[Mike O]

The words centripetal and centrifugal in physics have their dictionary meanings. centripetal=toward the centre, centrifugal=away from the centre. They are equal and opposite, action and reaction.

Most people get it wrong because they do not understand that the centripetal force acts on the stone, and the centrifugal force acts on the string (or the boy). We then get incorrect statements about the centrifugal force throwing the rock out and keeping the string taut and that sort of thing.

What keeps the string taut? The boy pulling on it.
What balances the centripetal force on the stone? Nothing, that's why it moves in a circle.

As a general rule, the more words used to describe the phenomenon,
the more errors will be introduced.

Interesting thread- although it seems like no one can clearly define all the issues/perspectives involved in clearly understanding the topic at hand. Although, some may take that personally- it's more just an observation regarding the thread. And I'm certainly not one who necessarily could clearly define all the issues involved- in fact I'd probably be more off base than most.

Just one piece of the puzzle and confusion- and I could be wrong but in relation to Newton's third law- "action reaction" and the notion of "centripetal" and "centrifugal"- there seems to be some discussion of the boy, string, rock- and some of it appears to me to be on the wrong track.

Newton's third law to me - just says that you can't create a force out of nothing. To me it's similar or it's "sister" concept is that you can't create or lose Matter - it just is. So if you push a car down the street in a straight line- then the equal and opposite force is your feet on the ground pushing against the earth in the opposite direction of the motion of the car. That's a simple example of the action/reaction concept.

I would think the proper context of this in relation to the golf swing would be to look at the "whole system". So imagine that you have the clubhead or rock moving in a straight line- attached to it is a golf shaft or string- now to prevent that thing from moving is a straight line you give a tug on the shaft or string and it curves/moves toward the tugging- that's the centripetal force. So in relation to Newton's third law- you ask what offsets or allows that inward tug to happen? Depending on the movement- i.e. amount of force involved etc.- how heavy the rock is compared to the mover it could be different- but to me the basic reaction forces would be the feet pushing against the earth- that force is away from center and allows the force towards center to take place.

So I'm limiting my comment to the specific issue of action/reaction forces- and I don't see it - in the larger picture - being the boy and the string or the boy and the rock, or the rock and the boy.

Hope I didn't further clutter the post- with something that isn't clear.