Torque: Applied Forces Pos/Neg?

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Discussion Overview

The discussion revolves around the concept of torque, specifically the assignment of positive and negative values to torques based on the direction of applied forces. Participants explore the implications of the right hand rule and how it relates to determining the direction of net torque in various scenarios, including homework problems involving circular motion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant notes that torque is positive for counterclockwise rotation and negative for clockwise rotation, questioning the assignment of negative values in specific homework problems.
  • Another participant asserts that negative values should not be assigned to forces when determining torque direction, emphasizing the use of the right hand rule.
  • A later post suggests that the direction of individual torques should be determined before calculating the net torque, mentioning that opposing torques will cancel each other out.
  • It is proposed that the convention of assigning positive and negative signs to torques based on their rotational direction is arbitrary, although counterclockwise is typically considered positive.
  • Some participants highlight the importance of understanding the geometry of the situation and ensuring the correct angle is used in calculations related to torque.

Areas of Agreement / Disagreement

Participants express differing views on the assignment of negative values to forces and torques, indicating that multiple competing perspectives exist regarding the application of the right hand rule and the conventions used in torque calculations. The discussion remains unresolved.

Contextual Notes

Participants mention the need to consider the geometry of the situation and the reference coordinate system when determining angles for torque calculations, but specific assumptions or definitions are not fully clarified.

DameLight
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Hi,

I have a question regarding the forces applied on an object that create the torque. I understand that the resulting torque is pos if the object turns counter clock wise and is neg if it turns clock wise and that you can also use the right hand rule to find the direction of the net torque.

I found the right answer for my homework by making some of the resulting torques negative, but I don't understand why they are negative.

Particularly one such problem where there is a circle. One of the forces is tangent and pointing up toward the E most point of the circle. The answer needs this to be neg, but the right hand rule makes it pos?

Was I suppose to be looking for the difference in the resulting torques? But that doesn't make any sense since the net torque is the sum of the torques found.
 
Last edited:
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Never assign a negative value to a force when determining the direction of the torque. Use right hand rule, clockwise is plus and counterclockwise is negative, or the other way around if you wish.
 
Conclusion:

When finding the torque, the magnitude is found through the equation
c22e37af0377d584aea9ff3cfadf7332.png
but to find the direction you must use the right hand rule for each individual torque before finding the net torque. The ones in opposite directions will cancel each other out and then you will use the right hand rule again to find the direction of the net torque.
 
The key is that you must assign opposite signs to torques on ccw and cw motion. It doesn't matter which sign you use although the convention is that ccw is positive and cw is negative.
 
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Sorry if this is obvious but... It's important not to just memorise that equation but to understand that it comes from the geometry of the situation. What I mean is when presented with a drawing that already has some random angle marked θ do check it's the correct angle to use in the calculation.
 
Last edited by a moderator:
CWatters said:
Sorry if this is obvious but...

Yes, I understand that the angle you need has to be correct in reference with your chosen coordinate system.
 

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