最后我们让小球自动运动或者你可以用鼠标拖动小球。

 var ball:MovieClip = _root.attachMovie("ball", "ball", 2, {_x:250, _y:100});
var elastic:MovieClip = _root.createEmptyMovieClip("elastic", 1);
var point1:MovieClip = _root.attachMovie("sling", "point1", 3, {_x:50, _y:200});
var point2:MovieClip = _root.attachMovie("sling", "point2", 4, {_x:450, _y:200});
var gravity = 0.1;
var angle1:Number = 0;
var angle2:Number = 0;
var radius:Number = 13.0;
// Or whatever half the width of your ball is.
var elasticCoefficient:Number = 0.001;
// This number will affect the stretchiness of the sling.  The greater the number
// the tighter the elastic will be.    
var released:Boolean = true;
var forced:Boolean = false;
var acc:Object = {x:0, y:0};
var vel:Object = {x:0, y:0};
_root.onMouseDown = function() {
    ball._x = _root._xmouse;
    ball._y = _root._ymouse;
    ball.startDrag();
    _root.released = false;
    vel.x = 0;
    vel.y = 0;
};
_root.onMouseUp = function() {
    ball.stopDrag();
    _root.released = true;
};
ball.onPress = function() {
    ball.startDrag();
    _root.released = false;
    vel.x = 0;
    vel.y = 0;
};
ball.onRelease = function() {
    ball.stopDrag();
    _root.released = true;
};
_root.onEnterFrame = function() {
    elastic.clear();
    elastic.lineStyle(3, 0xFF0000);
    if (released) {
        ball._x += vel.x;
        ball._y += vel.y;
    }
    if (ball._y>=187) {
        /* This area differs from the code in the tutorial.
        The reason for that is I didn‘t read the code, I just looked at the examples.
        I try to gain experience with problem-solving by doing things as close to by myself as possible. */
        forced = true;
        var x1:Number = ball._x-point1._x;
        var y1:Number = ball._y-point1._y;
        var x2:Number = point2._x-ball._x;
        var y2:Number = point2._y-ball._y;
        var distance1:Number = Math.sqrt(x1*x1+y1*y1);
        var distance2:Number = Math.sqrt(x2*x2+y2*y2);
        _root.angle1 = Math.atan2(y1, x1);
        _root.angle2 = Math.atan2(y2, x2);
        var xOffset:Number = Math.cos(angle1+Math.PI/2)*radius;
        var yOffset:Number = Math.sin(angle1+Math.PI/2)*radius;
        var xOffset2:Number = Math.cos(angle2+Math.PI/2)*radius;
        var yOffset2:Number = Math.sin(angle2+Math.PI/2)*radius;
        angle1 += Math.sin(radius/distance1);
        angle2 += Math.sin(radius/distance2)*-1;
        elastic.moveTo(point1._x, point1._y);
        elastic.lineTo(ball._x+xOffset, ball._y+yOffset);
        elastic.moveTo(point2._x, point2._y);
        elastic.lineTo(ball._x+xOffset2, ball._y+yOffset2);
    } else {
        forced = false;
        elastic.moveTo(point1._x, point1._y);
        elastic.lineTo(point2._x, point2._y);
    }
    acc.x = 0;
    acc.y = 0;
    acc.y = gravity;
    if (released && forced) {
        /* This section applies the force of the sling to the ball in terms of acceleration based on the stretching of
        the sling, in the direction of the displacement, scaled by a coefficient that also encapsulates the mass of
        the ball. */
        acc.x += distance1*Math.sin(angle2)*elasticCoefficient;
        acc.y += -distance1*Math.cos(angle1)*elasticCoefficient;
        acc.x += distance2*Math.sin(angle1)*elasticCoefficient;
        acc.y += -distance2*Math.cos(angle2)*elasticCoefficient;
    }
    if (released) {
        vel.x += acc.x;
        vel.y += acc.y;
    }
}; 

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