Scratch Project · Grade 5 · Chapter 7

Scratch Mini-Game Part 1: Player & Obstacles

Designing the engine for "Space Dodger" — responsive player movement and falling asteroid physics!

Learning Goals

What We'll Build Today

GOAL 1

Game Engine

Set up coordinate bounds, player movement, and smooth keyboard responsiveness.

GOAL 2

Random Spawning

Use pick random to drop falling obstacles from random top positions.

GOAL 3

Obstacle Resetting

Build infinite falling loops by resetting Y coordinates when reaching bottom.

Game Architecture

Deconstructing Arcade Games

Think of games like Space Invaders or Subway Surfers. What are the 3 main visual elements running constantly on screen?

Discussion: Player character, obstacles/enemies, and background backdrop!

Project Brief

"Space Dodger" Mini-Game Concept

Our hero, Astro-Cat, is navigating through an asteroid belt in space! Space rocks drop continuously from above. Astro-Cat must dodge left and right to survive!

Today in Part 1, we write the code for Astro-Cat's thrusters and the falling asteroid system!

Scratch Math

Understanding Stage Coordinates

The Scratch stage is a grid: X moves horizontally (-240 left to +240 right), Y moves vertically (-180 bottom to +180 top).

⬅️ Horizontal X-Axis

Center = 0 | Far Left = -240 | Far Right = 240

⬆️ Vertical Y-Axis

Center = 0 | Top Edge = 180 | Bottom Edge = -180

Setup Flowchart

Initializing the Player Sprite

1

Start Position

Set X: 0, Y: -130 (Bottom center of stage).

2

Set Size

Set size to 60% so dodging feels crisp.

3

Control Loop

Start forever loop reading left/right keys.

Code Implementation

Astro-Cat Movement Script

Smooth left/right movement with screen boundary checks:

      when green flag clicked

      go to x: (0) y: (-130)

      forever {

        if <key [right arrow v] pressed?> then { change x by (8) }

        if <key [left arrow v] pressed?> then { change x by (-8) }

      }

Game Physics

Preventing Off-Screen Escape

If player holds right arrow, Astro-Cat might disappear off screen! We add boundary caps:

      if <x position > 210> then { set x to (210) }

      if <x position < -210> then { set x to (-210) }

Obstacle Mechanics

How Falling Objects Work

To make an asteroid fall down continuously, we decrease its Y coordinate inside a forever loop!

1

Spawn Top

Set Y to 180 (top edge) & X to random (-200 to 200).

2

Fall Down

change y by (-6) repeatedly.

3

Reset Loop

If Y < -170, send back to top!

Code Implementation

Asteroid Falling & Respawning Script

      when green flag clicked

      forever {

        set y to (180)

        set x to (pick random (-200) to (200))

        repeat until <y position < -170> {

          change y by (-7)

          turn right (3) degrees // Spinning effect

        }

      }

Scratch Operators

The Magic of `pick random`

Without randomness, games become predictable and boring. pick random (-200) to (200) generates a new surprise X position every single drop!

Fun Fact: Game developers call this Procedural Generation!

Side-by-Side Comparison

Predictable Drop vs. Dynamic Surprise Drop

❌ PREDICTABLE DROP

Asteroid drops down middle (X=0) every time.

Player can just stand on the side and win effortlessly!

✅ DYNAMIC SURPRISE DROP

Asteroid picks new random X coordinate for every spawn.

Keeps player actively dodging and engaged!

Debunking Myths

Common Mini-Game Glitches

Glitch: "Stuck at Bottom"

If the check is y position = -180, fast falling objects might jump from -176 to -183 and miss the exact match! Always use < (less than)!

Glitch: "Sluggish Dodge"

Using move 2 steps feels super slow. Increase change step to 8 or 10 for snappy arcade controls!

Smartboard Voting

Predict the Speed!

Which script drops the asteroid faster?

Script A: change y by (-4)

Script B: change y by (-12)

Vote: Thumbs UP for A, Thumbs DOWN for B!

Spot the Bug

Why Is the Asteroid Teleporting Mid-Air?

forever [ set y to (180), change y by (-5) ]

Bug Analysis: Every cycle of the loop resets Y back to 180! Why is it missing the repeat until inner loop?

Think-Pair-Share

Designing Obstacle Variations

Imagine adding a second obstacle: a fast-falling Lightning Bolt!

Partner Task: How would the Lightning Bolt script differ from the Asteroid? (Speed, size, sound effects?)

Guided Practice

Assembling Player & Obstacle Live

Let's connect our scripts together on the Scratch stage!

Step 1: Choose Space backdrop & Astro-Cat sprite.
Step 2: Add smooth arrow controls & boundary limits.
Step 3: Create Rock sprite and attach falling random loop.

Hands-on Challenge

The Double Obstacle Rush!

Mission: Duplicate the Asteroid sprite. Give Asteroid 2 a smaller size (40%) and faster fall speed (Y change by -10)!

Scratch Goal: Test dodging TWO falling objects at different speeds simultaneously!

Quick Check · Question 1

Which block combination spawns an obstacle at the top edge at a random horizontal position?

Aset x to (180), set y to (0)

Bset y to (180), set x to (pick random (-200) to (200))

Cmove (10) steps, turn (90) degrees

Dgo to [mouse pointer v]

Click to reveal answer

Quick Check · Question 2

Why do we use `change y by (-8)` instead of `change y by (8)` to make an object fall?

APositive numbers make things fall down

BNegative Y changes move down toward the bottom of the screen

CNegative numbers make sprites bigger

DScratch doesn't allow positive Y numbers