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.
The first law, stating that for unforced motion an object moves at a constant velocity , is not on its face a law about statics. However , since it merely involves a system that moves at a constant velocity relative to an inertial frame, it is applicable to the statics of forces in the moving reference frame.It statics that, in an inertial reference frame (one that is at rest or moving st a constant velovity relative to the "fixed" stars), if there is net force then the velocity is constant.
this is often associated with the concept of inertia. However. note that if we are at rest relative to an inertial laboratory frame and we then walk at a constant velocity (v), then all other objects develop a velocity (v), independent of their inertial mass. Thus , from the viewpoint of kinematics at constant velocity. the inertial mass is irrelevant to the first law. To establish the first law requires a true force-measuring device , so that one can show that the net force is zero, and devices for measuring distance and time, so that can show that the velocity (dr/dt) is constant. Thus, even without the second law of motion there is a need for distance, force, and time units.
with this in mind, let us consider how one might establish Newton's laws of motion , beginning with the staticsassociated first and third laws.