How do I simulate the interaction of two point masses(gravitational pull) using the verlet algorithm?

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Hello. I am trying to implement verlet's algorithm to simulate the interaction of two stars with the same mass as the sun. They are initially 2 parsec away and symetric velocities perpendicular to the axis that passes through their centre.

Problems: 1. The trajectory of each star should be a parabola and instead it shows they stars are moving apart

What I have tried:

C++

<pre>

#include <iostream>
#include <math.h>
#include <stdlib.h>
#include <vector>
#include <cmath>
#include <random> 

using namespace std;

double Fifuntion(double mass1, double mass2, double r12 ){ 
double G= 4.3009125e-3*pow(1.02269032e-6,2); //4.30091(25)×10−3 pc⋅Msol–1⋅(km/s)2

// 1s=1 second =3.16887646 × 10-8 years => G= 4.30091(25)×10−3/(3.16887646 × 10-8)^2 pc⋅Msol–1⋅(km/year)2

//1 kilometer / second = 1.02269032 × 10-6 Parsecs / year
double Fi;
 //dist= sqrt[(x1-x2)^2+(y1-y2)^2]
 Fi= G*(mass1*mass2)/pow(r12,2);
return Fi;
}

double Uifuntion(double mass1, double mass2, double r12 ){ 
double G= 4.3009125e-3*pow(1.02269032e-6,2); //pc⋅Msol–1⋅(pc /year)2
double Ui;
 Ui= G*(mass1*mass2)/r12;
return Ui;
}

int main(){

//1 kilometer / second = 1.02269032 × 10-6 Parsecs / year => v= 2e-2*1.02269032 × 10-6 Parsecs / year
double vinicial=2e-2*1.02269032e-6; // Parsecs/year 


double d= 2; //parsec
double m1= 1; //solar mass
double m2= 1; //solar mass
//massa_solar= 1.9891 x 10^30 kg
//double V1= -G*(m1*m2)/r12;
//double F1= G*(m1*m2)/r12^2;
//double a1= G*(m2)/r12^2;
//double a2= G*(m1)/r12^2;

int N=2;
double b, j, t,r12;

vector<double> rixdtt_vector (N);
vector<double> rixt_vector (N);
vector<double> rixt_vectortemporary (N);
vector<double> rixtdt_vector (N);
vector<double> vix_vector (N) ;


vector<double> riydtt_vector (N);
vector<double> riyt_vector (N);
vector<double> riyt_vectortemporary (N);
vector<double> riytdt_vector (N);
vector<double> viy_vector (N) ;

vector<double> aixt (N) ;
vector<double> aiyt (N) ;


double deltat=100 ; 
double ttotal= 100000000; 

double Ek,Etotal,U;

 rixdtt_vector[0]= 0.0; // parsec // time t-dt
 riydtt_vector[0]= 6.0; // parsec // time t-dt
 rixdtt_vector[1]= 0.0; // parsec // time t-dt
 riydtt_vector[1]= 4.0; // parsec // time t-dt

 //dist= sqrt[(x1-x2)^2+(y1-y2)^2]=2=sqrt[(0-0)^2+(6-4)^2]=sqrt(4)=2

 rixt_vector[0]= rixdtt_vector[0] + pow(-1,0)*vinicial* (deltat); 
 riyt_vector[0]= riydtt_vector[0] + pow(-1,0)*vinicial* (deltat); 
 rixt_vector[1]= rixdtt_vector[1] + pow(-1,1)*vinicial* (deltat);
 riyt_vector[1]= riydtt_vector[1] + pow(-1,1)*vinicial* (deltat);

for(int b=0; b<N; b++){ //euler step
   //para t=0
   //((double)rand()/RAND_MAX)*(10-0.1)+0.1= number from 0.1 to 10
   rixtdt_vector[b]= ((double)rand()/RAND_MAX)*(10-0.1)+0.1; // time t+dt 
   riytdt_vector[b]=((double)rand()/RAND_MAX)*(10-0.1)+0.1; // time t+dt 
      
   vix_vector[b]=pow(-1,b)*vinicial ; // velocity in parsec/year //time t
   viy_vector[b]=pow(-1,b)*vinicial; //  velocity in parsec/year //time t

   
}
 
//a(t) = f(t) / m
while( t < ttotal){ 
   r12=pow(  pow(rixt_vector[1]-rixt_vector[0],2) + pow(riyt_vector[1]-riyt_vector[0],2)   ,0.5 ) ; 
   aixt[0]= (  Fifunction(   m1, m2, r12   ) *( (rixt_vector[1]-rixt_vector[0])/r12  ) )/m1 ;  //particle 1, x component
   aiyt[0]= ( (  Fifunction(   m1, m2, r12   ) *( (riyt_vector[1]-riyt_vector[0])/r12  ) )/m1 ); //particle 1, y component
   aixt[1]=( (  Fifunction(   m1, m2, r12   ) *( (rixt_vector[0]-rixt_vector[1])/r12  ) )/m1 ); //particle 2, x component
   aiyt[1]=( (  Fifunction(   m1, m2, r12   ) *( (riyt_vector[0]-riyt_vector[1])/r12  ) )/m1 ); //particle 2, y component
   
   U-=Uifunction(m1, m2, r12);
   //Verlet step     
   //x(t+dt) = 2*x(t) - x(t-dt) + ax(t)*dt*dt
   rixtdt_vector[0] = 2*rixt_vector[0] - rixdtt_vector[0] + aixt[0]*t*t ; 
   riytdt_vector[0] = 2*riyt_vector[0] - riydtt_vector[0] + aiyt[0]*t*t ; 
   rixtdt_vector[1] = 2*rixt_vector[1] - rixdtt_vector[1] + aixt[1]*t*t ; 
   riytdt_vector[1] = 2*riyt_vector[1] - riydtt_vector[1] + aiyt[1]*t*t ; 
      
   // v(t) = ( x(t +dt) - x(t-dt) ) / 2dt
   vix_vector[0]= ( rixtdt_vector[0] - rixdtt_vector[0] ) / (2*t) ;
   viy_vector[0]= ( riytdt_vector[0] - riydtt_vector[0] ) / (2*t) ;
   vix_vector[1]= ( rixtdt_vector[1] - rixdtt_vector[1] ) / (2*t) ;
   viy_vector[1]= ( riytdt_vector[1] - riydtt_vector[1] ) / (2*t) ;
   
   //Kinetic energy    
   Ek+= (pow(vix_vector[j],2)/2)+(pow(viy_vector[j],2)/2);  
   
   rixt_vectortemporary[0] = rixt_vector[0] ;  
   riyt_vectortemporary[0] = riyt_vector[0] ;
   rixt_vectortemporary[1] = rixt_vector[1] ;
   riyt_vectortemporary[1] = riyt_vector[1] ;   
      

   rixt_vector[0] = rixtdt_vector[0] ;  //x(t2)= x(t1+dt)
   riyt_vector[0] = riytdt_vector[0] ;
   rixt_vector[1] = rixtdt_vector[1] ;
   riyt_vector[1] = riytdt_vector[1] ;
   
   rixdtt_vector[0] = rixt_vectortemporary[0] ; //x(t2-dt)= x(t1)
   riydtt_vector[0] = riyt_vectortemporary[0] ;  
   rixdtt_vector[1] = rixt_vectortemporary[1] ; 
   riydtt_vector[1] = riyt_vectortemporary[1] ;
 
 
 Etotal= Ek + U ;

 Ek=0;
 U=0;
 t+=deltat; 
}

}

Posted 21-Nov-21 4:35am

CrystalNeves

Updated 21-Nov-21 4:38am

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OriginalGriff · Answer 1 · 2021-11-21T04:38:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!