--- title: "Projectiles" subtitle: "Arcade Foundations · Fire · Lesson 9" --- # Projectiles **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Space Invaders, Galaga and every 'shoot 'em up' since: you fire, and the game has to notice when a shot hits. :::teacher-note **In one sentence:** Students fire projectiles with A and find that a projectile is its own kind, so a hit needs a Projectile–Enemy overlap. **Students should be able to say:** I can fire a projectile from a sprite with the A button. I can explain why a hit needs a Projectile–Enemy overlap, not Player–Enemy. I can make a crash and a hit do different things. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict The overlap event names two kinds: **Projectile** and **Enemy**. ![The program as blocks](https://studio.learningwaypoints.com/tutorials/_pictures/makecode/code/01f0f80665ac.png?v=ecb699160bda9997) :::keep-together **You press A (space on the keyboard) once, with the ship right under the alien. What happens?** ☐ Only the alien disappears ☐ The shot goes through the alien ☐ The shot and the alien both disappear, and the score goes up by 1 ☐ The ship disappears My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** The shot and the alien both disappear, and the score goes up by 1. The shot is a Projectile, the alien an Enemy, so the Projectile–Enemy overlap runs: sprite is the shot, otherSprite the alien. ::: ## Run Run the program in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Investigate ![The program as blocks](https://studio.learningwaypoints.com/tutorials/_pictures/makecode/code/ec0b8ec8c734.png?v=4c1018ab2e9791d0) :::keep-together ### What does each part do? **Match each part of this game with what it does.** A. runs when any shot touches any alien B. makes a new alien each second C. the alien that was hit D. the shot that hit E. fires a shot straight up from the ship {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `projectile from ship with vx 0, vy -100` | | | `on game update every 1000 ms` | | | `on Projectile overlaps Enemy` | | | `sprite (in the overlap)` | | | `otherSprite (in the overlap)` | | ::: :::teacher-note **Answer:** projectile from ship with vx 0, vy -100 → E. fires a shot straight up from the ship; on game update every 1000 ms → B. makes a new alien each second; on Projectile overlaps Enemy → A. runs when any shot touches any alien; sprite (in the overlap) → D. the shot that hit; otherSprite (in the overlap) → C. the alien that was hit ::: :::keep-together ### A missed shot **A shot misses every alien and flies off the top of the screen. What happens to it?** ☐ It keeps flying for ever, off the screen ☐ The engine removes it ☐ It comes back at the bottom ☐ It turns into an alien ::: :::teacher-note **Answer:** The engine removes it A projectile is removed by the engine as soon as it leaves the screen, so missed shots do not pile up. ::: :::keep-together ### Ben's shots Ben's hit event: ![The program as blocks](https://studio.learningwaypoints.com/tutorials/_pictures/makecode/code/5133dae914b2.png?v=405c63bd4ba12d99) **Ben wrote his own hit event (below). He fires at the aliens, but every shot passes straight through. Why?** ☐ The overlap needs to be inside on A pressed ☐ A projectile is its own kind, Projectile, so a Player–Enemy overlap never notices a shot ☐ destroy only works on the player ☐ The shot is too fast to hit anything ::: :::teacher-note **Answer:** A projectile is its own kind, Projectile, so a Player–Enemy overlap never notices a shot The shot is made by the ship, but it is not the ship: it is a sprite of kind Projectile. Ben's event waits for a Player to touch an Enemy, which only happens if the ship itself crashes into an alien. The first kind should be Projectile. ::: :::typical ## Modify **Make the shots work.** The shots in this game pass through everything. Add the overlap that turns a shot into a hit. **Crash!.** Now the aliens can reach the ship without anything happening. Make a crash cost a life. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. **Make the shots work:** ![The program as blocks](https://studio.learningwaypoints.com/tutorials/_pictures/makecode/code/782d85a8cd0d.png?v=b06f7f0630ace2b9) **Crash!:** ![The program as blocks](https://studio.learningwaypoints.com/tutorials/_pictures/makecode/code/58419b4bafb4.png?v=7e1a631a695ed9b7) ::: ::: ## Mix it up ![The program as blocks](https://studio.learningwaypoints.com/tutorials/_pictures/makecode/code/6132064a0c4f.png?v=03c6438ef2599423) :::keep-together **The cat is at x 100 and the mouse at x 40. Which way does the cat go?** ☐ Both ways at once ☐ It stops ☐ Left (vx is set to -40) ☐ Right (vx is set to 40) ::: :::teacher-note **Answer:** Left (vx is set to -40) Is cat x (100) less than mouse x (40)? No, so the else part runs: vx -40, to the left, towards the mouse. ::: ## Make The aliens fall already. Make it a **shooter**: - **A** fires a shot straight **up** from the ship - a shot that hits an alien destroys **both** and scores **1** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. ![The program as blocks](https://studio.learningwaypoints.com/tutorials/_pictures/makecode/code/fbcf53e486c8.png?v=a5140b1ea1a6e561) ::: :::wider ## Change a firework show **A** launches a rocket into the night sky. Make the rockets launch faster, then make **B** launch a rocket that goes up at a slant. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Exit ticket :::keep-together **A shot hits an alien, but the game has only a Player–Enemy overlap event. What happens, and why?** {{lines-3}} ::: :::teacher-note **Answer:** Nothing. The shot is of kind Projectile, so a Player–Enemy event never runs for it. :::