Planets

This code is based on

g = gravitational force

m= mass of the dominant object

r= the distance from the center of the object's mass

g=m/r²

Code in frame 1 layer 1

//two planets
//simulate a space ship no friction
//add a planet with basic gravity
//experiment with a second planet

init();

function init(){
  //create the ship

  myShip.dx = 0;
  myShip.dy = 0;
  myShip.speed = 0;
  myShip.dir = 0;
  myShip.gotoAndStop("still");

  planet.gravity = 10000;
  planet2.gravity = 15000;

  //set up drawing
  _root.moveTo(myShip._x, myShip._y);
  _root.lineStyle(.5,0xffffff,100);

} // end init

myShip.onEnterFrame = function(){
  myShip.checkKeys();
  myShip.turn();
  myShip.move();
  myShip.gravitate(planet)
  myShip.gravitate(planet2)
} // end enter frame

myShip.checkKeys = function(){
  //check for left and right arrows
  if (Key.isDown(Key.LEFT)){
    this.dir -= 10;
    this.gotoAndStop("left");
    if (this.dir < 0){
      this.dir = 350;
    } // end if
  } else if (Key.isDown(Key.RIGHT)){
    this.dir += 10;
    this.gotoAndStop("right");
    if (this.dir > 360){
      this.dir = 10;
    } // end if
  } else if (Key.isDown(Key.UP)){
    //thrust on up arrow
    this.thrustSpeed = 1;
    this.gotoAndPlay("thrust");
  } else {
    this.thrustSpeed = 0;
    this.gotoAndStop("still");
  } // end if

  if (Key.isDown(Key.SPACE)){
    //clear drawings
    _root.clear();

    //restart drawing
    _root.moveTo(myShip._x, myShip._y);
    _root.lineStyle(.5,0xffffff,100);

  } // end if
} // end checkKeys

myShip.turn = function(){
  this._rotation = this.dir;
  //trace("I'm here...");

  //get new thrust vector
  degrees = this.dir
  degrees -= 90;
  radians = degrees * Math.PI / 180;
  thrustDX = this.thrustSpeed * Math.cos(radians);
  thrustDY = this.thrustSpeed * Math.sin(radians);

  //add thrust to dx and dy
  this.dx += thrustDX;
  this.dy += thrustDY; 

} // end turn

myShip.move = function(){
  this._x += this.dx;
  myShip._y += this.dy;

  //  let ship go off screen

  _root.lineTo(this._x, this._y);

} // end move

myShip.gravitate = function(focus){
   //pull this element to the focus object
   //assumes focus object has a gravity property

   //figure angle difference between ship and planet
   tempDX = this._x - focus._x;
   tempDY = this._y - focus._y;

   //calculate distance between ship and planet
   tempDistance = Math.sqrt(tempDX * tempDX + tempDY * tempDY);

   //normalize vector (make it length of one)
   tempDX /= tempDistance;
   tempDY /= tempDistance;

   //compensate for the planet’s gravitational pull
   tempDX *= focus.gravity / (tempDistance * tempDistance);
   tempDY *= focus.gravity / (tempDistance * tempDistance);

   //invert the vector so it pulls ship to planet
   tempDX *= -1;
   tempDY *= -1;

   //add vector to ship
   this.dx += tempDX;
   this.dy += tempDY;
} // end gravitate

1. Beginning Flash Game Programming For Dummies,A Reference for the Rest of Us !, A. Harris,ISBN 0-7645-8962-8