---
title: "init(mass:inertia:centerOfMass:)"
framework: realitykit
role: symbol
role_heading: Initializer
path: "realitykit/physicsmassproperties/init(mass:inertia:centerofmass:)"
---

# init(mass:inertia:centerOfMass:)

Creates a mass properties instance with the given settings.

## Declaration

```swift
init(mass: Float, inertia: SIMD3<Float> = SIMD3<Float>(x: 0.1, y: 0.1, z: 0.1), centerOfMass: (position: SIMD3<Float>, orientation: simd_quatf) = (SIMD3<Float>(x: 0, y: 0, z: 0), simd_quatf(ix: 0, iy: 0, iz: 0, r: 1)))
```

## Parameters

- `mass`: The mass in kilograms. If you use a mass of 0 or infinity, the simulation treats the object as doc://com.apple.RealityKit/documentation/RealityKit/PhysicsBodyMode/kinematic. That is, the object doesn’t respond to forces.
- `inertia`: The inertia in kilograms per square meter. The vector contains the diagonal elements of the diagonalized inertia matrix.
- `centerOfMass`: The center of mass and the orientation of the principal axes, defined in the local space of the rigid body. The position defines the center of mass with a default value of (0, 0, 0), which means that the local origin of the model is the center of mass. The orientation defines the principal axes, such the inertia matrix is a diagonal.

## See Also

### Creating custom mass properties

- [init()](realitykit/physicsmassproperties/init().md)
- [init(shape:density:)](realitykit/physicsmassproperties/init(shape:density:).md)
- [init(shape:mass:)](realitykit/physicsmassproperties/init(shape:mass:).md)