roblox-physics-and-constraints

Official sources (always check these for the latest):

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-client-server-networking-and-security/SKILL.md for network ownership security and server-authoritative validation.
roblox-fundamentals-and-services/SKILL.md for services and script locations.
roblox-testing-and-debugging/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.

Constraint	Use case
`WeldConstraint`	Rigidly lock two parts together, same relative transform
`RigidConstraint`	Same as WeldConstraint but attachment-based, supports bones
`HingeConstraint`	Doors, levers, rotating parts; motor/servo optional
`PrismaticConstraint`	Sliding doors, elevators, pistons
`CylindricalConstraint`	Slides + rotates, like hydraulic rams or landing gear
`SpringConstraint`	Springs, shocks, suspension
`TorsionSpringConstraint`	Rotational springs
`BallSocketConstraint`	Shoulders, ball joints
`UniversalConstraint`	Drive shafts, constant-velocity joints
`RopeConstraint`	Cables, winches, maximum length
`RodConstraint`	Fixed separation distance
`PlaneConstraint`	Constrain motion to a plane
`NoCollisionConstraint`	Disable 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:

Legacy	Modern	Use case
`BodyPosition`	`AlignPosition`	Move attachment to a position or another attachment
`BodyGyro`	`AlignOrientation`	Align orientation; `LookAtPosition` for tracking
`BodyVelocity`	`LinearVelocity`	Maintain constant velocity along vector/line/plane
`BodyAngularVelocity`	`AngularVelocity`	Maintain constant angular velocity
`BodyForce`/`BodyThrust`	`VectorForce`	Apply constant force
`RocketPropulsion`	`LineForce` + `AlignOrientation`	Follow + face target
—	`Torque`	Apply constant torque
—	`LineForce`	Force along line between two attachments
—	`AnimationConstraint`	Constraint 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

Roblox	Metric
1 stud	28 cm
1 RMU	21.952 kg
196.2 studs/s²	default "Classic" gravity
35 studs/s²	"Realistic" gravity (≈ 9.8 m/s²)

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

Decide if the object is a rigid mechanism (mechanical constraint) or force-driven (mover constraint).
Plan assemblies, root parts, and anchoring strategy.
Set network ownership for multiplayer responsiveness.
Tune forces/velocities using units and physical properties.
Profile with MicroProfiler and watch sleep/ownership visualizations.