# Throw a ball with a sling physics Flash tutorial: Part 2 by Kevin Ward

Only 2 days. That’s how much time Kevin Ward needed to solve and publish the solution of the problem posted in Throw a ball with a sling physics Flash tutorial.

You may know Kevin for his excellent post about a different approach to create a game like Line Rider, and now he come with the second part of the sling tutorial.

Take your honour, Kevin, you deserve it!!

This is the movie Kevin created:

And this is the actionscript, with comments in some critical parts:

```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;
// 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);
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;
}
};```

This is an excellent solution to the problem, and I think this should be an inspiration for the ball physics Flash game design competition.

214 GAME PROTOTYPES EXPLAINED WITH SOURCE CODE
// 1+2=3
// 10000000
// 2 Cars
// 2048
// Avoider
// Ballz
// Block it
// Blockage
// Bloons
// Boids
// Bombuzal
// Breakout
// Bricks
// Columns
// CubesOut
// Dots
// DROP'd
// Dudeski
// Eskiv
// Filler
// Fling
// Globe
// HookPod
// Hundreds
// InkTd
// Iromeku
// Lumines
// Magick
// MagOrMin
// Maze
// Memdot
// Nano War
// Nodes
// o:anquan
// Ononmin
// Pacco
// Phyballs
// Platform
// Poker
// Pool
// Poux
// Pudi
// qomp
// Racing
// Renju
// SameGame
// Security
// Sling
// Slingy
// Sokoban
// Splitter
// Sproing
// Stack
// Stringy
// Sudoku
// Tetris
// Threes
// Toony
// Turn
// TwinSpin
// vvvvvv
// Wordle
// Worms
// Yanga
// Zhed
// zNumbers