Workshop › Asteroids › Step 10 of 41

A speed limit

What we are doing

The speed is the length of the velocity vector, Math.hypot(vx, vy). Over the limit, both parts are scaled down by the same factor: the direction stays and only the speed shrinks.

```js
const MAX_SPEED = 6

  const speed = Math.hypot(ship.vx, ship.vy)
  if (speed > MAX_SPEED) {
    ship.vx *= MAX_SPEED / speed
    ship.vy *= MAX_SPEED / speed
  }
```

What it means

• Math.hypot(vx, vy) is the length of the velocity, Pythagoras.
• MAX_SPEED / speed is below 1 when too fast; multiplying both parts by it makes the length exactly MAX_SPEED.

Your turn

1. Under FRICTION write MAX_SPEED.
2. In update, above ship.x += ship.vx, write the speed limit.

Skills: Velocity and gravity

← A little friction Around the screen →

All steps

1. Black space
2. Where the ship is
3. Which way it points
4. A triangle ship
5. Which keys are down?
6. Turning
7. The game loop
8. Thrust and drift
9. A little friction
10. A speed limit
11. Around the screen
12. Making a bullet
13. Fire!
14. Bullets fly
15. A lifetime for bullets
16. Making a rock
17. Four big rocks
18. Drawing the rocks
19. Jagged rocks
20. Drifting rocks
21. When do two circles touch?
22. Breaking a rock
23. Bullets hit rocks
24. Points
25. Show the score
26. Losing a life
27. A rock hits the ship
28. Show the lives
29. What time is it?
30. A moment of safety
31. Blink while safe
32. Waves of rocks
33. The next wave
34. Faster every wave
35. One function to start
36. Playing or over
37. Game over
38. Best score
39. GAME OVER
40. Play again
41. Build it yourself: hyperspace