Pulley Simulation..HELP

[dazza-s]

Hi

I am trying to simulate a line being effected by a pulley.

At the moment this is very simple. I have 3 plotted points. 1 The begining point, 2 the pully and 3 the end point. A line is drawn to each point so point 1 to point 2, point 2 to point 3.

I want to say if I move point 1 down then point 2 will stay static but point 3 will move up to point 2 by the equal amount of travel applied to point 1. Can do this easily just on the x axis but when I start mixing in the y axis as well I can't figure out how to plot the new position of point 3. I am guessing I would have to use triganomatry (sp?)

Please help.

[dazza-s]

It's ok, I have figured it. I just use the x position of the pully and decrease the x position of point 3 whilst doing the same to the y until the x position of 3 is equal to that of the pully. The nearer I get to the pully the more x I decrease. I would like some help later as I will be applying forces and pressures to the pulley system so would like the pulleys to act in different ways. I.e The pulley could be attached to a weight and the top of the pulley connected to a tow point via a elastic chord. So the pulley would move somewhat until the elastic overcomes the weight.

Actually any thoughts and tips would be good

[hardwired]

Code:

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

public class PulleyTest extends JPanel implements ChangeListener {
    PulleyModel model = new PulleyModel(200,50,40,400);
    Rectangle weight = new Rectangle(65, 40);
    int bitterEndHeight = 100;

    public void stateChanged(ChangeEvent e) {
        bitterEndHeight = ((JSlider)e.getSource()).getValue();
        repaint();
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        // draw pulley
        g2.setPaint(Color.blue);
        int dia = model.diameter;
        Point p = model.loc;
        double x = p.x - dia/2;
        double y = p.y - dia/2;
        Ellipse2D.Double pulley = new Ellipse2D.Double(x, y, dia, dia);
        g2.draw(pulley);
        // mark center
        g2.setPaint(Color.orange);
        g2.fill(new Ellipse2D.Double(p.x-2, p.y-2, 4, 4));
        // 
        int h = getHeight();
        int bitterLength = h - bitterEndHeight - p.y;
        double loadLength = model.getLoadLength(bitterLength);
        //System.out.printf("bitterLength = %d  loadLength = %.1f%n",
        //                   bitterLength, loadLength);
        // check calculations
        double totalLength = bitterLength + Math.PI*dia/2 + loadLength;
        //System.out.printf("totalHeight = %.1f%n", totalLength);
        // locate and draw weight
        weight.setLocation(p.x+dia/2-weight.width/2, (int)(p.y+loadLength));
        g2.setPaint(Color.green.darker());
        g2.draw(weight);
        // draw line
        g2.setPaint(Color.black);
        x = p.x-dia/2;
        g2.draw(new Line2D.Double(x, p.y, x, p.y+bitterLength));
        x = p.x+dia/2;
        g2.draw(new Line2D.Double(x, p.y, x, p.y+loadLength));
    }

    private JSlider getSlider() {
        JSlider slider = new JSlider(JSlider.VERTICAL,
                                     10, 280, bitterEndHeight);
        slider.addChangeListener(this);
        return slider;
    }

    public static void main(String[] args) {
        PulleyTest test = new PulleyTest();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(test);
        f.add(test.getSlider(), "After");
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
class PulleyModel {
    int diameter;
    Point loc;
    int lineLength;

    PulleyModel(int x, int y, int dia, int line) {
        loc = new Point(x, y);
        diameter = dia;
        lineLength = line;
    }

    public double getLoadLength(int bitterLength) {
        // loadLength = lineLength - bitterLength - circumference/2
        return lineLength - bitterLength - Math.PI*diameter/2;
    }
}