# init(mass:inertia:centerOfMass:) Creates a mass properties instance with the given settings. ``` init(mass: Float, inertia: SIMD3 = SIMD3(x: 0.1, y: 0.1, z: 0.1), centerOfMass: (position: SIMD3, orientation: simd_quatf) = (SIMD3(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 [`PhysicsBodyMode.kinematic`](/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. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)