A few days ago I tried to simulate a scene of "wheel"s' interaction,but my code doesn't work out properly in the case of more than 3 wheels.I think its lack of speed status preservation may be the cause.Could you be kind enough to point out what's wrong?Thank you.
Codes are give below.

wheels wity;
wheel w1,w2,w3;
PFont ft;
void setup()
{
 size(500,500);
 w1=new wheel(50,50,50);
 w2=new wheel(50,150,50);
 w3=new wheel(50,250,50);
 wity=new wheels();
 wity.setwheel(w1);
 wity.setwheel(w2);
 wity.setwheel(w3);
 ft=loadFont("Albertus-Medium-48.vlw");
 textFont(ft,20);
}

void draw()
{
 background(200);
 w1.update();
 w2.update();
 w3.update();
}
class wheel
{
 float omega;
 float dx,dy;
 float theta;
 PVector center=new PVector();
 float radii;
 float flag;
 PVector d=new PVector(); //relative position vector
 PVector dm=new PVector();//motion vector
 PVector normd=new PVector();//normalized vector

   wheel(float x,float y,float _radii)
 {
   center.x=x;
   center.y=y;
   radii=_radii;
 }

 void displayatorigin()
 {

   ellipse(0,0,radii,radii);
   pushStyle();
   fill(0, 102, 153);
   text("oh",2,2);
   popStyle();// here encountered a situation where the ellipse covers the text
 }

 void spin()
 {
   pushMatrix();
   translate(center.x,center.y);
   rotate(theta+=omega);
   displayatorigin();
   popMatrix();
 }

 void movedbycursor()
 {
   dx=mouseX-pmouseX;
   dy=mouseY-pmouseY;
   center.x+=dx;
   center.y+=dy;

 }

 void movedbyothers()
 {

   if (this.cursorin()==true)// i is passive then continue
       return;


   for(int j=1;j<=wity.ws.length-1;j++)
   {
     if (wity.ws[j].flag==this.flag) continue;//not itself then continue

       d=PVector.sub(this.center,wity.ws[j].center);
     if(d.mag()<55)
     {
       dm.x=wity.ws[j].dx;
       dm.y=wity.ws[j].dy;
       normd=d.get();
       if(dm.dot(normd)>0)
       {
         normd.normalize();

         this.dx+=dm.dot(normd)*normd.x;
         this.dy+=dm.dot(normd)*normd.y;
         this.omega+=mag(dm.x-dm.dot(normd)*normd.x,dm.dot(normd)*normd.y);
       }
     }


     center.x+=dx;// Here ,dx not specified!!!
     center.y+=dy;
   }
   dx=0;
   dy=0;
 }

 boolean cursorin()//cursor seems to be an built in element
 {
   if (dist(center.x,center.y,mouseX,mouseY)<radii/2)
     return true;
   else return false;
 }

   void update()
   {
     spin();
     if (cursorin()==true)
     movedbycursor();
     else
     movedbyothers();
   }

}

class wheels
{
   wheel[] ws=new wheel[1];

   void setwheel(wheel w)
   {
     ws=(wheel[])append(ws,w);
     w.flag=ws.length-1;
   }

}

Thank you and my case is that if I have wheel A,B and C,and when I push wheel A to boost wheel B and at the same time let B boost C,the collision effect between B and C doen't exist,say,A-->B-->C fails.Maybe a tree to convey velocity is in need?Or multiple thread?

PFont ft;
MM wheelinter;
void setup()
{
 size(600,600);
 ft=loadFont("Albertus-Medium-48.vlw");
 textFont(ft,20);
 wheelinter=new MM();
}

void draw()
{
 background(200);
 wheelinter.changecolor();
}

void mouseDragged()
{
 wheelinter.moved();
}
class MM
{
 PVector d=new PVector();
 PVector dm=new PVector();
 PVector normd=new PVector();
 wheel[] wheels=new wheel[3];
 float [][] movelen=new float[2][3];
 MM()
 {
   for(int i=0;i<wheels.length;i++)
   {
     wheels[i]=new wheel(100,100*i+100,50);
     wheels[i].name=nf(i,1);
   }
 }

 void display()
 {
   for(int i=0;i<wheels.length;i++)
   {
     wheels[i].spin();
   }
 }

 void cd()
 {
   for(int i=0;i<wheels.length;i++)
   {
     if (wheels[i].cursorin()==true)
         continue;
     for(int j=0;j<wheels.length;j++)
     {
       line(wheels[i].center.x,wheels[i].center.y,wheels[j].center.x,wheels[j].center.y);
       if (j==i) continue;
         d=PVector.sub(wheels[i].center,wheels[j].center);
       if(d.mag()<55)
       {
         dm.x=wheels[j].dx;
         dm.y=wheels[j].dy;
         normd=d.get();
         if(dm.dot(normd)>0)
         {
           normd.normalize();

           wheels[i].dx+=dm.dot(normd)*normd.x;
           wheels[i].dy+=dm.dot(normd)*normd.y;
           wheels[i].rotv+=mag(dm.x-dm.dot(normd)*normd.x,dm.dot(normd)*normd.y);
         }
       }
     }
     movelen[0][i]=wheels[i].dx;
     movelen[1][i]=wheels[i].dy;
   }

   for(int i=0;i<wheels.length;i++)
   {
     wheels[i].dx=movelen[0][i];
     wheels[i].dy=movelen[1][i];
     wheels[i].movedbyothers();

   }

   for(int i=0;i<wheels.length;i++)
   {
     wheels[i].dx=0;
     wheels[i].dy=0;
   }
 }

 void changecolor()
 {
   for(int i=0;i<wheels.length;i++)
   {
     if (wheels[i].cursorin()==true)
       fill(88);
     else fill(200);
     wheels[i].spin();
   }
 }
 void moved()
 {

   for(int i=0;i<wheels.length;i++)
   {
     if (wheels[i].cursorin()==true)
     {
       wheels[i].movedbycursor();
     }
     cd();
   }
 }
}
class wheel
{
 float rotv;
 float dx,dy;
 float theta;
 String name;
 wheel(){
 };
 PVector center=new PVector();
 float radii;
 wheel(float x,float y,float _radii)
 {
   center.x=x;
   center.y=y;
   radii=_radii;
 }
 void display()
 {

   ellipse(center.x,center.y,radii,radii);
 }

 void displayatorigin()
 {

   ellipse(0,0,radii,radii);
   pushStyle();
   fill(0, 102, 153);
   text(name,2,2);
   popStyle();
 }

 void spin()
 {
   pushMatrix();
   translate(center.x,center.y);

   rotate(theta+=0.02);
   displayatorigin();
   popMatrix();
 }

 void movedbycursor()
 {
   dx=mouseX-pmouseX;
   dy=mouseY-pmouseY;
   center.x+=dx;
   center.y+=dy;

 }

 void movedbyothers()
 {
   center.x+=dx;
   center.y+=dy;
 }

 boolean cursorin()
 {
   if (dist(center.x,center.y,mouseX,mouseY)<radii/2)
     return true;
   else return false;
 }
}

You've said it before: jbox2d, definitely! :-)