code: import java.awt.*;
  import javax.swing.*;
  
  public class Pendulum extends JPanel implements Runnable {
  
      private double angle = Math.PI / 2;
      private int length;
  
      public Pendulum(int length) {
          this.length = length;
          setDoubleBuffered(true);
      }
  
      @Override
      public void paint(Graphics g) {
          g.setColor(Color.WHITE);
          g.fillRect(0, 0, getWidth(), getHeight());
          g.setColor(Color.BLACK);
          int anchorX = getWidth() / 2, anchorY = getHeight() / 4;
          int ballX = anchorX + (int) (Math.sin(angle) * length);
          int ballY = anchorY + (int) (Math.cos(angle) * length);
          g.drawLine(anchorX, anchorY, ballX, ballY);
          g.fillOval(anchorX - 3, anchorY - 4, 7, 7);
          g.fillOval(ballX - 7, ballY - 7, 14, 14);
      }
  
      public void run() {
          double angleAccel, angleVelocity = 0, dt = 0.1;
          while (true) {
              angleAccel = -9.81 / length * Math.sin(angle);
              angleVelocity += angleAccel * dt;
              angle += angleVelocity * dt;
              repaint();
              try { Thread.sleep(15); } catch (InterruptedException ex) {}
          }
      }
  
      @Override
      public Dimension getPreferredSize() {
          return new Dimension(2 * length + 50, length / 2 * 3);
      }
  
      public static void main(String[] args) {
          JFrame f = new JFrame("Pendulum");
          Pendulum p = new Pendulum(200);
          f.add(p);
          f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          f.pack();
          f.setVisible(true);
          new Thread(p).start();
      }
  }
  