What it shows
The ball bounces off the walls and the paddles by changing its direction. It teaches how direction angles work and how two players share one program.
The science
In Scratch a sprite’s direction is an angle measured clockwise from straight up. Moving steps in that direction gives the path. When a ball hits a vertical surface like a paddle, its sideways motion reverses while the up and down motion stays, which means the new direction is 0 minus the old direction. This matches the law of reflection used in light.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A computer, laptop or phone with a browser (free)
- Scratch 3 in the browser at scratch.mit.edu (free, no sign-in needed to code)
- Three sprites: a Ball and two paddles drawn as thin rectangles
- A notebook and a pencil
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Draw two paddles with the Rectangle tool, tall and thin, and place one at x: -220 and one at x: 220 using go to x: y: blocks.
Paddle 1: when w key pressed change y by 15, and when s key pressed change y by -15.
Paddle 2: when up arrow key pressed change y by 15, and when down arrow key pressed change y by -15.
Make variables p1 and p2 for the scores.
On the Ball: when green flag clicked, set p1 to 0, set p2 to 0, go to x: 0 y: 0, point in direction 75.
Add forever with move 6 steps and if on edge, bounce.
Inside the forever add if touching Paddle1? or touching Paddle2? then point in direction (0 - (direction)) and move 10 steps.
Add if x position < -230 then change p2 by 1 and go to x: 0 y: 0; add the mirror rule for x position > 230 changing p1. Play with a friend.
What you should see
The ball bounces between the top and bottom edges and off both paddles, and the score of the player who gets the ball past the other paddle goes up.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which angle does direction 0 mean?
Straight up.
2Why is the new direction 0 minus the old one?
A vertical bounce reverses the sideways motion, which mirrors the angle around the up direction.
3Why move 10 steps after the bounce?
To get out of the paddle so the touch is not detected again immediately.
4Where else do we see reflection like this?
Light bouncing off a mirror obeys the same equal-angle rule.
Take it further
Increase the ball speed by 0.5 after each paddle hit using a speed variable instead of the fixed move 6.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Computer & office
Computer Fundamentals Study Book
The complete study notes, practice questions and glossary for Computer Fundamentals