# Family gravity

## The contract in words, and how to choose

No hand-written part: how to choose among this family's laws, or to attach them together, is not written yet.

## Contract

What every member must satisfy, as the family's harness checks it on the member's own node.

- Sign rule: weight minus buoyancy lies along gravity, pointing with it when the body is heavier than the fluid it displaces
- Cases a member passes: `sign`, `frame`, `units`, `material reach`, `oracle`, `gradients`, `nodes`
- Settings that stand for a material's properties, each of which must reach the output: `mass`, `buoyant_volume`, `fluid_density`, `centre_of_buoyancy`
- Deliveries a two-way law makes equal and opposite: none
- Reference member a swap replaces: none
- Settings proportional to a material property: none

## Members

| Entry | Rung | Derivative |
|---|---|---|
| [`gravity`](../entries/interaction/gravity.md) 0.1.0 | none | `exact` |

## Comparison

This family has one member, so there is nothing to compare.