Rigid bodies
Bricks, crates, rocks and dice that tumble, stack and come to rest; bodies moved by hand; and joints for chains, doors, wheels and drawers.
A Rigid Body makes each particle of its event a solid body of the shape it shows: it falls, tumbles, hits the others, stacks and comes to rest.
Start from
The Rigid Bodies starter: a block born each frame for 60 frames, turned at random, falling into a pile.
The bodies' shapes
Each body is the shape its particle shows:
- With a Shapes collection (the Display panel's Shapes), its object's mesh, at the particle's Scale. A Shape operator picks among several; without one, all are the first.
- Born On Pieces, its own piece of the mesh: see Destruction.
- Otherwise, its sphere.
The Rigid Body's Shape says how closely the body follows that:
| Shape | Follows | For |
|---|---|---|
| Box | The box round it | Bricks, crates, books. Fastest |
| Ball | A ball round its longest side | Balls, marbles, anything that should roll |
| Hull | It, wrapped tight with no hollows | Rocks, debris, broken pieces |
| Concave | It, hollows and all | Cups, bowls, frames, a chair. Slowest |
Weight, grip and bounce
- Density in kg/m³: water 1,000, wood about 600, stone about 2,500, steel 7,850. A body weighs its volume at that density, so a big rock is heavy and a pebble light. It matters when bodies meet: a heavy one shoves light ones aside, and a liquid floats wood and sinks stone.
- Friction: low and they slide off each other; high and they stack.
- Bounce: near 0 for bricks and stone, more for rubber.
Stacks that stand
A wall or a tall stack needs its bodies to rest quietly on each other:
- Build it in place: Birth On Vertices of a mesh with a vertex where each brick goes, with a Shapes collection of one brick, or On Pieces.
- Rounds is how many times a step the contacts are worked out: raise it if a stack sags, jitters or creeps. Loose debris needs fewer.
- A little Friction helps a stack stand; bricks want 0.6 or so.
Moving bodies by hand: Kinematic
A Kinematic body doesn't fall or get pushed: it moves as the event's other operators say, through colliders, shoving the others aside. A wrecking ball, a paddle, a bulldozer's blade.
- Moved by an object: in the same event, a Pin with the object. The body goes and turns with it, so keyframe the object.
- Moved by a speed: a Speed, and a Spin for turning.
Held still until struck
A Struck Test on kinematic bodies sends those another body strikes to an event whose Rigid Body isn't kinematic: they're freed, and take the blow. A wall stands until a ball hits it.
Joints
A Bind between rigid bodies is a joint. Bind them Nearest, with a Distance a little over the distance between their centres, and pick the Joint:
| Joint | Bodies | For |
|---|---|---|
| Ball | Turn freely about the point between them | Chains, a crate on a rope, dangling things |
| Hinge | Turn only round Axis | Doors, wheels, seesaws, flaps, a drawbridge |
| Slider | Slide along Axis, without turning | Drawers, pistons, lifts |
| Fixed | Hold together as they are | Welds |
| Glue | Are one body, until struck or loaded too hard | Things to smash: see Destruction |
- A chain: links in a row, Bind Nearest with Max 2, so each holds the one before and after, and Joint Ball.
- Fixed to the world: bind the end to a particle held still by a Pin in another event. That event needs a Bind too, so the two find each other.
- A door: the door body, bound to a particle held by a Pin at its hinge edge, its Bind's Joint Hinge, Axis the door's own up axis. Limits keep it between Lower and Upper.
- A wheel or a motor: a Hinge with Motor Speed (radians a second) and Motor Strength (newton metres) drives it round.
- Breaking: Break Load in newtons (holding 1 kg takes 9.81) and Break Torque in newton metres. Load Over averages the load, so a sudden knock counts for less than a steady pull.
Bodies meet everything
- Bodies hit the rigid bodies of every event, and their own event's Collide objects and ground.
- Grains with Particle Physics push bodies and are pushed by them.
- A liquid floats them by their density, and carries them off.
- Cloth drapes over them.
What goes wrong
- Bodies pass through each other. They're too small or fast for the substeps: raise the bake's Substeps.
- They're all the same shape. Add a Shape operator to pick among the Shapes collection.
- Hull or Concave is an error. They need a Shapes collection, or pieces, to take the shape from.
- Bodies get the wrong shapes. The Shapes collection's names sort two ways: name them alike, Rock.001, Rock.002, as the editor asks.
- Can't share an event with Pin. Only Kinematic bodies can be pinned. Free bodies hang from a pin through a joint.
Related
Cloth, ropes and soft bodies
Flags, sheets, curtains, ropes and wobbly things: particles born on a mesh, bound along its edges, hung, torn and draped, played back as your own mesh.
Destruction
Cut a mesh into pieces with the Fracture tool, glue them into one body, and smash it: on the ground, with a ball, or one piece at a time.