roblox-physics

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

| Goal | Mechanism |

|---|---|

| sustained physical interaction | unanchored assembly + modern constraints/forces |

| instantaneous physical change | ApplyImpulse / ApplyAngularImpulse |

| kinematic platform/path | controlled pivot/transform with an explicit passenger policy |

| character locomotion | Humanoid/custom character controller (roblox-characters) |

| authoritative hit test | server raycast/overlap with filters and gameplay validation |

| cosmetic trail/recoil | local visual motion; no gameplay authority |

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.

1. Define interaction policy. Write the collision-group matrix and separately decide

CanCollide, CanTouch, and CanQuery. These flags are not interchangeable.

1. 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.

1. 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.

1. Set ownership deliberately. Server-own gameplay-critical loose assemblies when required;

client ownership can improve responsiveness but never authorizes gameplay results.

1. Bound cost and lifetime. Reuse query parameters, cap query frequency/result count, remove

temporary constraints/attachments, and disconnect event listeners.

1. 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.

Common failures

| Symptom | Likely cause | Remedy |

|---|---|---|

| welded mechanism will not move | one part anchored | inspect full assembly; anchor only intentional world roots |

| force behaves too strongly/weakly | assembly mass ignored | inspect AssemblyMass; tune force/impulse by intended acceleration |

| hit misses fast projectile | discrete touch sampling/tunneling | swept query — WorldRoot:Blockcast(), Spherecast(), or Shapecast() — plus physics-tuning; do not rely only on .Touched |

| ray hits shooter/effects | filters/collision group absent | reuse explicit params and query group |

| same target damaged many times | overlap returned multiple body parts | deduplicate by target model and enforce attack ID/cooldown |

| exploit fires impossible touch | client owns relevant physics | server query/context validation; deliberate ownership |

| invisible trigger blocks or cannot query | three flags conflated | set CanCollide, CanTouch, CanQuery independently |

| mechanism leaks attachments | temporary constraint lifecycle missing | own and destroy constraints, attachments, and connections together |

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