/*
 * fbwl.java - revised 9 Jan 08 - width 501, height 361
 * @author jackord@kw.igs.net
 * particle sliding in a bowl with friction
 */

import java.applet.Applet;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class fbwl extends Applet 
                   implements ActionListener {
  int kk=0;                                            // Declarations
  double gr, u, s, r;
  String bs="Plot"; Button b=new Button(bs);

  public void init() {
    setLayout(new FlowLayout(FlowLayout.LEFT));
    setBackground(new Color(211, 211, 211));           // Light Gray Background
    add(b);
    b.addActionListener(this);
  }

  public void pin(Graphics g) {                        // Initialize the Graph
    g.setFont(new Font("TimesRoman", Font.PLAIN, 16));
    g.setColor(Color.black);
    g.drawRect(88, 12, 400, 300); g.drawLine(88, 162, 488, 162);
    g.drawString("0", 85, 332);
    g.drawString("20", 481, 332);
    g.drawString("Time", 268, 344);
    g.drawString("-90", 64, 318);
    g.drawString("0", 72, 168);
    g.drawString("90", 68, 18);
    g.drawString("Theta", 16, 93); g.drawString("deg", 24, 243);
  }

  double acc(double v) {                               // Newton's Law
    return -gr*Math.sin(s/r)+u*(gr*Math.cos(s/r)+v*v/r);
  }

  public void paint(Graphics g) {
    int x1, y1, x2, y2, rr, cc, del;
    double dt, pi, t, v, dv, vo, theta;
    String z;
    pin(g);
    if (kk==1) {
      gr=9.8; r=10; u=.12; dt=.02; rr=220; cc=300;
      z="turns"; t=0; s=r*Math.PI/2; v=0;
      g.setColor(Color.blue); x1=88; y1=12;
      long tt=System.currentTimeMillis(); del=5;
      dv=-gr*dt; v=v+dv/2; vo=v;                       // Feynman half-step
      do {
        t=t+dt; s=s+v*dt; dv=acc(v+dv/2)*dt; v=v+dv;
        theta=s/r*180/Math.PI;
        x2=88+(int)(20*t); y2=12+(int)(5*(90-theta)/3);
        g.drawLine(x1, y1, x2, y2); x1=x2; y1=y2;
        if (vo*v<0) {
          if (Math.abs(u/Math.tan(s/r))>1) { z="stops"; }
          g.drawString(z+" at theta = "+(float)theta, cc, rr); rr=rr+20; u=-u;
        }
        vo=v;
        tt=tt+del; while (tt>System.currentTimeMillis()) { }
      } while ((z=="turns")&&(kk>0));
    }
    kk=0;
  }

  public void actionPerformed (ActionEvent e) {        // Button
    String tst;
    tst=e.getActionCommand();
    if (bs.equals(tst)) {
      kk=1; 
      repaint();
    }
  }
}