2D Physics - Water Simulator (Difficulty: 3)

CreatiCode

Introduction

In this tutorial, you will use the 2D physics engine to build an interesting animation: simulating water in a bucket.

Step 1 - Initialize the 2D Physics World

Please create a new project on the CreatiCode platform, and delete the dog sprite. In the empty sprite, add the following blocks to initialize a 2D physics world.

The physics world is not visible to us, and it is managed by the 2D physics engine. We can not use any other blocks in the “2D Physics” category without this step.

In this program, we are setting a gravity in the Y direction for -100. Since the number is negative, it means objects will be pulled down along the Y direction.

Step 2 - Add a Ball Costume

We will use small balls to represent water drops. Please switch to the costume tab and add the “Ball-b” costume:

Please also make the sprite smaller by setting its size to 30%:

Step 3 - Make the Ball Follow the Mouse

Next, let’s make the ball move to right above our mouse pointer’s position whenever we press down the mouse. We can use a forever loop that checks if the mouse button is down repeatedly, and whenever that’s true, we move the ball sprite to the mouse’s X position and 10 plus the mouse’s Y position. We are adding 10 to the Y position to make sure the ball does not go right under the mouse pointer, which would switch it to the “dragging” mode.

The ball should follow your mouse whenever you press down and drag the mouse button:

Step 4 - Create Clones

We will use many clones of the ball sprite to represent small water drops. We can keep creating new clones after moving the ball:

Now we are “drawing” a trail with the clones of the ball:

Step 5 - Add Physics Bodies to Each Clone

Now we are going to hand over these clones to the physics engine, which will help us make the balls behave like real balls. Whenever a new clone is created, we will use the “behave as” block to convert the clone to a circle-shape object:

Now our clones become “alive”: the physics engine will make them fall due to gravity, and also collide with each other.

Step 6 - Hide the Original Sprite

Since we only use clones of this ball sprite to represent water drops, the original sprite is left on the stage at a fixed position:

We can hide the original sprite using the “hide” block, and then “show” the clones after they are created:

Step 7 - Draw the Bucket Sprite

Next, let’s create a bucket to hold these “water drops”. Create a new empty sprite named “Bucket”, and draw a bucket using 3 boxes like this:

Note that at the end you need to select all boxes, and shift their center to the center of the canvas.

Step 8 - Send a Message to Create the Bucket

To start the program in the bucket sprite, we should send a message from the “Empty1” sprite. This makes sure we create the bucket AFTER the physics world has been initialized.

In the “Empty1” sprite, broadcast the “add bucket” message:

Then in the “Bucket” sprite, receive the message:

Step 9 - Move the Bucket to the Center

In the Bucket sprite, let’s first reset the position and direction of this sprite:

Step 10 - Add a Physics Body to the Bucket

Next, we’ll tell the physics engine how we want the bucket to behave in the physics simulation. The bucket is a “concave” shape, since its top edge curves into itself. Therefore, we need to use a compound shape to represent it:

We will set the bucket to be a “movable” object. This is because we don’t need the bucket to fall due to gravity, so it does not need to be “dynamic”. But we do want to rotate it ourselves, so it should not be “fixed” either.

Since we set the “debug” parameter to “Yes”, we can see the actual shape created by the physics engine, which is composed of 3 red boxes. This is exactly what we need, so we can set the “debug” parameter back to “No” to hide these red boxes.

At this point, our bucket should be able to hold some “water” now:

Step 12 - Make the Bucket Rotate

Lastly, to make the animation more interesting, let’s make the bucket rotate around itself.

Here is the final result:

Creative Ideas

Please try to create some new animations based on what you learned in this tutorial. Here are some ideas for you to try out:

Colored Balls: You can add multiple ball costumes with different colors, and switch the ball’s costume before creating new clones.
Different Bucket Shapes: You can draw some different shapes for the bucket.
Make the Bucket Sway Left and Right: Currently the bucket only rotates one way. You can try to make it “sway” side to side, so it does not lose the water drops.
Update Physics Properties: Try to change the gravity in the physics world and the friction/restitution of the objects. Observe and explain the differences they make.