Java Simple Physics

Well with classes over, I wanted to at least do something with the year of University Physics torture I have endured. Thus I present Java Simple Physics. It is just a quick physics example that uses the kinetic and potential relationship to have balls with different gravity constants drop then bounce with no loss of energy. It took a few hours to get right, mostly because I either did not know what I was doing, or was over complicating things. I hope to slowly improve on this concept till I have a VERY simple physics engine. Hey, it can only help to have a physics engine laying around, one day I might actually need one.

import javax.swing.*;
import java.awt.*;

public class Physics extends Thread
{
 private int worldx = 200, worldy = 900, worldtime = 15;
 private JFrame frame = new JFrame();
 private Ball ball,ball1;

    public Physics() 
    {
     // Set up frame
     frame.setSize(worldx,worldy);
     frame.setVisible(true);
     // Make new ball
     ball = new Ball(400,450,9.8,worldx,worldy,worldtime);
     ball1 = new Ball(400,450,3.8,worldx,worldy,worldtime);
    }
    
    public void run()
 {
  while(true)
  {
   try
   {
    // Cover old draw with fresh screen
    frame.getGraphics().fillRect(0,0,worldx,worldy);
    
    // Move and paint ball
    ball.move();
    ball.draw(frame.getGraphics());
    ball1.move();
    ball1.draw(frame.getGraphics());
    Thread.sleep(worldtime);
    
   }
   catch(Exception e){System.out.print(e);}
  }
 }
    
    public static void main(String args[])
    {
     Physics p = new Physics();
     p.start();
    }
}

class Ball
{
 private double gravity = 9.8, t;
 private double x,y,v,a,m,dx,dy,dv,k,u;
 private int worldx,worldy,dir=-1,width = 90;
  
 public Ball(int xin, int yin, double g, int wx,int wy, int time)
 {
  // Set Constants
  worldx = wx; worldy = wy-width; t = time;
  // Reduce gravity to fit with drawing refresh rate
  gravity = g * .02;
  
  // Center ba;; on x axis
  x = worldx/2 + width/2;
  
  // Set varables
  y = yin;
  v = 0;
  a = 0;
  m=.8;
  
  // Preset u and k values based on varables
  k = (m * gravity * (worldy - width)) - (m * gravity * y);
  if(k < 0){ k = 0; }
  u = m * gravity * y;
 }
 
 // Catch and reverse
 public void move()
 {
  // Calculate u and k
  k += u - (m * gravity * y);
  u = m * gravity * y;
  //Find dv based on k = .5mv^2
  dv = ((2 * k) / m) - v;
  
  // Catch and reverse on k or u < 0
  if(k <= 0 && dir == 1)
  {
   dir = -1;
   v = v*(-1);
   a = a*(-1);
   k = 1;
   u = m * gravity * worldy-width/10;
   dv = -1;
  }
  else if(u <= 0 & dir == -1)
  {
   dir = 1;
   v = v*(-1);
   a = a*(-1);
   u = 0;
   k = m * gravity * worldy-width/10;
   dv = ((2 * k) / m);
  }
  
  // Finalise new movement
  y += ((1+dv)*dir)/t;
  
  // Print out values when needed
  //System.out.println("x:"+x+" y:"+y+" u:"+u+" k:"+k+" - dv"+dv*dir);
 }
 
 
 
 public void draw(Graphics g)
 {
  // Draw to screen
  g.setColor(Color.GREEN);
  g.fillOval(worldx - (int)x, worldy - (int)y - width/3, width, width);
 }
 
}