2. Establishing newton's laws of motion in a world with a different history
we first note that Newton's first and third laws involve statics, whereas the second law does not.
The third law, of action and reaction, is a statement about static forces in some reference frame, and assumes that even when there is motion relative to that reference frame, the law holds (although the values of the forces may change). To establish this law quantitatively requires an absolute forcemeasuring device, such as a spring scale (or even a digital weight scale. which is force-based), as opposed to a balance scale, such as often found in a doctor's office , which can only measure relative weights. In principle, one can establish that the third law holds even in non-inertial reference frames.