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.
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.

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