const

   MS_STOP = 1,

   MS_LEFT = 2,

   MS_RIGHT = 3;

 var body;

 var moveState;

 var EXAMPLE = 0;

​

 function setup() {

   createCanvas(400, 300);

   //30 is the conversion factor for pixel coordinates

   //box2d recommends shape edges between 0.1 and 10

   b2newWorld(30, b2V(0, 9.8));

   new b2Body('chain', false, b2V(width / 2, height - 8),

     [b2V(-width / 2 + 8, -height + 16), b2V(-width / 2 + 8, 0), b2V(width / 2 - 8, 0), b2V(width / 2 - 8, -height + 16), b2V(-width / 2 + 8, -height + 16)]);

   body = new b2Body('box', true, b2V(width / 2, 16), b2V(20, 20));

   moveState = MS_STOP;

 }

​

 function keyPressed() {

   switch (key) {

     case 'q': //move left

       moveState = MS_LEFT;

       break;

     case 'w': //stop

       moveState = MS_STOP;

       break;

     case 'e': //move right

       moveState = MS_RIGHT;

       break;

     case 'g': //switch from first example to fifth

       EXAMPLE = ++EXAMPLE % 6;

       break;

   }

 }

​

var states = ['absLinearVelocity', 'gradualLinear', 'force', 'gradualForce', 'impulse', 'gradualImpulse'];

 function draw() {

   var force;

   background(227);

   text('state=' + states[EXAMPLE], 20, 30);

   text('type g to step through the 6 states', 170, 30);

   text('type q to move left, w to stop, e to move right',120,45);

   text(body.toString(), 20, 45);

   //inside Step()

   var vel = body.linearVelocity;

   if (EXAMPLE == 0) {

     switch (moveState) {

       case MS_LEFT:

         vel.x = -5;

         break;

       case MS_STOP:

         vel.x = 0;

         break;

       case MS_RIGHT:

         vel.x = 5;

         break;

     }

     body.linearVelocity = vel;

   }

   if (EXAMPLE == 1) {

     switch (moveState) {

       case MS_LEFT:

         vel.x = max(vel.x - 0.2, -5.0);

         break;

       case MS_STOP:

         vel.x *= 0.98;

         break;

       case MS_RIGHT:

         vel.x = min(vel.x + 0.2, 5.0);

         break;

     }

     body.linearVelocity = vel;

   }

   if (EXAMPLE == 2) {

     force = 0;

     switch (moveState) {

       case MS_LEFT:

         if (vel.x > -5) force = -50;

         break;

       case MS_STOP:

         force = vel.x * -10;

         break;

       case MS_RIGHT:

         if (vel.x < 5) force = 50;

         break;

     }

     body.force = b2V(force, 0);

   }

   if (EXAMPLE == 3) {

     var desiredVel = 0;

     switch (moveState) {

       case MS_LEFT:

         desiredVel = -5;

         break;

       case MS_STOP:

         desiredVel = 0;

         break;

       case MS_RIGHT:

         desiredVel = 5;

         break;

     }

     var velChange = desiredVel - vel.x;

     force = body.body.getMass() * velChange / (1 / 60.0); //f = mv/t

     if (force != 0)

       body.force = b2V(force, 0);

   }

   if (EXAMPLE == 4) {

     var desiredVel = 0;

     switch (moveState) {

       case MS_LEFT:

         desiredVel = -5;

         break;

       case MS_STOP:

         desiredVel = 0;

         break;

       case MS_RIGHT:

         desiredVel = 5;

         break;

     }

     var velChange = desiredVel - vel.x;

     var impulse = body.body.getMass() * velChange; //disregard time factor

     if (impulse != 0)

       body.impulse = b2V(impulse, 0);

   }

   if (EXAMPLE == 5) {

     var desiredVel = 0;

     switch (moveState) {

       case MS_LEFT:

         desiredVel = max(vel.x - 0.2, -5.0);

         break;

       case MS_STOP:

         desiredVel = vel.x * 0.98;

         break;

       case MS_RIGHT:

         desiredVel = min(vel.x + 0.2, 5.0);

         break;

     }

     var velChange = desiredVel - vel.x;

     var impulse = body.body.getMass() * velChange; //disregard time factor

     body.impulse = b2V(impulse, 0);

   }

   b2Update();

   b2Draw();

 }