Workshop › Pool › Step 31 of 33

Where the hit ball goes

What we are doing

The hit ball leaves along the line from the ghost ball's centre through its own centre: that is the normal of the collision. We draw that line too, so you can aim a ball into a pocket.

```js
// The ball that is hit goes off along the line from the ghost ball through its centre.
const dx = g.ball.x - g.x
const dy = g.ball.y - g.y
ctx.beginPath()
ctx.moveTo(g.ball.x, g.ball.y)
ctx.lineTo(g.ball.x + dx * 3, g.ball.y + dy * 3)
ctx.stroke()
```

What it means

• (dx, dy) goes from the ghost ball's centre to the hit ball's centre: 2R long, pointing where it will go.
• The line starts at the hit ball and goes three times that far: 54 pixels.

Your turn

In the if (g) block, under the ghost ball's ctx.stroke(), write the comment and the six lines.

Skills: Velocity and gravity, Drawing on a canvas

← Draw the ghost ball Your best →

All steps

1. The room
2. The table
3. The cue ball
4. Draw the balls
5. The rack
6. Numbers on the balls
7. An aim line
8. Turn the aim
9. A shot is a velocity
10. Roll
11. Friction
12. Cushions
13. Aiming and rolling
14. Hide the aim while rolling
15. Count the shots
16. Balls that touch
17. Swap the push
18. Six pockets
19. In the pocket
20. Scratch
21. Balls left
22. Clear the table
23. Choose the power
24. A power bar
25. A slingshot
26. Press to aim
27. Pull back and let go
28. Smaller steps
29. Predict the hit
30. Draw the ghost ball
31. Where the hit ball goes
32. Your best
33. Build it yourself: two players