Newton 0

Although it is not traditionally specified explicitly as part of Newton's laws, our conceptual idea of object egotism plays a critical role in being able to disentangle complex situations. It says that to understand a situation where objects are interacting with each other, we focus on a particular object.  That object knows nothing except what is being done to it at a given instant.  It then responds to all those influences (forces) by changing its state of motion or not.

We will specify this formally as a pre-law to the standard set of Newton's laws and set it out as an explicit principle:

Newton's 0th Law: Any object (or part of an object) responds only to the forces that are being exerted on it by other objects and it responds to them at the instant it feels them.

Dangerous bends

We have added law to the standard canon in part to help you avoid some "dangerous bends" — errors that are easy to slip into and not notice that one is making them.  Here are a couple of things to watch out for. 

• Me, me, me: By our core idea of reciprocity, every time an object feels a force it also exerts one back on the object that is exerting the force on it.  But that force acts on the other object and affects its motion.  This is confusing.  Let's give our objects some names.  Let the object whose motion we are considering be called A.  Suppose it feels a force exerted by B.  The force $F_{B\rightarrow A}$  affects the motion of object A.  But A exerts a force back on B:  $F_{A\rightarrow B}$.  That force does NOT affect the motion of A, but it does affect the motion of B.
• Right now: An object doesn't change its velocity as a result of forces that it felt in the past, but if it has changed its velocity as a result of past forces it does keep that velocity.  Remember that forces don't maintain velocities, they change them. So if an object has changed its velocity, say from rest, as a result of forces it felt a few moments ago, it's easy to associate that force with the velocity and forget that the velocity continues even after the force that caused it is gone.

Joe Redish 9/22/11