Workshop › Asteroids › Step 8 of 41

Thrust and drift

What we are doing

Up is thrust: it does not set the speed, it adds a little velocity in the direction the ship faces. Then the ship moves by its velocity every frame. It keeps drifting the way it was going, even after you turn: that is inertia.

```js
const THRUST = 0.12

  if (keys.ArrowUp) {
    ship.vx += Math.cos(ship.angle) * THRUST
    ship.vy += Math.sin(ship.angle) * THRUST
  }
  ship.x += ship.vx
  ship.y += ship.vy
```

What it means

• While up is held, the direction (cos, sin) times THRUST is added to the velocity.
• Every frame the position changes by the velocity, whether or not a key is held.

Your turn

1. Under TURN write THRUST.
2. In update, under the two turning lines, write the thrust block and the two moving lines.

Skills: Velocity and gravity, Keyboard and mouse input

← The game loop A little friction →

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