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First Order ODE solution with ODE interface and ODEsolver post and replies

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Simulation of nucleus decay by ODEsolver - Aug 29, 2012 at 12:04PM
Martin Jola
7 Posts

To learn more about ODE interface and ODEsolver I tired to solve the ODE of nuclear decay:
dN/dt=-lambda*N. Analyitical solution is easy, but to learn ODE/ODEsolver I would like to solve it numerically. I followed the example of FallingParticle in the CMS book, but failed. It's not a proble to compile the code, no error messages and the control panle opens with the correct vaiables. However when I press calculate I get a lot of error messages, the first one like NullPointerException followed by many other. I guess it has to do with the state and rate. Can somebody give me an example of how to use ODE interface and ODEsolver for a first order ODE like the nucleus decay?


Replies to Simulation of nucleus decay by ODEsolver

Re: Simulation of nucleus decay by ODEsolver - Aug 30 2012 7:51PM
Wolfgang
70 Posts

Have you tried to run the examples in the OSP Eclipse workspace?

http://www.compadre.org/osp/items/detail.cfm?ID=7147

There are many ODE examples in the first few chapters.



Re: Re: Simulation of nucleus decay by ODEsolver - Aug 31 2012 5:10AM
Martin Jola
7 Posts

Dear Wolfgang

yes I tried the ODE examples of the book, they all work fine.But they are all for second order ODE's. I have a few first order ODE's I would like to splve with ODE/ODEsolver.  Based on the book Open Source Physics (a User Guide with Examples) and the examples csm book I found out how to use ODe/ODEsolver for first order ODE's. Please have a look on the code below. May be you have some comments for improvements.

Thanks for your help

Martin

import org.opensourcephysics.numerics.ODE;
public class NucDecODE implements ODE{
double [] state =new double[3];
double lambda=0.01;
public  NucDecODE(double x){
state[0]=x;
state[1]=-lambda*state[0];
state[2]=0;
}
public double[] getState(){
return state;
}
public void getRate(double[] state,double[] rate){
rate[0]=state[1];
rate[1]=0;
rate[2]=1;
}
}
public class NucDecODEApp {
public NucDecODEApp(double x0) {
}
public static void main(String[] args) {
double dt=0.1;
double x0=100.0;
NucDecODE ode = new NucDecODE(x0);
ODESolver ode_solver=new Euler(ode);
ode_solver.initialize(dt);
while(ode.state[0]>90.0){
ode_solver.step();
}
System.out.println("final time= " +ode.state[2]);
System.out.println("final x =" +ode.state[0]);
}
}



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