import java.awt.*;

/**
    Program:  Level29
    Purpose:  extend custom class + solve d.e. most naively
    @author:  Paul Garrett, garrett@math.umn.edu
    @version: 29, Thu Feb 20 12:17:45 CST 1997
*/

public class Level29 extends TimedApplet1 {

  int t;
  float xold, yold, xmed, ymed, xnew, ynew;
  int theWidth, theHeight;
  float increment = 1F;
  final float gravity = 60000F;

  final float xDotDot(float x, float y) {
    float rad = (float) Math.sqrt(x*x + y*y);
    return gravity * ( - x) / (rad * rad * rad);
  }

  final float yDotDot(float x, float y) {
    float rad = (float) Math.sqrt(x*x + y*y);
    return gravity * ( - y) / (rad * rad * rad);
  }

  public void init() {
    theWidth = this.size().width;
    theHeight = this.size().height;
    t = 0;
    xnew = 100F;
    ynew = 100F;
    xmed = 90F;
    ymed = 110F;
  }

  public void run() {
    while (true) {
      pause(100);
      xold = xmed;
      yold = ymed;
      xmed = xnew;
      ymed = ynew;
      xnew = 2F * xmed - xold + xDotDot(xmed,ymed);
      ynew = 2F * ymed - yold + yDotDot(xmed,ymed);
      repaint();
      t++;
    }
  }
  
  public void paint(Graphics g) {
    
    g.setColor(Color.yellow);
    g.fillOval((theWidth/2),(theHeight/2),10,10);
    
    g.setColor(Color.black);   
    g.drawLine((theWidth/2) + (int) xmed,
	       (theHeight/2) - (int) ymed,
	       (theWidth/2) + (int) xnew,
	       (theHeight/2) - (int) ynew);
  }

  public void update(Graphics g) {
    paint(g);
  }

} // the end