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The science
Same model as the 3D page. Drawing the pouch back by d stores Hooke-law energy E = (1/2) k d^2 with draw strength k in N/m. On release it converts to kinetic energy, so the launch speed is v0 = d sqrt(k / m) along the line from pouch to anchor.
In flight the projectile feels gravity and a small multiplicative air damping, v ← v * 0.999^(60 dt), as in the 3D code. Without damping the range on level ground is R = v0^2 sin 2 theta / g, maximal at 45 degrees; here the launch point is 1.2 m above the floor, which moves the best angle a little below 45.
Floor contact uses restitution 0.4 and horizontal friction factor 0.7 per bounce. Simplifications: the crates in this 2D page are removed when hit (the 3D page simulates the full stack); the band is massless and perfectly elastic; no band recoil energy loss.
What to try
- Drag the pouch back and down-left; the preview shows where it will land.
- Double the mass at the same strength: v0 falls by 1/sqrt(2), energy stays the same.
- Lower gravity (Moon is 1.6) and watch the range explode.
- Fire at 45 degrees with max pull and look at the energy graph exchange.