roblox-physics

Official sources (always check these for the latest):

• https://create.roblox.com/docs/physics
• https://create.roblox.com/docs/physics/assemblies
• https://create.roblox.com/docs/physics/mechanical-constraints
• https://create.roblox.com/docs/physics/mover-constraints
• https://create.roblox.com/docs/physics/network-ownership
• https://create.roblox.com/docs/physics/sleep-system
• https://create.roblox.com/docs/physics/adaptive-timestepping
• https://create.roblox.com/docs/physics/units
• https://create.roblox.com/docs/workspace/collisions

This skill covers the modern constraint-based physics system, not deprecated BodyMover objects. It focuses on building correct, stable, multiplayer-safe mechanisms.

When to use this skill

Activate when:

• Building vehicles, doors, elevators, cranes, swings, suspension, or platforms.
• Moving objects with forces instead of setting CFrame every frame.
• Tuning stability for complex mechanisms or multiplayer physics.
• Choosing between mechanical and mover constraints.
• Debugging why assemblies sleep, jitter, or behave unexpectedly.

Cross-reference:

• roblox-networking/SKILL.md for network ownership security and server-authoritative validation.
• roblox-core/SKILL.md for services and script locations.
• roblox-testing/SKILL.md for profiling physics with MicroProfiler.

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 (PhysicsService:RegisterCollisionGroup, 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.

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

Mover constraints

Modern replacements for deprecated BodyMovers:

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

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

Common mistakes this skill prevents

• 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.