Workshop › Angry Birds-style Slingshot › Step 12 of 47

Gravity

What we are doing

Gravity pulls every body down: each frame it adds a little to vy. The sideways speed never changes, the fall keeps speeding up, and together they draw a curve: a parabola.

```js
const GRAVITY = 0.25

  for (const b of bodies) {
    b.vy += GRAVITY
    b.x += b.vx
```

What it means

• GRAVITY is added to vy every frame, before the body moves.
• Going up, vy is negative and shrinks towards 0; then it grows and the body falls faster and faster.

Your turn

1. Under const GROUND = 290 write const GRAVITY = 0.25.
2. In step, write b.vy += GRAVITY as the first line inside the for loop.

Skills: Velocity and gravity

← Move by the velocity Land on the ground →

All steps

1. The sling
2. Sky and ground
3. Draw it again and again
4. The aim: an angle and a pull
5. A bird in the sling
6. Pull the bird back
7. The rubber band
8. Aim with the arrow keys
9. Bodies
10. Launch!
11. Move by the velocity
12. Gravity
13. Land on the ground
14. Aiming or flying
15. An empty sling in flight
16. Gone off the screen
17. Wait for calm
18. The next bird
19. See the path before you shoot
20. From a point to an aim
21. Grab the sling
22. Drag and let go
23. Wood and stone
24. A level as data
25. Push overlapping boxes apart
26. Bounce by mass
27. Every pair meets
28. Friction
29. Smaller steps
30. Hit points
31. Hard hits hurt
32. Things break
33. Pigs
34. Score
35. Show the score
36. Cracks
37. Three birds
38. Count the birds
39. Won or lost
40. The end panel
41. Play again
42. Three levels of data
43. Start any level
44. On to the next level
45. The best score
46. The right words
47. Build it yourself: the yellow bird