# Simulate Projectile Motion with ActionScript 3.0

In this tutorial I’m going to take you through the process of simulating basic projectile motion using Flash and ActionScript. Our simulation will dynamically animate the initial trajectory of a projectile, and its subsequent bounces against the ground.

## The Final Result

Here's a quick preview of what we'll be working towards:

## Introduction

One of the elements that makes modern video games so realistic is their physics simulation. By applying concepts of classical mechanics to our Flash animations, we can create stunning visual effects that enhance the overall user experience.

In this tutorial I'm going to take you through the process of simulating basic projectile motion using Flash and ActionScript. Our simulation will dynamically animate the initial trajectory of a projectile, and its subsequent bounces against the ground. To keep things simple I have not included air resistance, so the example projectile will only experience one force; gravity. This tutorial assumes you have some basic knowledge of object-oriented programming and ActionScript. You may also benefit from having some experience with quadratic functions, and basic concepts of motion like *acceleration* and *velocity*. Although I am mostly going to be covering the application of these concepts as they apply to Flash and ActionScript, I want to first take some time to briefly explain them as they apply to classical mechanics.

Classical mechanics is the branch of physics based on Newton's laws of motion. In this tutorial we're going to look at two concepts of classical mechanics: projectile motion, and the coefficient of restitution. Projectile motion describes the path (trajectory) of an object with an initial velocity, while experiencing acceleration from gravity (and in most cases air resistance). You're probably familiar with the story of Newton formulating his theory of gravity while sitting under an apple tree. Gravity plays a big role in projectile motion, since it adds a constant acceleration of -9.8 m/s^{2} in the y-direction.

A good example of projectile motion is a baseball being thrown in the air. When you throw a baseball, it has an initial speed and direction at the instant it leaves your hand. These two properties make up the ball's initial velocity. After the ball leaves your hand, it follows an arc shaped path, also known as its trajectory. The general trajectory of a projectile can be described by a quadratic function:

This function gives us the position of a projectile with respect to time (*d(t)* is the position after *t* seconds have passed). The little triangle means "change in", so *Δt* means "number of seconds passed".

To use this function we need to know:

- The acceleration,
*a*, of the projectile; - The initial velocity (speed in a particular direction),
*v*, of the projectile;_{1} - The initial position,
*d*, of the projectile._{1}

The path of a projectile in the x and y directions are independent. This means that in order to calculate the position in a 2-dimensional space, we need to use the function twice.

The other concept we're going to be looking at is something called the *coefficient of restitution*. The coefficient of restitution is a ratio for describing the difference in an object's velocity before and after a collision.

For example, if you bounce a basketball against the floor, the height it bounces decreases with each subsequent bounce. This change in height (directly related to the change in velocity), is constant, and can therefore be described by the *coefficient of restitution*. Calculating the coefficient of restitution for a moving object and a stationary surface is simple:

Here, *v _{1}* is the velocity of the ball just before it hits the ground, and

*v*is the velocity of the ball just after it hits the ground and starts bouncing upwards again.

_{2}Since the *coefficient of restitution* is different for every combination of objects, you will need to find your own, depending on what you are simulating.

## Step 1: Setting up the FLA Document

Let's start building this thing! Open up Flash, and create a new document. In the example I will be using the default dimensions of 550 x 400, and a frame rate of 50 FPS. Save this file under a name of your choice.

Next, we need to create a document class. Create a new Actionscript file, and add the following:

package { import flash.display.MovieClip; public class Main extends MovieClip { public function Main():void{ } } }

Save this file in the same directory as our FLA. Call it *Main.as*.

The last thing we need to do is link the document class with the FLA. Inside the FLA, find the *Properties* panel. Next to *Document class*, enter the name of the document class, *Main*.

For more information about using a document class, please refer to this Quick Tip.

## Step 2: Creating the Main Loop

Now that our flash document is set up, we can start coding. The first thing we need to do is create a loop that will run at a fixed time interval. Generally speaking, there are two trains of thought regarding timed loops: using the *ENTER_FRAME* event, or using the AS3 *Timer* class. Both methods have different pros and cons, but for this example I am going to use the *Timer* class.

We need to start by importing two classes: the *Timer* class, and its respective *TimerEvent* class. Inside *Main.as* add the following lines.

import flash.utils.Timer; import flash.events.TimerEvent;

Before we set up the *Timer*, I want to add another method called, *mainLoop* to the document class. As its name suggests, this method will serve as our loop.

public function mainLoop(event:TimerEvent):void{ }

Now we can set up the Timer! We're going to start by adding two properties to the class: *masterTimer*, and *interval*. The masterTimer's purpose should be obvious, but the interval property will be used to store the length (in seconds) of a Timer interval.

public var masterTimer:Timer; public var interval:Number;

Next we're going to populate these properties inside our constructor method, *Main*. In Step 1 we set our document's frame rate to 50 FPS. This frame rate can be referenced with *stage.frameRate*, and will be used as the basis of calculating the Timer interval.

// Get the document's frame rate var fps:int = stage.frameRate; // The timer interval in seconds interval = 1/fps; // Create a new instance of the timer, first parameter requires the interval in milliseconds masterTimer = new Timer(1000/fps);

The Timer generates a *TimerEvent.TIMER* event every interval (in our case, 50 times a second, or once every 20 milliseconds). By adding an event listener to the Timer, we can run our *mainLoop* method every time this event is generated, thus creating our loop.

// Connect our loop method to the timer. masterTimer.addEventListener(TimerEvent.TIMER, mainLoop);

The only thing remaining is to start the Timer.

// Start the Timer masterTimer.start();

## Step 3: Testing the Main Loop

If we test the movie as is, nothing will happen. Since we want to know if the loop is working, we are going to add a line to the mainLoop method that outputs the amount of time the SWF has been running. By default the Timer can only return the current number of intervals that have passed. Since we already calculated the length of an interval, we simply multiply the two values to get the time. Please note that I am rounding the result to avoid issues with floating point arithmetic.

// Calculate the current time var currentTime:Number = int((event.target.currentCount * interval) * 1000) / 1000;

If we trace *currentTime*, we can see the Timer working.

// Test the loop by tracing the current time trace("Current Time: " + currentTime);

Test the movie. Your output log should look similar to this:

Since we know the Timer is working, we can remove the trace statement.

## Step 4: Projectile.as Creating the Class

Our next step is to create a class that represents a projectile (the basketball). Create another ActionScript file, and once again add our basic class structure. Save this file as *Projectile.as*.

package { import flash.display.MovieClip; public class Projectile extends MovieClip { public function Projectile():void{ } } }

We're going to start by adding some properties to keep track of the projectile's characteristics. I will explain the animation specific properties later. The rest you should recognize from the equations at the start of the tutorial.

// Initial Velocity private var v1X:Number; private var v1Y:Number; // Acceleration private var aX:Number; private var aY:Number; // Initial Position private var d1X:Number; private var d1Y:Number; // Animation-specific private var startTime:Number; private var moving:Boolean;

## Step 5: Projectile.as Convenience Methods

Since this project uses a lot of calculations, it makes sense to write a set of convenience methods for the tedious calculations which will be performed numerous times. Below are six convenience methods that will be referenced throughout the rest of the tutorial.

The first method, *solveQuadratic*, return the roots of a quadratic function (in standard form: Ax^{2} + Bx + C; this method will return the value of x) using the quadratic formula.

private function solveQuadratic(a:Number, b:Number, c:Number):Array{ // Method paramters format: Ax² + Bx + C var solution:Array = new Array(2); var discriminant:Number = (b * b) - (4 * a * c); // b^2 - 4ac solution["root1"] = ((-1 * b) + Math.sqrt(discriminant)) / (2 * a); // (-b + squareroot(b^2 - 4ac)) / 2a solution["root2"] = ((-1 * b) - Math.sqrt(discriminant)) / (2 * a); // (-b - squareroot(b^2 - 4ac)) / 2a return solution; }

The second method, *getComponents*, returns an array with the individual x and y components of a Euclidean vector.

private function getComponents(angleDegrees:Number, magnitude:Number):Array{ var components:Array = new Array(2); // Trig functions require angles in radians (1 radian = PI/180 degrees) components["x"] = magnitude * Math.cos(angleDegrees * Math.PI / 180); components["y"] = magnitude * Math.sin(angleDegrees * Math.PI / 180); return components; }

The third method, *getPosition*, will return the position at a given time. We're using the position formula from the introduction; this is *d(t)*.

private function getPosition(time:Number, acceleration:Number, initialVelocity:Number, initialPosition:Number):Number{ var position:Number; // d(t) = (1/2)*a*(t^2) + v_1*t + d_1 position = (0.5 * acceleration * time * time) + (initialVelocity * time) + (initialPosition); return position; }

The fourth method, *getTimes*, is the opposite of *getPosition*. It returns the time taken for the projectile to reach a given position.

private function getTimes(finalPosition:Number, initialPosition: Number, acceleration:Number, initialVelocity:Number):Array{ var time:Array; // Solve quadratic position function: 0 = (1/2)*a*(t^2) + v_1*t + d1-d2 time = solveQuadratic(0.5 * acceleration, initialVelocity, initialPosition - finalPosition); return time; }

The fifth method, *getVelocity*, uses the first derivative of our position function to return the velocity of the projectile at a given time.

private function getVelocity(acceleration:Number, initialVelocity:Number, time:Number):Number{ var velocity:Number; // Velocity function is first derivative of position function: d' = a*t + v1 velocity = acceleration * time + initialVelocity return velocity; }

The sixth method, *getVelocityDirection*, returns the direction (angle) of the velocity at a given time. In other words, it returns the direction that the projectile is moving in at a specific instant.

private function getVelocityDirection(accelerationX:Number, accelerationY:Number, initialVelocityX:Number, initialVelocityY:Number, time:Number):Number{ var angle:Number; var velocityX:Number = getVelocity(accelerationX, initialVelocityX, time); var velocityY:Number = getVelocity(accelerationY, initialVelocityY, time); angle = Math.atan2(velocityY, velocityX) * 180 / Math.PI; return angle; }

## Step 6: Projectile.as Initialization Method

In step 4, we added a bunch of properties to the Projectile class. If you noticed, all these properties are private. I did this because we don't want to access any of them directly. We still need a way to set and preform the necessary calculations on these properties, so we're going to add an *init* method to do so.

public function init(velocityDirection:Number, initialVelocity:Number, initialPositionX:Number, initialPositionY:Number, acceleration:Number = 0, accelerationDirection:Number = 0, gravity:Number = -9.8):void{ }

The names of the different parameters should be fairly obvious. The *velocityDirection* parameter takes the angle of the *initialVelocity* in degrees. The acceleration parameters are all optional. If you set a value for *acceleration*, you will need to provide a separate value for *accelerationDirection* as we did with velocityDirection.

I mentioned earlier that we're dealing with components, however our *init* method only accepts a vector (magnitude and direction). This is where our convenience methods come into play.

// Get components for our velocity and acceleration vectors var vComponents:Array = getComponents(velocityDirection, initialVelocity); var aComponents:Array = getComponents(accelerationDirection, acceleration);

Now we have an array containing the individual components of the vectors. We can now store them to their corresponding properties.

// Store these vectors in the corresponding properties v1X = vComponents["x"]; v1Y = vComponents["y"]; aX = aComponents["x"]; aY = aComponents["y"] + gravity; // This is where we add the additional force of gravity in the y-direction

The points for initial position do not need to be changed.

// Store the initial position d1X = initialPositionX; d1Y = initialPositionY;

## Step 7: Projectile.as Control Methods

Before we continue, I need to explain the animation-specific properties we created in Step 4. I'll start with the *moving* property. Since our projectile has a rest state, we use the moving property as a flag so the loop only animates when the projectile is in motion. The *startTime* property is used to account for the difference between the global time of the Timer, and the local time of the projectile's position function. Nothing too complicated.

Our control methods, for lack of a better term, will, for the most part, be called from within the loop. These first three methods are really simple, and are used to interact with the animation-specific properties.

public function begin(currentTime:Number):void{ startTime = currentTime; moving = true; } public function end():void{ moving = false; } public function isMoving():Boolean{ return moving; }

The last method we're going to add deserves most of our attention. As its name suggests, the *positionAtTime* method returns the position of the projectile at a given time. We can use this position inside the loop to animate our MovieClip on the stage.

public function positionAtTime(currentTime:Number):Array{ var relativeTime:Number = int((currentTime - startTime) * 1000) / 1000; var position:Array = new Array(2); position["x"] = getPosition(relativeTime, aX, v1X, d1X); position["y"] = getPosition(relativeTime, aY, v1Y, d1Y); return position; }

If you look inside this method, you should notice a couple of things. This first thing is the use of a *relativeTime* variable. I mentioned earlier that we need to account for the difference in time between the *Timer* and *Projectile*. The beginning of the projectile's trajectory starts at a relative time of 0, so we're only interested in the the amount of time that has passed on the Timer since this trajectory began. This is why we store the *startTime*. The second thing is the use of our *getPosition* method from step 5. We use this method twice to calculate the position of the Projectile in both axes.

Save the Projectile class.

## Step 8: Projectile.as Linking to a MovieClip

Before we can animate, we need something to animate. If you noticed, our *Projectile* class extends the *MovieClip* class, which means we can link it to a *MovieClip* in our library.

Open the main flash document (the FLA), and draw a circle on the stage. Don't worry too much about the size, since we're going to scale it down later in code.

Next, convert the circle to a *MovieClip*. The MovieClip should appear in your library, once it does, delete it from the stage since we will be adding it programmatically in the next step.

Now we need to link our Projectile class with the MovieClip. In your library right click (Control click on the Mac) your MovieClip and select *Linkage...*

Check the box beside *Export for ActionScript*. Enter *Ball* for *Class*, and *Projectile* for *Base class*.

Click *OK*. You may see a dialogue box telling you that a class definition could not be found, and that one will be generated automatically. Click *OK* again.

Finally, save the document.

## Step 9: Animating!

We have a loop, we've written our Projectile class, and we've linked it with a MovieClip. Now it's time to animate the projectile! To begin, open our document class, *Main.as*.

We're gonna start by adding a few more properties.

public var scale:int; public var ball:Ball; public var startFlag:Boolean;

The purpose of these properties is pretty straightforward. The *scale* property is going to represent a scale between Flash's pixel position and our calculated metre position. The *ball* property represents the MovieClip we created in the previous step. You might notice that its type is set to *Ball*, this is the class name we set in the previous step. Finally the *startFlag* property is used as a flag to tell the loop when our projectile should begin its trajectory.

Inside the constructor method, we're going to add the following lines to setup our Ball object.

// 1 metre = "scale" pixels scale = 100; //i.e. 100 pixels = one metre, 1 pixel = 1cm // Create a new instance of the Ball ball = new Ball(); // Set the size of the ball, in this case it's 0.5m x 0.5m ball.height = 0.5 * scale; ball.width = 0.5 * scale; // Position the ball on the stage ball.x = 50; ball.y = 350; // Add the ball to the stage stage.addChild(ball);

If you test the movie, your ball should appear in the bottom-left corner of the stage.

Next we need to run the *init* method from step 6. If you look at the method declaration, you'll see that it requires a minimum of 4 paramters: *velocityDirection*, *initialVelocity*, *initialPositionX*, and *initialPositionY*. Staying in the constructor method, let's set those up!

// Run the projectile init method to set the properties of our projectile ball.init(80, 7, ball.x / scale, (stage.stageHeight - ball.y) / scale);

The parameters for initial position require some simple calculations before they're passed on to the method. The Projectile methods all use metres as the unit of distance, so we need to use our scale. The *initialPositionY* requires an additional step. Since Flash's origin exists in the top-left corner, and increasing y moves *down* the screen, we need to subtract our y-position from the *stage.stageHeight*, to move our origin to the bottom-left and allow increasing y to move *up* the screen.

Now that our projectile is configured, we can set the *startFlag*.

// Set the starting flag, so the loop puts the ball in motion startFlag = true;

Let's move on to setting up the loop. The first thing we need to do is add a conditional statement that checks for the *startFlag*.

// Start motion if the flag has been flagged if(startFlag == true){ ball.begin(currentTime); startFlag = false; }

There's a few things worth noting. If you recall, we already created a variable called *currentTime* in Step 3. Also, since we only want to call the *begin* method once, we set the *startFlag* property back to *false*.

It would be pointless for the loop to calculate the position of the projectile if it's at rest, so we need to add another conditional statement that checks whether the projectile is moving.

// Animate if ball is in motion if(ball.isMoving() == true){ }

Inside this statement we can implement the actual animation. The first thing we need to do is get the projectile's position.

// Get the current position var currentPosition:Array = ball.positionAtTime(currentTime);

Once we know the position, we simply update the *MovieClip*'s position. Similar to the *init* method, we need to take into account our scale, and Flash's origin.

// Move our custom MovieClip ball.x = currentPosition["x"] * scale; ball.y = stage.stageHeight - currentPosition["y"] * scale;

We've finally reached a milestone! If you test the movie now, you can see the basic effect!

## Step 10: Making the Ball Bounce

We've animated the trajectory of our projectile, but as you can see the ball keeps moving indefinitely. To enhance this simulation, we're going to create a *ground* on which the ball can bounce. For simplicity's sake, we're going to make our *ground* an imaginary line, at the bottom of the stage.

Our approach here involves implementing collision detection. Generally speaking we have two options for our collision detection: frame-based and predictive. Ultimately we are going to be using predicative collision detection, but I think its worth explaining the theory behind each option:

The idea behind frame-based collision detection is simple. Inside the loop, we would check whether the ball's y-position is below the ground's y-position. If the condition is met, a collision has taken place...sounds good right? Not really. Chances are that a collision will not happen on an exact *Timer* interval. This means that the time and position of the bounce will be inaccurate. Consider this diagram:

The actual collision would have taken place in between the two frames. But our frame-based collision detection code would only have detected the collision in the second frame, when the projectile should already be on its new trajectory.

We can overcome this problem by using predictive collision detection. It can be slightly more challenging to implement, but it gives us a more accurate simulation. Predictive collision detection works as follows. We first calculate, in advance, the exact time that the projectile will collide with the ground. Inside the loop we check whether this time has passed. If it has, we set the projectile on a new trajectory, starting at that exact time of collision.

First thing's first, we need to add a few more methods to our *Projectile* class. We'll start with *timeOfCollision*.

public function timeOfCollision(ground:Number):Number{ var time:Number; var times:Array; times = getTimes(ground, d1Y, aY, v1Y); // We don't want the negative time result, so use the larger time time = Math.max(times["root1"], times["root2"]); // We need to consider the startTime as well since we're checking the loop time = time + startTime return time; }

(Remember, *getTimes* returns the time at which the projectile will be at a specified position. It will actually return two times, one negative and one positive, because these are the roots of a quadratic equation.)

We're also going to need to know the magnitude and direction of our velocity at this time, so add these two methods for calculating that as well.

public function velocityAtTime(currentTime:Number):Number{ var relativeTime:Number = int((currentTime - startTime) * 1000) / 1000; var velocity:Number; var velocityX:Number = getVelocity(aX, v1X, relativeTime); var velocityY:Number = getVelocity(aY, v1Y, relativeTime); // Pythagorean theorem velocity = Math.sqrt(velocityX * velocityX + velocityY * velocityY); return velocity; } public function velocityDirectionAtTime(currentTime:Number):Number{ var relativeTime:Number = int((currentTime - startTime) * 1000) / 1000; var angle:Number; angle = getVelocityDirection(aX, aY, v1X, v1Y, relativeTime); return angle; }

Before we finish up with this class, I want to add one more method called *getStartTime*. We will be using it in the next step.

public function getStartTime():Number{ return startTime; }

Save the Projectile class and return to the document class. The first thing we need to do is add a couple of properties. A *collisionTime* property, and a *cor* property to represent our coefficient of restitution.

public var collisionTime:Number; public var cor:Number;

The first thing we need to do is set our *cor*. We can set this in the constructor method. I've picked 0.8, but feel free to experiment with different values to see the effects.

// Set the coefficient of restitution. Must be a value between 0 and 1 cor = 0.8;

Now for the fun part! Inside our main loop, we need to implement our bouncing methods. For starters, find the condition for *startFlag*. Inside add the following line. This will calculate our initial collision time.

// Since the position is the centre point of the ball, the y-position of the ground is actually at half the height of the ball. collisionTime = ball.timeOfCollision((ball.height / 2) / scale);

Next find the condition that checks *ball.isMoving()*. We're only going to check for collisions if the ball is in motion, so we'll put our collision detection statements in there. Add the following to the top of the condition.

// Check if a collision happened if(currentTime >= collisionTime){ }

Inside that collision condition, we need to execute the bounce. We achieve this by setting the projectile on a new trajectory, starting at *collisionTime*. The first thing we need to do is calculate the direction and magnitude of the velocity at *collisionTime*.

var newVelocityDirection:Number = -1 * ball.velocityDirectionAtTime(collisionTime); var newVelocity: Number = ball.velocityAtTime(collisionTime) * cor;

Next we need to run the appropriate methods for setting the new trajectory. For the *timeOfCollision* method, we need to set our *ground* y-position to half the height of the ball. We do this because the y-position of the ball is actually at its centre point, not its base.

// Set the new trajectory ball.init(newVelocityDirection, newVelocity, ball.x / scale, (stage.stageHeight - ball.y) / scale); ball.begin(collisionTime); collisionTime = ball.timeOfCollision((ball.height / 2) / scale);

We've reached another milestone! If you test the movie, you should see the bouncing effect.

## Step 11: But the Bouncing Never Stops!

If you take a look at the above example, the ball never comes to a complete stop. Towards the end it appears to have a quick, jittery motion. This phenomenon is caused by the following line:

var newVelocity: Number = ball.velocityAtTime(collisionTime) * cor;

I'll explain. Assuming *cor* is not equal to 0 or 1, our projectile's velocity will only *approach* zero, getting slower and slower, but never actually reaching it. Since the reality is that our projectile does come to rest, we need to implement a solution for this problem.

Here's how. Since the *cor* is constant, we can look at the bounces as a geometric series:

- a
_{1}: Time taken for first bounce - a
_{2}: Time taken for second bounce - a
_{3}: Time taken for third bounce - ...

Each *a* term in the sequence represents the length of time for a bounce. We then refer to the cumulative time taken like so:

- S
_{1}: Time taken for first bounce - S
_{2}: Time taken for first bounce*and*second bounce - S
_{3}: Time taken for the three first bounces, in total - ...

From here, we can use the formula for the sum of an infinite geometric series to calculate our stopping time:

The formula itself is simple. The *a _{1}* variable, represents the first term in the series, in our case, the length of the first bounce. The

*r*variable represents the series' ratio, in our case, the

*cor*.

As far as the implementation goes, we'll start by adding two more properties to the document class.

public var stoppingTime:Number; public var bounce:int;

We need to know when the first bounce occurs, so the *bounce* property will be used as a simple counter. Inside the loop's *startFlag* condition (this if-statement: *if(startFlag == true){*) add the following after our declaration of *cor*:

// Reset the bounce count bounce = 0;

Next, add the following to the end of our collision condition.

// Increase number of bounces bounce++; // Calculate stopping time on first bounce if(bounce == 1){ // Sum of infinite geometric series: S = a_1 / 1 - r stoppingTime = (collisionTime - ball.getStartTime()) / (1 - cor) + ball.getStartTime(); }

Every time a collision occurs we need to check whether *stoppingTime* has passed. If it has we can bring the projectile to rest, by calling the *end* method. The loop is still going to reposition the ball one last time, so we need to set the ball to its correct final position. The loop already set the projectile on a new trajectory, so we're going to get around this by setting *currentTime* to *ball.getStartTime()*.

// If we reach the stopping time, end the motion if(bounce > 1 && collisionTime > stoppingTime){ ball.end(); // Since the ball is already on a new trajectory, force it to its start position currentTime = ball.getStartTime(); }

That's it! The projectile now bounces and comes to a complete stop.

## Step 12: Adding Basic UI Control

When we play the movie, the projectile fires immediately. This is pretty useless for most applications, so let's change things a bit and add some basic UI controls. To keep things simple, we're going to add two buttons: a start button, and a reset button.

Our first step is similar to Step 8. In the FLA document, draw two buttons for Start and Reset. Convert each button to a symbol, this time make them *Buttons* as opposed to *MovieClips*. This time we are going to keep our buttons on the stage, so position them as desired. Similarly to Step 8, open the *Linkage Properties* for one of the buttons. Check the box beside *Export for ActionScript*, this time however, leave the default class names. Click OK and repeat this for the other button.

On the stage, select the Start button. Open up the *Properties panel*. Where it says <Instance Name> enter *startButton*. Repeat this for the reset button, calling it *resetButton*.

Save this file, and open the document class. Inside, we need to import the *MouseEvent* class.

import flash.events.MouseEvent;

Inside the constructor method remove the following lines.

// Run the projectile init method to set the properties of our projectile ball.init(85, 6, ball.x / scale, (stage.stageHeight - ball.y) / scale); // Set the starting flag, so the loop set's the ball in motion startFlag = true;

We're going to replace this by adding event listeners for each of the buttons we created. Notice that we can reference them with the *<Instance Name>* we set earlier.

// Add event listeners for our start and reset buttons startButton.addEventListener(MouseEvent.CLICK, startEvent); resetButton.addEventListener(MouseEvent.CLICK, resetEvent);

Finally, we can wrap everything up by adding the two methods we're calling from those event listeners.

public function startEvent(event:MouseEvent):void{ // Don't start if its already in motion if(ball.isMoving() == false){ // Run the projectile init method to set the properties of our projectile ball.init(85, 6, ball.x / scale, (stage.stageHeight - ball.y) / scale); // Set the starting flag, so the loop set's the ball in motion startFlag = true; } } public function resetEvent(event:MouseEvent):void{ // End the motion ball.end(); // Move ball back to original position ball.x = 50; ball.y = 350; }

## The Final Result

## Conclusion

Although the final result is simple, it works, and it shows how easy it is to implement real, dynamic physics concepts into your flash projects. How you choose to apply them comes down to your own creativity. If you're looking for a more complete, intuitive solution, it might make more sense to check out an existing physics engine like QuickBox2D.

To summarize: we've created a timed loop, written a class that models the trajectory of a projectile and added the necessary collision detection to simulate bounce. We covered a lot of content, but ultimately, we've barely scratched the surface of what we can do with classical mechanics. In projectile motion alone, there is still room to add things like drag, air resistance, and terminal velocity.

I hope you have enjoyed this tutorial. Thanks for reading!

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