What is the role of rotation in mechanics?

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Discussion Overview

The discussion revolves around the role of rotation and relative motion in mechanics, particularly focusing on the perspectives of two particles, A and B, where one is at the center of a circle and the other is rotating around it. Participants explore concepts of rotation, revolution, and the implications of these motions in different reference frames.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that if A is at the center of a circle and B is rotating around A, then A would see B rotating, while B would see A rotating around itself.
  • Others argue that the definitions of rotation and revolution need clarification, suggesting that rotation refers to turning about an axis, while revolution refers to moving in a circular path around another object.
  • One participant mentions that both A and B would perceive centripetal forces acting on each other in their respective reference frames, raising questions about the equality of these forces.
  • There is a discussion about the inertial and non-inertial frames, with some participants questioning whether circular motion can be considered inertial, citing differing views from a professor's teachings.
  • Another participant challenges the idea that uniformly revolving frames can be treated as inertial, suggesting that this is a simplification that may not hold in all scenarios.

Areas of Agreement / Disagreement

Participants express differing views on the definitions of rotation and revolution, the nature of inertial frames in circular motion, and the implications of centripetal forces. There is no consensus on these points, indicating ongoing debate and exploration of the concepts.

Contextual Notes

Some participants note that the discussion may depend on specific definitions and assumptions regarding motion, reference frames, and the application of Newton's laws in non-inertial frames.

santo35
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well if i understand relative motion quite well but while thinking about it i had this doubt-
if A is in center of circle and if B is rotating in that circle, A would see B rotating rite? now , how would B see A?... there are more questions to follow up based on the answer...
 
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Do you mean B is revolving around A in a circular path?
Or
Do you mean that B is revolving around A in a circular path and also rotating about its own axis?
You should be more specific in your question, as the answer will depend upon the actual scenario that you have envisioned.
 
santo35 said:
well if i understand relative motion quite well but while thinking about it i had this doubt-
if A is in center of circle and if B is rotating in that circle, A would see B rotating rite? now , how would B see A?... there are more questions to follow up based on the answer...

B would also see A rotating around B.
 
256bits said:
Do you mean B is revolving around A in a circular path?
Or
Do you mean that B is revolving around A in a circular path and also rotating about its own axis?
You should be more specific in your question, as the answer will depend upon the actual scenario that you have envisioned.

well, let us consider they are particles ( for simplicity )...so thus there is no sense to talk about revolving...
 
ank160 said:
B would also see A rotating around B.

great ! i wanted this ans to pop out.. so now both A and B would see the other having centripetal forces in their respective reference frames rite??

for A B's Fcent = Mass of b * velocity(b)^2 / radius
for B A's Fcent = Mass of a * velocity(a)^2 / radius

since they are relative both the Fcent are equal rite? or am i wrong somewere?
 
santo35 said:
well, let us consider they are particles ( for simplicity )...so thus there is no sense to talk about revolving...
Usually 'rotate' refers to turning on an axis. For example: The Earth rotates about its axis.

Usually 'revolve' means moving in a (perhaps) circular path around something. For example: The Earth revolves around the sun.

So for a particle, rotation about its own axis wouldn't make sense, but revolving about some point would.

So if I understand your initial post: B revolves around A. And looked at from B's perspective, A revolves around B as well. (Of course, B would not be in an inertial reference frame, assuming A is at rest in one.)
 
Doc Al said:
Usually 'rotate' refers to turning on an axis. For example: The Earth rotates about its axis.

Usually 'revolve' means moving in a (perhaps) circular path around something. For example: The Earth revolves around the sun.

So for a particle, rotation about its own axis wouldn't make sense, but revolving about some point would.

So if I understand your initial post: B revolves around A. And looked at from B's perspective, A revolves around B as well. (Of course, B would not be in an inertial reference frame, assuming A is at rest in one.)

yes i ment the same...if i am not rong when i read about the atom modal, people said that as any moving charged particle would lose energy and thus the negative charge would consequently fall down in the nucleus if not for the bhor's energy level...am i rite?
 
lets suppose the to particles are not rotating about its axis. B is revolving with A in the cetre. A is not revolving around B. We can take it as Earth which is not rotaing is revolving around the sun. In such a situation, B will see A to be rotating about its axis but not revoling around B.
 
Doc Al said:
...So if I understand your initial post: B revolves around A. And looked at from B's perspective, A revolves around B as well. (Of course, B would not be in an inertial reference frame, assuming A is at rest in one.)

One of A or B cannot be in an inertial frame. One is accelerating while the other is not; or at least not necessarily.

I ask this because I have heard from my professor that movement in a circle is considered to be inertial. I thought that was just an ignorant remark. If you are saying it is true, can you explain why?
 
  • #10
saim_ said:
One of A or B cannot be in an inertial frame. One is accelerating while the other is not; or at least not necessarily.
Right.

I ask this because I have heard from my professor that movement in a circle is considered to be inertial. I thought that was just an ignorant remark. If you are saying it is true, can you explain why?
You'll have to ask your professor what he meant by that remark. Anything accelerating is not in an inertial frame.
 
  • #11
Actually, he not only remarked it once, we used this concept (that uniformly revolving frame is inertial) throughout this semester. It seems its common practice in engineering dynamics to make this simplification, so much so that our professor was adamant that it was no simplification but that by definition uniform revolution is inertial :D
 
  • #12
santo35 said:
great ! i wanted this ans to pop out.. so now both A and B would see the other having centripetal forces in their respective reference frames rite??

for A B's Fcent = Mass of b * velocity(b)^2 / radius
for B A's Fcent = Mass of a * velocity(a)^2 / radius

since they are relative both the Fcent are equal rite? or am i wrong somewere?

i guess ur basic r not clear that how to work wid non inertial frames when Newtons laws are applied in it...newayys...just see...

B is a non-inertial frame having acceleration a = v^2/r, in direction from B to A radially. Now if u want to work 4m this frame then u will have to apply a force of -ma, where 'm' is the mass of ne body that ur seeing 4m that frame and a is the acceleration as mentioned above. Now consider A, so when u see body A 4m frame of B, then u will have to apply a force of -(mass of A)*a = mA*v^2/r in direction A to B radially, and this is nothing but centripetal force which A is experincing.

Hope things will get clear 2 u.
 
  • #13
saim_ said:
It seems its common practice in engineering dynamics to make this simplification, so much so that our professor was adamant that it was no simplification but that by definition uniform revolution is inertial :D

Either you misunderstood what your prof meant, or he is talking nonsense.

The ONLY time you that can ignore the rotation (whether it is uniform or not) is when its effects are small compared with what you are interested in.

For eaxmple in lab experiments in dynamics you can often ignore the rotation of a reference frame fixed to the earth. But if you do a lab experiment with a Foucault pendulum and ignore the Earth's rotation, you will have a very hard time understanding what happened.
 

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