Centrifugal Force on a stone tied to a thread

  • Thread starter Cromptu
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  • #26
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All forces are toward the centre, including the force on the person causing holding the other end of the rope.
False. "All net forces" are toward the center in this case, which is not at all the same as "all forces".

I was distinguishing tensions from forces (F=ma).
The F in that equation is the net force.

If the forces on the rope and the the stone were equal and opposite there would be no acceleration, which is not the case.
This is nonsense. A third law pair act on different objects and they are never summed to determine a second law net force or acceleration.

I suggest that "centrifugal" should not be used to describe the 3rd law pair to the force on the stone. In my view, it simply adds confusion
You should stick with this objection. Your technical objections are flat out wrong. If you don't like a term then you don't need to use it. If it causes confusion or is redundant or has other semantic problems then explain that. But like it or not the term is well defined and the concept has no technical deficiencies. The only possible objections are semantics and personal preferences.
 
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  • #27
Andrew Mason
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This is nonsense. A third law pair act on different objects and they are never summed to determine a second law net force or acceleration.
I did not say otherwise. See my response in post #17. My comment was in response to A.T.s statement: "If string and stone are mentioned as separate objects, then there are equal but opposite forces between them in a free body diagram." I was not objecting to that statement. I was just saying it was incomplete. There are other forces on the rope (ie. that you would have to show in a free-body diagram). If there were not, you could not have the (centripetal) acceleration that is observed.

You should stick with this objection. Your technical objections are flat out wrong. If you don't like a term then you don't need to use it. If it causes confusion or is redundant or has other semantic problems then explain that. But like it or not the term is well defined and the concept has no technical deficiencies. The only possible objections are semantics and personal preferences.
I disagree. "Centrifugal" has significant technical deficiencies. There is no possible way that the third law pair to the centripetal force on the stone will ever cause the rope to flee the centre of rotation. You have only to look at the Wikipedia article on "centrifugal reaction force" to see the confusion that surrounds that term.

AM
 
  • #28
A.T.
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"If string and stone are mentioned as separate objects, then there are equal but opposite forces between them in a free body diagram." I was not objecting to that statement. I was just saying it was incomplete.
It is complete, as far Newtons 3rd is concerned. Your deflections about accelerations are just conflation of forces and net forces.


"Centrifugal" has significant technical deficiencies. There is no possible way that the third law pair to the centripetal force on the stone will ever cause the rope to flee the centre of rotation.
Irrelevant from the technical standpoint. Newtons Laws don't care about causes. Acceleration is related to net force not just any force. As DaleSpam said, it is just semantics and personal preferences.
 
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  • #29
A.T.
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So, I ask again, what's the issue?
As far I can say this is the core of the issue:

I was distinguishing tensions from forces (F=ma).
Confusion of "force" and "net force" (as in F=ma). All the other wrong claims follow from that: That there are no outward forces, because there is no outward acceleration. That action & reaction in Newtons 3rd must be actual accelerations. And so on.
 
  • #30
Doc Al
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If I ever get to be world dictator, my first act will be to execute everybody who has ever used terms like "reactive centrifugal force" as if they actually meant something :biggrin:
:rofl: :approve:
 
  • #31
A.T.
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If I ever get to be world dictator, my first act will be to execute everybody who has ever used terms like "reactive centrifugal force" as if they actually meant something :biggrin:
To me the "reactive"-part is more misleading than the "centrifugal"-part.
 
  • #32
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I think someone referred to a free body diagram. It should be the first requirement in discussions of this sort.
I would require my students to produce a free body diagram as part of the explanation of this elementary A level physics, you can tell at a glance whether they understand or not.
Let me offer my diagram.
C is the centripetal force. Can you identify the 3rd law pairings? (hint: they are different colours)
 

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  • #33
WannabeNewton
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Confusion of "force" and "net force" (as in F=ma). All the other wrong claims follow from that: That there are no outward forces, because there is no outward acceleration. That action & reaction in Newtons 3rd must be actual accelerations. And so on.
Ah ok, thanks for pointing out the issue. Yes as you say it is important to keep a distinction between individual forces and the net force.
 
  • #34
Andrew Mason
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I think someone referred to a free body diagram. It should be the first requirement in discussions of this sort.
I would require my students to produce a free body diagram as part of the explanation of this elementary A level physics, you can tell at a glance whether they understand or not.
Let me offer my diagram.
C is the centripetal force. Can you identify the 3rd law pairings? (hint: they are different colours)
It is somewhat arbitrary to speak about third law reaction pairs when what you have is an array of tensions, the existence of each of which depends all the existence of all the others.

We simplify things by treating the person and stone as point masses (located at the respective centres of mass) so we don't have to worry about analysing all the tensions within the person and within the stone. We also assume that the mass of the rope is negligible. If we do that, we can say, quite legitimately I think (although others on this board seem to disagree), that the stone is exerting a force on the person (via the rope) and the person is exerting an equal and opposite force on the stone. These can be considered third law force pairs in our simplified system. Each of those forces is toward the centre of rotation. There is no need to complicate matters by introducing the misleading, and - as far as I can tell - useless, concept of "centrifugal reaction force".

AM
 
  • #35
Andrew Mason
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As far I can say this is the core of the issue:


Confusion of "force" and "net force" (as in F=ma). All the other wrong claims follow from that: That there are no outward forces, because there is no outward acceleration. That action & reaction in Newtons 3rd must be actual accelerations. And so on.
There is no confusion about net force. I never said that Newton third law pairs must be actual accelerations. A box resting on the earth's surface undergoes no acceleration but the box and earth are still exerting equal and opposite forces on each other (a pair of gravitational forces and a pair of mechanical forces).

What I have said (in another thread) is that Newton's third law is really about changes in motion (momentum conservation), not about static forces although I must admit that Newton was not very clear in his writing on this point. If Newton's third law is about changes in motion it becomes a universal law. If it is about equal and opposite tensions it is not universal. A good example of where the latter fails is in electrodynamics: a photon can exert a force on an atom but the atom cannot exert a force on the photon. However, there are always equal and opposite changes in momentum.

AM
 
  • #36
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I disagree. "Centrifugal" has significant technical deficiencies. There is no possible way that the third law pair to the centripetal force on the stone will ever cause the rope to flee the centre of rotation. You have only to look at the Wikipedia article on "centrifugal reaction force" to see the confusion that surrounds that term.
That is not a technical objection, it is a semantic complaint. You cannot paint a rose with red color charge, you cannot win a woman's heart with a charm quark, there is no sense in which a top quark is on top, and so forth. You can object to those terms on similar grounds, but those are not technical deficiencies.

Anyway, we have already had this argument in full previously and I see nothing to be gained by rehashing it.
 

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