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# momentOfInertia

The body’s moment of inertia, expressed in the local coordinate system of the node that contains the body.

```
var momentOfInertia: SCNVector3 { get set }
```

## Discussion

A body’s moment of inertia determines how it responds to torques (that is, forces with a rotational component). Each component of this vector is the moment of inertia for the corresponding principal axis (in the coordinate system containing the physics body). For example, if the x-component value of the moment vector is less than the y-component value, the body rotates more freely about its x-axis than its y-axis.

By default, SceneKit automatically determines the body’s moment of inertia based on its shape and mass. Use this property to define a custom moment of inertia (for example, to model an object of non-uniform density). Using a custom moment of inertia requires setting the [`usesDefaultMomentOfInertia`](/documentation/SceneKit/SCNPhysicsBody/usesDefaultMomentOfInertia) property to <doc://com.apple.documentation/documentation/Swift/false>.

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