-
I thought the middle bit of the Wikipedia article was reasonably clear:
In the case of a ball held in circular motion by a string, the centripetal force is the force exerted by the string on the ball. The reactive centrifugal force on the other hand is the force the ball exerts on the string, placing it under tension.
IOW:
The second form of Newton's 3rd law is
mutual actions of two bodies upon each other are always equal, and directed to contrary parts
The two bodies are the ball and the string
The centripetal force is the string pulling the ball towards the centre of rotation
The reactive force is the ball causing the tension in the string
Both of those are visible in an inertial reference frame, although the directions are constantly changing
I was reading https://en.m.wikipedia.org/wiki/Centrifugal_force and found this in the intro:
Then this long-established article: https://en.m.wikipedia.org/wiki/Reactive_centrifugal_force :
Is my brain creaking more than I thought, or is this just wrong? I don't get the jump from 'an object' to the key bit of Newton's third law, 'mutual actions of two bodies upon each other are always equal'. Neither do I understand the discussion on the talk page.
Edit: there are hundreds of pages elsewhere with claimed graduates arguing about this. I think it comes down to the authors fundamentally not getting what reference frames are, especially rotating (i.e. accelerating) ones.