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roblox-physics

Skill25 starsChanged 56 days agoMITView on GitHub

Build or debug Roblox vehicles, mechanisms, and moving objects. Use for assemblies, mechanical or mover constraints, network ownership, collision filtering, and simulation performance.

4 other projects use a version of this.

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We split this skill into the rules it gives an agent

5 rules. 0 are common, 3 are distinctive, and 2 are very specific to this project.

  • 0

    Common

    Most of the 4 edited versions we read keep these.

  • 3

    Distinctive

    This author's own choices. Worth a look.

  • 2

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Use mechanical constraints to connect parts and mover constraints to apply forces or velocity targets.

    line 20
  2. Very specific:

    Use collision groups (Workspace:RegisterCollisionGroup (inherited from WorldRoot), BasePart.CollisionGroup) or NoCollisionConstraint for …

    line 44
  3. Very specific:

    All mechanical constraints connect one or two Attachments (or Bones), except WeldConstraint and NoCollisionConstraint which use Part0/Part1.

    line 48
  4. Distinctive:

    If a slow mechanism falls asleep, increase motion or use actuated joints (motor/servo constraints) which get stricter sleep thresholds.

    line 99
  5. Distinctive:

    Use the MicroProfiler to check island distribution.

    line 108

“Common” is measured: kept by at least 6 in 10 of the 4 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

roblox-physics

Official sources:

Use mechanical constraints to connect parts and mover constraints to apply forces or velocity targets. For older code, use the BodyMover migration table below.

Core concepts

Assemblies

An assembly is one or more parts connected by rigid welds or movable joints, simulated as a single rigid body.

Key BasePart properties (same for any part in the assembly):

  • AssemblyLinearVelocity / AssemblyAngularVelocity: prefer constraints or ApplyImpulse over direct assignment for realistic motion.
  • AssemblyCenterOfMass: force here produces pure linear acceleration.
  • AssemblyMass: sum of all part masses; infinite if any part is anchored.
  • AssemblyRootPart: automatically chosen root for replication and network ownership.

Root-part priority: anchored > non-massless > higher RootPriority > size/name heuristics.

Anchoring

  • Anchoring one part in an assembly makes it the root; the rest are implicitly anchored.
  • Anchoring multiple parts in the same assembly splits it.
  • To anchor an entire assembly, only anchor the root part.

Collision filtering

Use collision groups (Workspace:RegisterCollisionGroup (inherited from WorldRoot), BasePart.CollisionGroup) or NoCollisionConstraint for part-to-part filtering. CanCollide, CanTouch, and CanQuery control different behaviors; see references/collisions-and-filtering.md.

Mechanical constraints

All mechanical constraints connect one or two Attachments (or Bones), except WeldConstraint and NoCollisionConstraint which use Part0/Part1.

ConstraintUse case
WeldConstraintRigidly lock two parts together, same relative transform
RigidConstraintSame as WeldConstraint but attachment-based, supports bones
HingeConstraintDoors, levers, rotating parts; motor/servo optional
PrismaticConstraintSliding doors, elevators, pistons
CylindricalConstraintSlides + rotates, like hydraulic rams or landing gear
SpringConstraintSprings, shocks, suspension
TorsionSpringConstraintRotational springs
BallSocketConstraintShoulders, ball joints
UniversalConstraintDrive shafts, constant-velocity joints
RopeConstraintCables, winches, maximum length
RodConstraintFixed separation distance
PlaneConstraintConstrain motion to a plane
NoCollisionConstraintDisable collisions between two specific parts

See references/mechanical-constraints.md for creation, orientation, motor/servo tuning, and limits.

Mover constraints

Modern replacements for deprecated BodyMovers:

LegacyModernUse case
BodyPositionAlignPositionMove attachment to a position or another attachment
BodyGyroAlignOrientationAlign orientation; LookAtPosition for tracking
BodyVelocityLinearVelocityMaintain constant velocity along vector/line/plane
BodyAngularVelocityAngularVelocityMaintain constant angular velocity
BodyForce/BodyThrustVectorForceApply constant force
RocketPropulsionLineForce + AlignOrientationFollow + face target
-TorqueApply constant torque
-LineForceForce along line between two attachments
-AnimationConstraintConstraint driven by animation/transform

See references/mover-constraints.md for force modes (Magnitude vs PerAxis), relativity frames, rigidity, and reaction forces.

Network ownership

  • The server owns anchored parts.
  • Unanchored parts near a player character are automatically owned by that client.
  • Set ownership explicitly with BasePart:SetNetworkOwner(player) (server only). Reset with SetNetworkOwnershipAuto().
  • Assign vehicle/driver ownership carefully: the first seated player may own the whole assembly otherwise.

Security: clients can exploit owned parts (teleport, fake collisions). Validate gameplay-critical events server-side. See references/network-ownership.md.

Sleep system

Assemblies stop simulating when still to save performance. They wake on collisions, property changes, impulses, or gravity/wind changes.

  • If a slow mechanism falls asleep, increase motion or use actuated joints (motor/servo constraints) which get stricter sleep thresholds.
  • Visualize sleep states with Awake parts in Studio's Visualization Options.

Physics stepping method

Workspace.PhysicsSteppingMethod:

  • Fixed (default, 240 Hz): best general choice; use for racing, destruction, tanks, or when most parts already solve at 240 Hz.
  • Adaptive: assigns assemblies to 60/120/240 Hz islands for up to ~2.5× performance in suitable experiences.

Use the MicroProfiler to check island distribution.

Units quick reference

RobloxMetric
1 stud28 cm
1 RMU21.952 kg
196.2 studs/s²default "Classic" gravity
35 studs/s²"Realistic" gravity (≈ 9.8 m/s²)

See references/units-and-physical-properties.md.

Common mistakes

  • Using deprecated BodyMovers instead of modern constraints.
  • Setting CFrame every frame instead of using forces/constraints.
  • Anchoring every part in an assembly (splits it and hurts performance).
  • Ignoring network ownership on vehicles and then wondering why input lags.
  • Trusting Touched events from client-owned parts for damage/authority.
  • Letting actuated joints fall asleep prematurely.

Scripts

  • scripts/VehicleController.lua: chassis setup with HingeConstraint steering and motor drive, with client ownership and server validation.
  • scripts/DoorHinge.lua: motorized/servo door with limits and state machine.
  • scripts/PlatformMover.lua: AlignPosition + AlignOrientation platform with configurable waypoints.
  • scripts/Suspension.lua: spring-damper suspension using SpringConstraint.

How to proceed

  1. Decide if the object is a rigid mechanism (mechanical constraint) or force-driven (mover constraint).
  2. Plan assemblies, root parts, and anchoring strategy.
  3. Set network ownership for multiplayer responsiveness.
  4. Tune forces/velocities using units and physical properties.
  5. Profile with MicroProfiler and watch sleep/ownership visualizations.
<!-- catalog:references:start -->

Reference index

  • collisions-and-filtering.md: Configure collision groups, touch events, or spatial query filters.
  • mechanical-constraints.md: Connect parts with hinges, springs, ropes, or other mechanical constraints.
  • mover-constraints.md: Apply forces or velocity targets, or migrate BodyMover code.
  • network-ownership.md: Assign physics simulation to the server or a player and debug ownership.
  • units-and-physical-properties.md: Tune mass, density, friction, force, or simulation stepping.
<!-- catalog:references:end -->

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