# Someone helps me please

1. May 26, 2008

### dmg_80

someone helps me please!!

Hello all,

This is the first time Ive stumbled across this site, but it appears to be extremely helpful. I am an elettronic engineering grad student, and in my research, I have run across the following 2nd order non linear differential equation. It is of the form:

x'' + x = sinwt + s(x)

where s(x) = abs(5*x+1)-abs(5*x-1) (saturation)
and t=(0 2000)
and w=[0:0.001:2.^0.5] (it's a variable )

My questions are :
1) How can I write and solve this problem using Matlab ?
2) How can I plot this differential equation as a function of w ?
after the plot, How can I find the max Local (Relative) Maxima and put these results in another graph always considering them as a function of w ?

I hope there's someone who helps me...
thanks

2. May 27, 2008

### HallsofIvy

Staff Emeritus
If you wanted to know how to solve that equation, I could tell you. Unfortunately, I know nothing about using Matlab.

3. May 28, 2008

### dmg_80

Thank you....but i have to solve this problem only using matlab...

4. May 28, 2008

### eys_physics

Hey

First you need to transform your problem to a system of (two) first order differential equations by the substitution
$$x_{1}=x$$
and
$$x_{2}=x'$$

You will then get a problem of the type
$$x'_{1}=x_{2}, x'_{2}=f(x)$$

After that create a m-file containing

function xprim=xprim(t,x)
xprim=[x_2;f(x)]​

When try to use the function ode45 or some other solver to solve problem.
and use plot to plot the result

5. Jun 18, 2008

### dmg_80

I have another problem :
I have to find, after solving that differential equation,a vector based only with the maximum values of its results...After that i have to plot this vector as a fuction of w...how can i do that ?

6. Jun 18, 2008

### shoehorn

Let me give you some invaluable advice: read the Matlab documentation. It's superb, and explains in detail how to approach such basic problems.

7. Jun 18, 2008

### dmg_80

Thank you for your "invaluable advice"...:uhh:

8. Jun 18, 2008

### shoehorn

You're enrolled in an electronic engineering graduate programme but are incapable of using the detailed examples given by typing 'doc ode45'? Sounds like the advice I gave you really would be invaluable if only you'd take it.

Ninety-five percent of the time, a question about Matlab is answered in the Matlab documentation. For instance, precisely all of your questions are answered, with examples, in the documentation. Use it. It'll give you a better answer than anyone here can.

9. Jun 18, 2008

### dmg_80

it's hard for me twice : first of all i haven't time to study all the Matlab documentation and second i'm an italian girl that not understand english very well..i'm trying to do it by myself...trying to traslate all i read...so please don't kidding me ...

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