Roblox physics

Choose deliberately between simulation, character control, hit detection, and visual-only motion; they are different jobs. Targets Roblox's rolling platform APIs. Pair with physics-tuning for engine-neutral stability and feel.

When to use

• Use for BasePart assemblies, constraints, collision/query policy, ray/overlap queries, forces, impulses, moving physical objects, network ownership, or physics cleanup.
• Use when Touched is unreliable/security-sensitive, parts tunnel or jitter, a mechanism breaks when anchored, or old BodyMover patterns appear.

When not to use: ordinary Humanoid lifecycle/control belongs to roblox-characters; remote validation belongs to roblox-networking; decorative UI/world motion may only need a tween.

Decide the system first

Workflow

1. Inspect the mechanism. In Studio, visualize assemblies, anchors, constraints, collision groups, massless parts, and network owners. Identify the assembly root and intended authority.
2. Define interaction policy. Write the collision-group matrix and separately decide CanCollide, CanTouch, and CanQuery. These flags are not interchangeable. Register groups in the Studio Collision Groups editor or with Workspace:RegisterCollisionGroup() and Workspace:CollisionGroupSetCollidable(); the PhysicsService collision-group methods are deprecated and forward to Workspace.
3. Choose simulation or query. Do not use .Touched as a universal hit detector. Use a ray for a path/line, an overlap query for a volume, and simulation contacts when physical response is actually required.
4. Apply motion at assembly level. Forces on a part affect its assembly. Use modern LinearVelocity, AngularVelocity, VectorForce, AlignPosition, and AlignOrientation constraints as appropriate; migrate deprecated BodyMovers when changing that system.
5. Set ownership deliberately. Server-own gameplay-critical loose assemblies when required; client ownership can improve responsiveness but never authorizes gameplay results.
6. Bound cost and lifetime. Reuse query parameters, cap query frequency/result count, remove temporary constraints/attachments, and disconnect event listeners.
7. Verify under load and multiplayer. Test anchored/unanchored transitions, mass extremes, collision matrix, fast motion, multiple clients, ownership changes, streaming, and cleanup.

Pattern: filtered server raycast

local Workspace = game:GetService("Workspace")

local params = RaycastParams.new()
params.FilterType = Enum.RaycastFilterType.Exclude
params.FilterDescendantsInstances = {shooterCharacter}
params.IgnoreWater = true
params.CollisionGroup = "WeaponQuery"

local direction = aimDirection.Unit * MAX_RANGE
local result = Workspace:Raycast(muzzlePosition, direction, params)
if result then
    local model = result.Instance:FindFirstAncestorOfClass("Model")
    local humanoid = model and model:FindFirstChildOfClass("Humanoid")
    if humanoid and serverCanDamage(shooter, model, result.Position) then
        humanoid:TakeDamage(serverWeaponDamage(shooter))
    end
end

The server must validate the origin/direction against server-known character/weapon state; do not accept an arbitrary client origin and treat the raycast itself as validation.

Pattern: overlap volume with explicit policy

local params = OverlapParams.new()
params.FilterType = Enum.RaycastFilterType.Exclude
params.FilterDescendantsInstances = {sourceCharacter}
params.CollisionGroup = "DamageQuery"
params.MaxParts = 64

local seen: {[Model]: boolean} = {}
for _, part in Workspace:GetPartBoundsInBox(hitboxCFrame, hitboxSize, params) do
    local model = part:FindFirstAncestorOfClass("Model")
    if model and not seen[model] then
        seen[model] = true
        validateAndApplyHit(model)
    end
end

Bounds queries use bounding boxes and can include multiple parts from one target; deduplicate and perform exact/gameplay checks as needed. For exact geometry use WorldRoot:GetPartsInPart(part, overlapParams) only when its additional cost is justified. Note OverlapParams.RespectCanCollide decides whether a query honours CanCollide or CanQuery — set it deliberately, or it silently overrides the flag policy below. OverlapParams.Tolerance controls contact slop.

Assemblies, force, and ownership

• Welded parts form one rigid assembly; force, impulse, velocity, mass, and ownership operate on that assembly. Anchoring a part changes simulation/ownership and can make an assembly effectively infinite mass.
• Apply an impulse for a one-time change; use a force or velocity constraint for sustained control. Setting AssemblyLinearVelocity is an immediate state change, not a continuous force model.
• Prefer attachments plus modern constraints over BodyPosition, BodyVelocity, BodyGyro, and other deprecated BodyMovers when authoring or revising a mechanism.
• Automatic ownership may move nearby unanchored assemblies to clients. Use SetNetworkOwner(nil) conservatively for critical objects, then measure responsiveness/server cost. Visualize network owners in Studio.
• A client owner can manipulate physical results and .Touched observations. The server validates consequential hits, positions, timing, and permissions independently.
• If Workspace.AuthorityMode is Server, the server owns simulation and clients predict parts near their own character with rollback (RunService:SetPredictionMode(), client-only, overrides per instance). Drive predicted forces and constraints from RunService:BindToSimulation() on both sides rather than from Heartbeat or client-only scripts; custom physics scripts need adapting when a place switches modes. See roblox-networking.

Common failures

Resources

• Read references/queries-and-ownership.md for collision/query matrices, assembly debugging, ownership security, migration choices, and the physics verification matrix.

Related skills

• physics-tuning — timestep, jitter, tunneling, mass ratios, and stability methodology.
• roblox-characters — Humanoid/custom movement and respawn lifecycle.
• roblox-networking — authoritative validation of client-requested physical actions.
• roblox-studio-workflow — visualization, Output, and multi-client verification.

Primary references

• https://create.roblox.com/docs/physics/assemblies
• https://create.roblox.com/docs/physics/network-ownership
• https://create.roblox.com/docs/workspace/raycasting