# Family hydrodynamic

## 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: the resistance dissipates: V^T R V > 0 for every nonzero twist, R symmetric positive definite
- Cases a member passes: `sign`, `frame`, `units`, `material reach`, `oracle`, `gradients`, `swap`, `nodes`
- Settings that stand for a material's properties, each of which must reach the output: `viscosity`
- Deliveries a two-way law makes equal and opposite: none
- Reference member a swap replaces: none
- Settings proportional to a material property: `viscosity`

## Members

| Entry | Rung | Derivative |
|---|---|---|
| [`drag`](../entries/interaction/drag.md) 0.4.0 | [`surrogate`](../entries/interaction/drag.md#rung-surrogate) | `piecewise_jump` |
| [`drag`](../entries/interaction/drag.md) 0.4.0 | [`mrs`](../entries/interaction/drag.md#rung-mrs) | `exact` |

## Comparison

| Member | Validity | Checked against | Cost | Derivative | Needs a GPU |
|---|---|---|---|---|---|
| [`drag surrogate`](../entries/interaction/drag.md) | 9 declared lines, free text, not parsed | not measured | not measured | `piecewise_jump` | not measured |
| [`drag mrs`](../entries/interaction/drag.md) | 9 declared lines, free text, not parsed | not measured | not measured | `exact` | not measured |

A cell reads `not measured` until a measurement record or a claim holds it, and each is counted.