Gist: The quantity you want to measure dictates the instrument you need. Fix the geometry and the required precision follows from it.
Why it matters: To clock a bullet slower than sound ( m/s) across a gate of cm, even the fastest crossing lasts about ms — so a timer resolving th of a millisecond pins it down to roughly . The apparatus is not a free choice; the physics sets its specification.
Detail: Lecture 2, p.3
Related: Average speed, Measuring g by free fall, Stroboscopic measurement, Physics