2 Reactions of 2 objects are not at equilibrium

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    Equilibrium Reactions
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Discussion Overview

The discussion revolves around the concept of equilibrium in the context of forces acting on a body placed on a smooth surface. Participants explore the conditions under which a body is considered to be in equilibrium, particularly focusing on the reactions between the body and the surface it rests on.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the reactions from two objects are not at equilibrium, suggesting a misunderstanding of the conditions for equilibrium.
  • Another participant requests clarification and examples to better understand the initial question regarding equilibrium.
  • A later reply attempts to clarify that if a body is on a smooth horizontal surface, the forces exerted by the surface and the body should result in equilibrium, provided all force vectors cancel out.
  • It is noted that Newton's Third Law states that every action has an equal and opposite reaction, but this does not imply that the forces acting on different objects contribute to equilibrium.
  • One participant explains that the coffee cup on a table is in equilibrium because the forces acting on it (the upward force from the table and the downward gravitational force) add up to zero, not because the forces between the cup and the table are equal.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of equilibrium, with some asserting that the situation described does represent equilibrium, while others seek further clarification on the initial question and its implications.

Contextual Notes

There is a lack of consensus on the definition of equilibrium as it pertains to the forces acting on the body and the reactions involved. The discussion highlights potential confusion regarding the application of Newton's Third Law in this context.

kira506
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Why are the reactions from two objects not at equilibrium ?
 
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I don't understand your question. In particular, I don't understand what you mean by "reactions [...] not at equilibrium". Can you please give a specific example?
 
jtbell said:
I don't understand your question. In particular, I don't understand what you mean by "reactions [...] not at equilibrium". Can you please give a specific example?


sorry , I was in a hurry XD
I meant if a body is placed on a smooth surface ,the surface exerts a reaction on the body and so does the body so why are they not at equilinrium , I know its because of the weight mg , I know , but I can't understand why , I mean the 2 reactions are enough bec. According to Newton) 3rd law , every action has a reaction equal in magnitude but opposite in direction , unless the values of reactions are not equal ?
 
I still cannot make sense of your question because "if a body is placed on a smooth surface ,the surface exerts a reaction on the body and so does the body" (assuming the surface is horizontal) then we do have an equilibrium situation. As long as all force vectors cancel, so there is no net force, then we have equilibrium. I also do not understand "unless the values of the reactions are not equal". That is certainly NOT part of Newton's 3rd law.
 
kira506 said:
so why are they not at equilinrium

You still haven't said what you mean by "not at equilibrium." Like HallsofIvy, I don't understand what you're really asking about.

Right now, my coffee cup is resting on the table next to my elbow. The table exerts an upwards force on the coffee cup. The coffee cup exerts a downwards force on the table. These two forces are equal in magnitude and opposite in direction, according to Newton's Third Law. The coffee cup is also in equilibrium, because it's stationary (not accelerating).

Note however, that the reason why the coffee cup is in equilibrium is not because the two forces named above add up to zero, because those two forces act on different objects. (Maybe this is what is confusing you?)

The coffee cup is in equilibrium because the forces that act on it add up to zero. One force is the upwards force exerted by the table. The other force is the downwards gravitational force exerted by the earth. These two forces add up to zero, therefore the coffee cup is in equlibrium.
 

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