Centrifugal force in rotating frame of reference

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Discussion Overview

The discussion centers around the concept of centrifugal force as observed in a rotating frame of reference, particularly in relation to the behavior of water in such a frame. Participants explore the implications of viewing forces from both rotating and inertial frames, addressing the nature of fictitious forces and their role in explaining observed phenomena.

Discussion Character

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

Main Points Raised

  • One participant questions the explanation of centrifugal force in a rotating frame, expressing confusion over its necessity.
  • Another participant attempts to clarify that a molecule of water in uniform circular motion experiences a centripetal force, and in a rotating frame, an equal and opposite centrifugal force must be considered.
  • A different participant highlights the difficulty in reconciling the explanation with the observed parabolic shape of the water's surface, suggesting that gravity alone cannot account for this shape.
  • It is noted that centrifugal force is a fictitious force that arises in non-inertial frames, and that analyzing the situation from an inertial frame can simplify the understanding of the forces at play.
  • Participants discuss how the resultant forces in both rotating and inertial frames differ, with the resultant force in the rotating frame being normal to the water's surface due to the balance of gravity and centrifugal force.
  • One participant acknowledges the complexity of the topic but begins to understand the role of fictitious forces in explaining the curvature of the water's surface.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and interpretation of centrifugal force in rotating frames. While some agree on the role of fictitious forces, others remain uncertain about the implications for the shape of the water's surface and the overall understanding of forces involved.

Contextual Notes

Participants mention the need for modifications to Newton's laws in non-inertial frames and the dependence of the analysis on the chosen frame of reference. The discussion highlights the complexity of reconciling observations in different frames without reaching a consensus on the explanations provided.

andrevdh
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Can someone enlighten me why it is mentioned in the thread
https://www.physicsforums.com/showthread.php?p=769852"
that when the water is viewed from a rotating frame of reference it experiences an outward centrifugal force. This does not make sense to me.
 
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I'm going to attempt an explanation, but I'm not sure it's accurate. It would be nice if it were because it would make explaining centrifugal forces easy.

Let's focus on one molecule of water, instead of the whole cylinder.

This molecule is decribing uniform circular motion. Hence, there is a centripetal force towards the center acting on the molecule. But in a rotating frame of reference, the molecule is at rest. Hence, there is must be an equal and opposite force acting on it: the centrifugal force.
 
Yes, the universe has a nasty way of making things difficult for us to understand. It is us humans who try to make the universe easier for us to comprehend. The problem with reconciling your explanation with what was said in the thread is that there would then be only gravity to control the shape of the surface of the water, and since it is shaped parabolically in the rotating frame your suggestion would not explain the effect, or am I understanding you incorrectly?
 
Centrifugal force only appears when analyzing the situation from a rotating frame of reference. Since such a frame is non-inertial, Newton's law do not apply without modification. One modification is the introduction of "fictitious" forces (such as centrifugal force) which are entirely an artifact of the acceleration of the frame of reference. Some problems are much easier to analyze when viewed from a rotating frame.

Of course, the parabolic shape of the water does not depend on what frame you view it in. It is perfectly OK to analyze the problem from an ordinary inertial frame in which the water is rotating and thus centripetally accelerating. I give such an analysis in the referenced thread.
 
In the rotating frame of reference the horizontal outwards force is a fictitious one (meaning it does not really exist), the centrifugal force, that we introduce in order to explain an effect that we know exist, even in the rotating frame of reference - is a manifestation of the accelerating frame of reference. The resultant force will then be pointing into the water, perpendicular to it's surface. Which explains everything well in this frame. In the inertial frame of reference the resultant force will however point somewhat along the surface of water due to the known required centripetal force?
 
andrevdh said:
In the rotating frame of reference the horizontal outwards force is a fictitious one (meaning it does not really exist), the centrifugal force, that we introduce in order to explain an effect that we know exist, even in the rotating frame of reference - is a manifestation of the accelerating frame of reference.
Right.
The resultant force will then be pointing into the water, perpendicular to it's surface. Which explains everything well in this frame.
In the rotating frame, the resultant force (on an element of the water surface) is zero, since it does not accelerate in that frame. The combination of gravity plus centrifugal force will be normal to that parabolic surface.
In the inertial frame of reference the resultant force will however point somewhat along the surface of water due to the known required centripetal force?
In the inertial frame of reference, the resultant force is centripetal.
 
And here I thought it would get easier as we go along... , but it is getting clearer! I forgot about the supporting force of the surrounding water on a water element. That is why I was thinking that the resultant force in the inertial frame points along the surface. I understand the zero resultant force in the rotating frame now - we are ignoring the fictitious force in this regard, since it does not really exist - but to explain the curvature of the water (which cannot appear out of nowhere) we use it to get the apparent gravity perpendicular to the surface. We cannot explain the curvature the same way in the inertial frame. We just say that the curvature is a result of the spinning.
 

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