Physics — Moving Parts and Payload

With physics off, parts are moved by the conveyors alone: they follow the belt, they queue up, and at the end of a line they simply stop. With physics on, a part that leaves a conveyor becomes a real body — it falls, slides down a chute, tips over, and stacks in a bin.

Two things follow from that, and they are what this page is about: machine parts that move can now push those bodies around, and what they carry comes back as a load on the drive.

Moving parts push, lift and carry

Any part the twin moves itself — a part on a drive axis, or a link of a mechanism — takes part in the physics as soon as physics is on. A pusher pushes a box off the line. A lift table lifts what stands on it. A gripper pad carries a part from one station to the next, and lets go when it opens.

There is one condition, and it is worth authoring deliberately:

A moving part needs a collider. Author a Box Collider or a Mesh Collider on it in Unity. Unlike fixed scenery, a moving part gets no automatic fallback to its bounding box — a guessed box on a gripper finger would be silently wrong in exactly the place where it matters most, so a part without a collider stays passive instead.

What the two collider types become:

Authored	What the physics sees
Box Collider	The box, with its offset, rotation and scale as authored.
Mesh Collider	The convex hull of the shape — a shrink-wrap around it. Pockets, slots and forks are filled in.

A hull is used even when the mesh is authored as concave; the message log says so. If a fork or a pocket has to grip something, model that region as its own box collider rather than relying on the mesh.

Colliders marked disabled or is trigger in Unity stay out of the physics, exactly as elsewhere.

The payload shows up in the drive forces

When a mechanism link carries something, the weight is measured where it touches and fed into the mechanism’s force analysis. Open Quick Edit → Mechanism → Forces while the machine is running and the drive figures are those of the loaded machine, not the empty one — the peak and RMS values a motor gets sized on include the part.

An extra column, Ext. load, appears in the same table as soon as any link carries something. It shows the force in newtons pressing on that link right now, and it disappears again when nothing does — a column of dashes would say nothing worth the width.

Part weight. A part weighs 1 kg by default. Set a different weight per zone, or per part through the part’s PhysicsMassKg field. This matters more than it sounds: without a weight the physics engine would size a 200 mm cube at about 8 grams, and every payload figure would be noise.

What this does not do yet

The figures are honest to a point, and the point is worth knowing before a drive is ordered against them:

Friction from stacked parts is not passed down. If a part rests on another part, its weight reaches the link correctly. The friction it exerts on the part beneath it does not travel further down. A sliding stack therefore under-reports the sideways load.
A tipping part’s spin is ignored. Its contact forces and their lever arms are counted; the rotational inertia of the tipping itself is not.
The load is one simulation step old. The physics runs after the mechanism each step, so a load measured now reaches the force analysis on the next one — invisible at 60 steps per second, but it is why a hard impact shows up one step late.
Parts that drive into a zone later. If the model authors explicit physics zones instead of using the whole scene, a machine part is assigned to a zone when the model loads. A part that only later travels into another zone will not collide there. With the default whole-scene zone this never applies.
Physics never moves the mechanism. A mechanism follows its drives and its joints — always. A load can raise the force a drive has to produce; it can never bend, stall or drag the machine. The same motion without any parts produces exactly the same poses.

Checking that it is working

The Simulation settings tab shows a small diagnostics line while physics is running, for example:

2 zones / 17 bodies (4 kinematic) / 0.4 ms step

Bodies are the free parts; kinematic are the moving machine parts. If a gripper visibly travels through a part, that number is the first thing to read — no kinematic bodies means no colliders were authored on the moving parts, and the parenthesis is simply absent.