---
name: ferret-physics-friction-impact
description: Use to dial friction and collisions per scope in Ferret: ice and sticky zones, grip per body, rolling and pivot resistance, anisotropic skates and skis; restitution scale, rest threshold, bounce falloff, sticking bodies and the physics:impact event.
---

# Friction and impact

## When to use
Slippery floors, rails, skis; bouncy or dead bodies; clay and magnets; sound or damage from hits.

## The model
The kernel already applies Coulomb friction, rolling resistance and restitution from the materials. `friction` and `impact` add what a game dials **per scope**, so a patch of ice can be a zone as well as a material. The value is read at the **dynamic body's** entity and the contact point. `impact` has a restitution `scale`, a rest `threshold`, loss of bounce at high speed, `stick`, and emits `physics:impact` with its energy.

## Steps
1. Enable the capability; set values at zone or entity scope.
2. Make things settle: raise `threshold`, or `scale` below 1.
3. Sound and sparks: listen to `physics:impact` (respect `minEventSpeed`).
4. Damage from hits belongs to `strength` (`ferret-physics-strength`), driven by the same contacts.
5. Rolling and pivot friction need the material's `rollingFriction`.

## Check afterwards
A block on a slope creeps or holds as `μ` against `tan(angle)` predicts; a dropped ball's bounce height follows `e² h` for spin-free spheres.

## What usually goes wrong
- Nothing slides on ice: capability off, or the scope key is not the dynamic body's.
- A ball never stops rolling: no `rollingFriction`, and `rollingScale` multiplies zero.
- Anisotropy, pivot friction and `stick` do nothing on Havok.
- `scale` above 1 gives restitution above 1: energy appears.

## Files

- `reference/friction.md` — Friction by scope: slippery zones, per-body grip, rolling and pivot resistance, and anisotropic friction for skates, rails and skis.
- `reference/impact.md` — Collisions as a dial: restitution scale, rest threshold, bounce falloff at speed, sticking bodies, and the impact event with its energy.
- `reference/api-physics-friction.md` — every export of `@ferret/physics-friction` with signature and doc comment
- `reference/api-physics-impact.md` — every export of `@ferret/physics-impact` with signature and doc comment