Can there be force couples in a particle system of equilibrium?

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Discussion Overview

The discussion revolves around the concept of force couples in a particle system that is in equilibrium, specifically addressing whether such force couples can exist without violating the conditions of equilibrium. The scope includes theoretical considerations of static equilibrium, translational and rotational dynamics, and the implications of net forces and moments.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant recalls their teacher stating that a system in equilibrium cannot have momentum and questions the validity of disregarding force couples in this context.
  • Another participant clarifies that static equilibrium requires both translational and rotational equilibrium, suggesting that while net force can be zero, a net moment can still exist.
  • A participant proposes that for a force couple to exist in static equilibrium, there must be a negative moment equal to the magnitude of the force couple to maintain rotational equilibrium.
  • There is a discussion about the possibility of angular momentum change during a force couple, with some participants asserting that a net moment of zero would prevent such changes.
  • Participants explore the idea that a system can have zero net force and moment while still moving uniformly, indicating a nuanced understanding of motion in equilibrium.

Areas of Agreement / Disagreement

Participants express differing views on the implications of force couples in equilibrium, with some asserting that they can exist under certain conditions while others emphasize the constraints imposed by equilibrium definitions. The discussion remains unresolved regarding the precise conditions under which force couples can be considered in equilibrium.

Contextual Notes

Limitations include potential misunderstandings of equilibrium definitions, the dependence on specific conditions for force couples, and the unresolved nature of angular momentum changes in relation to net moments.

Inertigratus
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My teacher said that if a system is in equilibrium, then there cannot be any momentum. I asked, what if there are force couples?
He said that fource couples are disregarded, or they that are not possible in a system of equilibrium. If this is wrong, then I probably don't remember what he said exacty.
Hoping for a better explanation here. Can't there be two opposite forces of equal magnitude affecting the particle and therefor making the particle rotate while the system is in equilibrium?
 
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Static equilibrium generally means both translational and rotational equilibrium. Thus, about any point the net moment of all forces would be zero.

But it's certainly possible to have the net force equal to zero while having a net moment, as you point out.
 
So to have a force couple in a static equilibrium it means that there also has to be a negative moment the same magnitude as that of the force couple?
 
Inertigratus said:
So to have a force couple in a static equilibrium it means that there also has to be a negative moment the same magnitude as that of the force couple?
Yes. Otherwise it will not be in rotational equilibrium.
 
@doc atleast during the couple there will be momentum change (angular) or watever...
 
phyeinstein_c said:
@doc atleast during the couple there will be momentum change (angular) or watever...
Not if the net moment is zero everywhere.
 
oh yeah.. :P I am tubelight
 
it is also possible that net force (moment/torque ) is 0 but it continues in uniform motion??
 
phyeinstein_c said:
it is also possible that net force (moment/torque ) is 0 but it continues in uniform motion??
Definitely.
 
  • #10
ohkk... :)
 

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