Workshop › Pool › Step 12 of 33

Cushions

What we are doing

A cushion reflects the ball: past the left edge, put it back on the edge and flip vx. Real cushions absorb some energy, so the bounce keeps only 80% of the speed.

```js
const BOUNCE = 0.8 // speed kept when hitting a cushion

    // Cushions: reflect the velocity and lose a little speed.
    if (b.x < LEFT + R) [b.x, b.vx] = [LEFT + R, -b.vx * BOUNCE]
    if (b.x > RIGHT - R) [b.x, b.vx] = [RIGHT - R, -b.vx * BOUNCE]
    if (b.y < TOP + R) [b.y, b.vy] = [TOP + R, -b.vy * BOUNCE]
    if (b.y > BOTTOM - R) [b.y, b.vy] = [BOTTOM - R, -b.vy * BOUNCE]
```

What it means

The ball touches the left cushion when its centre is closer than R to LEFT.
[b.x, b.vx] = [LEFT + R, -b.vx * BOUNCE] sets two values in one line: the position back on the edge, and the speed reversed and reduced.

Your turn

Under FRICTION write BOUNCE.
In step, inside the loop, under b.y += b.vy, write the comment and the four cushion lines.

Skills: Velocity and gravity, Collision detection

← Friction Aiming and rolling →

All steps