[Mike O]

Nice post Mike.

You've got a good handle on things from my point of view, but by looking at the system as a whole (like yoda did with the string) it leaves greater scope for misunderstanding and it is harder to identify where the misunderstanding lies.

There is a nice student exercise that beginning physics students use about a man standing on bathroom scales whilst a lift accelerates upward. I'll run through it if anybody thinks it will help, but to go back to your examples...

If you push on your car the equal and opposite reaction is that the car pushes on you with the same force and in the opposite direction. You push on the earth and the earth pushes back, but this pair of equal and opposite forces will be different in magnitude to the push on car/car pushes back. The second phase of analysis is to look at each object and ask what forces are acting. On the car, your push. On you, the car's push and the ground's push. On the ground, your push.

With the rock and the string, if the mass of the string is important then we can't perform a correct analysis without separating considering the "string on rock/rock on string" and "boy on string/string on boy" pairs separately because they will have different magnitudes. We can't just say "boy on rock/rock on boy" without things getting very muddled.

This is where the man on scales in lift will help if you need more detail.

Golf Sceptic- Just my perspective but it appears to me- that you have a significant communication problem. Don't know if you are aware of it or not. I don't say that in a derogatory tone or sarcastic tone- it's just the feedback I would give you - in order to improve your performance in the future. I'm assuming you know your subject but can't communicate it clearly. You've got 25 somewhat extensive posts on this thread/subject matter and it's my feeling that no one (at least myself) has made any progress in understanding your perspective or the context and importance of your point. Every new post doesn't get you any closer to the answer than the previous post.

My only guess to the problem is that the foundation of concepts that supports your viewpoint - that seems obvious to you is not obvious to your audience (me). When you build a concept, idea, system, theory- you can't get to the theory and take everything as self-evident- especially for teaching or describing it's functioning- you've got to essentially retrace the original route - in principle- not point by point- to it's basis- starting reference points- those things that you can see, touch, smell, hear. Homer had a similar problem- so you've got company. You've also got to understand when you mention any particular point- how other people might mis-interpret it and explain and what the wrong turns could be at any turn- so that you keep the reader on track.

So that's why very few people stuck it out with Homer- and very few will stick it out with you- (like me)- so when you ask "This is where the man on scales in lift will help if you need more detail." I'm thinking no thanks- because that post is going to be like the last 25 - not going to get me any closer to you answering and me understanding whatever you were talking about when this thread started.

That's just my feedback- hope it helps you.