accurately know of the quantities appearing in F=ma, so this law could be employed to obtain a standard for (a), and then for time (t). Simlar considerations apply if distance is the least accurately know quantity appearing in F=ma. Clearly, the use of Newton's F=ma depends upon what is known and how accurately it is know. (some other works that take the view that F=ma should be established experimentally , rather than postulated , are given by Refs.12-14)