Understanding Torque and Reference Frames in Rotational Motion

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Discussion Overview

The discussion revolves around the concepts of torque and reference frames in rotational motion, specifically how torque is perceived differently depending on the chosen pivot point. Participants explore the implications of changing reference frames on the sign and magnitude of torque.

Discussion Character

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

Main Points Raised

  • One participant describes a scenario involving a rod with a pivot point and questions how torque changes when the reference frame is shifted from one end of the rod to the other.
  • Another participant agrees that the sign of torque will change with the reference frame but notes that the convention for positive torque typically designates counter-clockwise rotation as positive.
  • A different participant asserts that the motion of the bar will still be clockwise from the perspective of the unanchored end, suggesting that the fictitious torque in this frame will match the original torque's magnitude and direction.
  • One participant expresses confusion about how the torque could remain in the same direction when visualizing the rotation, asking for further clarification.
  • Another participant attempts to clarify by explaining that rotating the left end of the bar clockwise results in the same relative position of the ends, regardless of which end is fixed.

Areas of Agreement / Disagreement

Participants exhibit some agreement on the consistency of torque sign changes with reference frames, but there is disagreement regarding the visualization and interpretation of torque direction in different frames.

Contextual Notes

Participants reference conventions for defining positive torque and the implications of fixed points in rotational motion, but there are unresolved visualizations and interpretations of the scenarios presented.

tomizzo
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Hello,

I have a question regarding the concept of torque and reference frames.

Say for example I have a rod of length L and that the rod has it's right side anchored to act as a pivot point. Now let's say that I input a force on the left side of the bar in an upward direction. Assuming that positive torque corresponds to clockwise rotation, the torque is the force x L (and is positive) with respect to the right pivot point..

Now let's say I do this same experiment, but this time, I want to use a frame of reference such that the left side of the rod appears to be anchored. From what I can deduct, the torque as seen by this virtual pivot point will have the same torque magnitude as the last experiment, except it will be negative now...

Is my reasoning correct? I'm kind of just curious and don't know if phenomenon has a specific name.

Thanks for any help!
 
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You are correct in the sense that as long as you consistently designate one direction as positive and the other as negative the results will be consistent up to that change in sign. Usually one designates counter-clockwise rotation as positive (that is, opposite of your choice above) since this corresponds to the right-hand rule [1], but that is just a practical convention that gives a consistent relation between linear direction of the axis with direction of rotation around those axis.

[1] http://en.wikipedia.org/wiki/Right-hand_rule
 
The motion of the bar from the point of view of the unanchored end will also be clockwise rotation. So, the fictitious torque in this frame will have the magnitude and direction as the true torque in the original frame.
 
Hi Perok,

I can't visualize how the torque would be in the same direction... I keep picturing the bar would have to be rotated counter clockwise to keep the left end in place for our reference frame. Could you explain further?
 
If the bar starts horizontal and you rotate the left end clockwise by 90°, then the left end is above the right end.

If you imagine the left end is fixed, and rotate the right end clockwise by 90°, then again the left end is above the right. Same as before.

I'm not sure what you're picturing.
 

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