RK4 гравитационная ориентация

Я использовал RK4 для физики в небольшой 2-мерной игре, которую я сделал, но когда дело доходит до гравитации, все объекты растягиваются до 0,0, как я могу это изменить? (Извините за отсутствие комментариев, еще не было времени, чтобы прокомментировать это)

    #include "Gravity.h"
gravity::accelerationReturn gravity::acceleration(state & state , float time) //Returns a struct instead easier to put into a vector.
{
accelerationReturn return1;
const float k =9.8;
const float b = 1;
return1.Xaccel =  -k * state.pos.getX() - b* state.vel.getX();
return1.Yaccel = -k * state.pos.getY() - b* state.vel.getY();
return return1;
}gravity::derivative gravity::evaluate(state  & initial, float time, float deltaTime,derivative & Derivative)
{
state state;
state.pos.setXY(initial.pos.getX() + Derivative.dpos.getX()*deltaTime,
initial.pos.getY() - Derivative.dpos.getY()*deltaTime);

state.vel.setXY(initial.vel.getX() + Derivative.dvel.getX() * deltaTime,
initial.vel.getY() + Derivative.dvel.getY() * deltaTime);

derivative output;
output.dpos.setXY(state.vel.getX(),state.vel.getY());
output.dvel.setXY(acceleration(state,time + deltaTime).Xaccel,
acceleration(state,time + deltaTime).Yaccel);
return output;

}

void gravity::integrate(state & State , float time, float deltaTime)
{

derivative a = evaluate(State ,time,0.0f,derivative());
derivative b = evaluate(State,time,deltaTime * 0.5f,a);
derivative c = evaluate(State,time,deltaTime * 0.5f,b);
derivative d = evaluate(State,time,deltaTime,c);

Vector dpdt;
Vector dvdt;
dpdt.setXY((1.0f/10.0f * (a.dpos.getX() + 2.0f * (b.dpos.getX() + c.dpos.getX()) + d.dpos.getX())),
(1.0f/10.0f * (a.dpos.getY() + 2.0f * (b.dpos.getY() + c.dpos.getY()) + d.dpos.getY())));
dvdt.setXY((1.0f/10.0f * (a.dvel.getX() + 2.0f * (b.dvel.getX() + c.dvel.getX()) + d.dvel.getX())),
( 1.0f/10.0f * (a.dvel.getY() + 2.0f * (b.dvel.getY() + c.dvel.getY()) + d.dvel.getY())));State.pos.setXY ((State.pos.getX() + dpdt.getX() * deltaTime),(State.pos.getY() + dpdt.getY() * deltaTime));
State.vel.setXY ((State.vel.getX() + dvdt.getX() * deltaTime),(State.vel.getY() + dvdt.getY() * deltaTime));

}

CPP

#ifndef GRAVITY_H
#define GRAVITY_H

#include "2DVector.h"

class gravity :Vector
{
private:float time; //current time
float deltaTime; // previous time
public:
struct accelerationReturn //for returning acceleration
{
float Xaccel;
float Yaccel;
};
struct state //current state of object.
{
Vector pos; //position
Vector vel; //velocity
};
struct derivative
{
Vector dpos; // derivative of posistion is velocity.
Vector dvel; // derivative of velocity is acceleration.
};

accelerationReturn acceleration (state &,float); //calculates the new acceleration
derivative evaluate ( state & , float , float ,  derivative &); //gets derived values
void integrate ( state &, float , float);};
#endif

главный

#include "Gravity.h"#include <iostream>
#include "SFML\Graphics.hpp"#include "SFML\System.hpp"
using namespace std;

int main()
{gravity grav;
gravity::state marrio;
marrio.pos.setXY(500,500);sf::RenderWindow mwindow(sf::VideoMode(800, 800), "my window");
sf::CircleShape shape(50);
shape.setFillColor(sf::Color(100,250,250));

sf::Time timeC;
float time;
float dTime = 0.01f;

sf::Clock timePhysics;
while(mwindow.isOpen())
{

timeC = timePhysics.getElapsedTime();
time = timeC.asSeconds();
grav.integrate(marrio,time,dTime);
time += dTime;
cout<< "pos     vel"<<endl<<marrio.pos.getX()<<" "<<marrio.pos.getY()<<" "<<
marrio.vel.getX()<<" "<<marrio.vel.getY()<<endl;
shape.setPosition(marrio.pos.getX(),marrio.pos.getY());mwindow.clear();
mwindow.draw(shape);
mwindow.display();
}

system("PAUSE");
return 0;
}