java - Pendulum simulation only performing one period -

i making simulation of pendulum, performs 1 swing before sending close random positions bob at. essentially, not go backwards.

i have tried change direction using goingforward boolean, still doesnt work.

public class animationpane extends jpanel {     // start changeable variables     private double startangle = -60.0; // degrees     private double mass = 1; // kilogrammes     private int radius = 10; // m     private double gravity = 9.80665; // m/s^2 // on earth: 9.80665     // end changeable variables     private bufferedimage ball;     private bufferedimage rope;     private int pointx = 180;     private int pointy = 50;     private double endangle = math.abs(startangle); // absolute value of startangle     private double angle = startangle; // current angle     private double circum = (2 * math.pi * radius); // m     private double distance = 0; // m     private double velocity = 0; // m/s     private double totalenergy = ((radius) - (math.cos(math.toradians(angle)) * radius)) * gravity * mass + 0.00001;     private double previouse;     private int xpos = 0; // program     private int ypos = 0; // program     private boolean goingforward = true;     private double height = 0;      public animationpane() {         try {             ball = imageio.read(new file("rsz_black-circle-mask-to-fill-compass-outline.png"));             timer timer = new timer(100, new actionlistener() {                 @override                 public void actionperformed(actionevent e) {                     double anglerad = math.toradians(math.abs(angle));                      double potentiale = ((radius) - (math.cos(anglerad) * radius)) * gravity * mass;                     double pe = new double(potentiale);                     height = (radius - (math.cos(anglerad) * radius));                     double kinetice = totalenergy - pe;                      if (kinetice <= 0 || angle >= endangle) {                          if (goingforward == true) {                             goingforward = false;                         }                         else                          {                             goingforward = true;                          }                         kinetice = 0.1;                          angle = 60;                     }                      velocity = math.sqrt(2 * kinetice / mass);                     double ratio = distance / circum;                      if (goingforward == true) {                                                    distance = distance + (velocity / 10);                         angle = startangle + (360 * ratio);                     }                     else {                         distance = distance - (velocity / 10);                         angle = startangle - (360 * ratio);                     }                                              double angles = math.toradians(angle);                      double xdouble = math.sin(angles) * (radius * 10);                     double x = new double(xdouble);                     xpos = x.intvalue() + 150;                      double ydouble = math.cos(angles) * (radius * 10);                     double y = new double(ydouble);                     ypos = y.intvalue() + 50;                      repaint();                 }              });             timer.setrepeats(true);             timer.setcoalesce(true);             timer.start();         } catch (ioexception ex) {         }     }      @override     protected void paintcomponent(graphics g) {         super.paintcomponent(g);          g.drawline(xpos + 20, ypos + 20, pointx, pointy);         g.drawimage(ball, xpos, ypos, this);      }  } 

i appreciate getting work. thank you

i can't follow code. if had thought problem is, distance variable, , consequent use in ratio: seems defined being 0 when pendulum starts. when switch direction of pendulum, think needs rebased zero... somehow.

you should separate out gui code simulation. can make print out x , y coordinates while debugging. instance, can write values csv file, can visualize them in matlab (or whatever) ensure simulation looks right on time.

i make 2 suggestions simulation:

1. don't model in terms of energy, scalar quantity, meaning have use additional state "direction flag" model way going;
2. you going need additional state variable. storing x , y redundant, since these can derived directly angle (and r, although constant). modelling second-order system, need second state variable models movement of system @ instant, position. example, model angle , angular velocity on time.

and, of course, don't mess around degrees - stick radians :)

(on matter of velocity, variable of name speed - don't know direction moves in, , information highly relevant dynamics of system).

the derivation of method using angle , angular velocity quite straightforward, although maths rusty enough caution use without checking carefully!

the rotational form of newton's second law of motion is:

torque = moment of inertia * angular acceleration 

• the torque on bob given -mgr sin angle (m mass, g gravity, r length of pendulum, angle angular displacement of pendulum vertical). minus sign because force tends return bob vertical;
• the moment of inertia of point mass mr^2 - reasonable approximation if pendulum long compared radius of bob.

therefore angular acceleration -g sin theta / r. should simple harmonic motion - acceleration proportional distance equilibrium (for small theta, sin theta approximately theta) , directed towards it.

you can put numerical scheme simulate pendulum. example, using euler's method:

new angle            = old angle                + dt * old angular velocity new angular velocity = old angular velocity vel - dt * g * sin(angle) / r 

where dt time step.

you can, of course, use higher-order methods runge-kutta reduce numerical errors in simulation, (slightly) more involved.