57.4 A 'normal slice' of a stationary event is the slice of it cut off between any two normal cross-sections. An 'oblique slice' of a stationary event is the slice of it cut off between any two parallel oblique cross-sections. A normal slice of a stationary event is itself a stationary event in the same time-system.
[58. Motion of Objects]. 58.1 A material object is 'motionless' within a duration when throughout that duration the material object and its extensive components are all situated in stationary events.
In the case of a motionless material object, Law I for uniform objects can be made more precise, as follows:
If
be a material object motionless in the duration
and
be the stationary event extending throughout