under evolution
dvx/dt =  ( fx=calf(0,vx,vy)/m)
dvy/dt = ( fy=calf(1,vx,vy)/m)


custom:
public double calf(int xid,double vx,double vy){
cta=Math.atan2(vy,vx);
if(xid==0)return f*Math.cos(cta+pi/2);
else return f*Math.sin(cta+pi/2);
}





Question:
why is it so complicated, need int xid ?

is it correct to just let
under evolution
dvx/dt =   f/m*Math.cos(cta+pi/2)
dvy/dt = f/m*Math.sin(cta+pi/2)