---
name: ferret-physics-stretch
description: Use for things that give way in Ferret: springs, ropes that sag and snap, bungees with yield and break strain, cloth and flags by position-based dynamics with colliders and wind, and soft jelly bodies. Includes budgets and what to do when cloth does not move.
---

# Springs, ropes, cloth, soft bodies

## When to use
A rope bridge, a bungee, a curtain, a flag, a jelly.

## The model
Springs and ropes are links with `E`, area, damping, rest length, `yieldStrain` (permanent set) and `breakStrain` (snap); strength held still is `breakStrain × E × area`. Cloth and soft bodies are **not** kernel bodies: particles advanced by their own solver with a fixed step and a fixed number of passes, so replays are identical. They meet the world through colliders you give them and read gravity and wind from the host.

## Steps
1. Enable `stretch`; build with `cap.rope({...})`, `Cloth`, `SoftBody`.
2. Pin the top row for a curtain (`pinned: 'top'`); add colliders (`sphere`, `plane`, `box`, rigid bodies) to `cloth.colliders`.
3. Name links you care about; events `stretch:yield`, `stretch:break`.
4. Keep cloth to a handful on screen; check `cap.particles()` against the budget.

## Check afterwards
A rope hangs `F L / E A` longer than rest; a curtain hangs about its length with the strain limit held; a load over the rating parts the rope.

## What usually goes wrong
- Cloth does not move: over the particle budget, capability off, or hand-stepped without calling `step`.
- Rubber cloth: raise `stiffness`, lower `strainLimit` (1.05).
- Passes through a body: not in `cloth.colliders`, or `thickness` smaller than a step's travel: raise `substeps`.
- A rope does not sag: `rope: false`, or too stiff for the load.
- Results differ between runs: variable `dt`.

## Files

- `reference/stretch.md` — Springs, ropes that sag and snap, cloth and flags, soft jelly bodies: everything that gives way instead of staying rigid.
- `reference/api-physics-stretch.md` — every export of `@ferret/physics-stretch` with signature and doc comment
- `reference/commands.md` — 2 commands of these packages with their arguments
- `recipes/structures-and-ropes.md` — Six recipes: a rope that holds a load or snaps, a shear pin that protects the rest, a crate that breaks only at speed, a bungee that springs back, a wall that breaks under a blast, a curtain that hangs.