y(pi/2) = -1 and y'(pi/2) = 0 using the annihilator method and got :

Ytotal = (16/3 - pi)cos(t) - 11/3*sint - 8/3*cos(2t) + 2tcos(t)

If you get a chance, can you take a look and see if you come out with the same thing because i've gotten different answers with other methods and i'm not sure if i'm doing this one right.

Thanks again for the help!
-A

Last edited: Nov 9, 2004