BaseParts are deceptively simple instances. At the most basic level, they are pieces of 3D geometry with a position and orientation defined by their CFrame, as well as a size.
An assembly is one or more BaseParts connected by rigid WeldConstraints or Motor6Ds (animated rigid joints).¹
From a physics perspective, an assembly is considered a single rigid body, meaning no force can push or pull the connected parts from each other, and they will move as a single unit. All forces applied to a specific BasePart are applied to its assembly — for instance, BasePart:ApplyImpulse() applies impulse to the assembly at BasePart.AssemblyCenterOfMass.
¹ The joints that combine multiple parts into assemblies are only active in the Workspace or another WorldModel instance. If the parts are stored elsewhere, all of the welds/connections that combine parts into assemblies will be non-functional.
The following BasePart properties show data regarding its assembly. Their values will be the same for any part in the same assembly, so it doesn't matter which part you use.
|BasePart.AssemblyLinearVelocity||The linear velocity vector of the part's assembly. Setting the velocity directly may lead to unrealistic motion, so usage of a VectorForce or LinearVelocity constraint is preferred, or BasePart:ApplyImpulse() for an instantaneous change in linear velocity.|
|BasePart.AssemblyAngularVelocity||The angular velocity vector of the part's assembly. Setting the velocity directly may lead to unrealistic motion, so usage of a Torque or AngularVelocity constraint is preferred, or BasePart:ApplyAngularImpulse() for an instantaneous change in angular velocity.|
|BasePart.AssemblyCenterOfMass||A read-only position calculated via the mass and position of all the parts in the assembly. A force applied to the center of mass will not cause angular acceleration, only linear.|
|BasePart.AssemblyMass||The sum of the BasePart.Mass of all parts in the assembly. If the assembly has an anchored part, the assembly's mass is considered infinite.|
|BasePart.AssemblyRootPart||The part automatically chosen to represent the assembly's root part.|
Assembly Root Part
Every assembly has a root part indicated by its AssemblyRootPart property. This is the part that doesn't move when Motor6D transforms are updated, as well as the part used to keep consistent physics replication and network ownership.
You cannot explicitly set the root part, but the following factors affect probability from highest to lowest:
An Anchored part will always be assigned as the root part.
Parts with Massless set to false (default) take precedence.
Higher RootPriority values take precedence.
Precedence based on the part's size, with multipliers for parts with specific names.
When one of an assembly's parts is anchored, that part becomes the root part and all of the other parts become implicitly anchored with it. The following sequence illustrates this behavior.
Below, four parts are welded together with WeldConstraints (green bars) to form a single assembly, as indicated by the matching colored outlines.²
² To enable this visualization, go to Studio Settings → Physics and enable Are Assemblies Shown.
If just one part in the assembly is anchored, the assembly will not change, other than the root part potentially changing (the anchored part always has the highest priority for becoming the root part).
If more than one part is anchored, the assembly will split. Below, both the left and top parts are anchored, so the original assembly splits into two assemblies as shown by the colored outlines. Also, the WeldConstraint between the two assemblies deactivates, since you cannot have an active weld between two anchored assemblies.