Newton's Third Law: Action Equals Opposite Reaction

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Discussion Overview

The discussion revolves around Newton's Third Law of Motion, specifically the interpretation of action and reaction forces in various scenarios. Participants explore the implications of the law in contexts involving friction and acceleration, questioning the logic behind the application of forces on objects.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant asserts that if a force of 10N is applied to an object, it should not move due to an equal and opposite reaction from friction, questioning the correctness of this logic.
  • Another participant challenges this view, suggesting that the formulation of Newton's Third Law needs to consider the interaction between two bodies, emphasizing that the friction force is not the reaction to the applied force.
  • Several participants explain that the equal and opposite reaction refers to the force exerted by the object back on the person applying the force, which does not prevent the object from moving if other forces, like friction, are also acting on it.
  • There is a discussion about the difference in outcomes when applying force in different environments, such as on a smooth surface versus a sticky table, highlighting the role of friction in the interaction.
  • Participants note that the forces acting on an object determine its acceleration, and the reaction forces do not directly affect the object's motion.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of Newton's Third Law and its application in scenarios involving friction. Multiple competing views remain, particularly concerning the role of friction and the nature of action-reaction pairs.

Contextual Notes

Some participants highlight the importance of context, such as the presence of friction or the environment (e.g., deep space) when discussing the implications of Newton's Third Law. There are unresolved assumptions about the definitions of forces and interactions in different scenarios.

Ajit Kumar
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(a) Newtons III law : Every action has equal and opposite reaction

By (a), if I apply a force of 10N on an object, it sholudnt move, as it has equal reaction, by friction of -10N so as to stop it.

Isnt that logic correct?
 
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Ajit Kumar said:
(a) Newtons III law : Every action has equal and opposite reaction
You should find a more precise formulation. How many bodies are involved in Newtons III Law, and how many in your scenario?
 
The equal and opposite reaction is due to the second body acting on the first.

Another statement of Newton's third law for two objects A and B would be...
"the force on A due to B is equal and opposite to the force on B due to A"

If I push on a block of ice on a smooth floor, the ice will accelerate.

The forces on that block of ice, are:
-a large applied force due to my pushing,
-a smaller friction force in the opposite direction,
-gravity pulling down,
-and the smooth surface pushing up on the object, so that it does not sink through the floor.

However, as I apply a 10 N force on the block, the block is applying a 10 N force on me, pushing back on me exactly as much as I on it.

There might seem to be a sort of paradox here, but the important thing to consider is that it's only the forces acting on an object that will determine its acceleration. The forces it imparts on other objects have no direct effect.
 
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Ajit Kumar said:
(a) Newtons III law : Every action has equal and opposite reaction
That is a very poor formulation of Newton's third law, which states that "When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body .

Ajit Kumar said:
By (a), if I apply a force of 10N on an object, it sholudnt move, as it has equal reaction, by friction of -10N so as to stop it.

Isnt that logic correct?
It is totally wrong. The friction is not reaction to your push. What happens if you push something on ice? will not it move?
Friction is force of interaction between bodies in contact. That force opposes relative motion.
 
jfizzix said:
The equal and opposite reaction is due to the second body acting on the first.

Another statement of Newton's third law for two objects A and B would be...
"the force on A due to B is equal and opposite to the force on B due to A"

If I push on a block of ice on a smooth floor, the ice will accelerate.

The forces on that block of ice, are:
-a large applied force due to my pushing,
-a smaller friction force in the opposite direction,
-gravity pulling down,
-and the smooth surface pushing up on the object, so that it does not sink through the floor.

However, as I apply a 10 N force on the block, the block is applying a 10 N force on me, pushing back on me exactly as much as I on it.

There might seem to be a sort of paradox here, but the important thing to consider is that it's only the forces acting on an object that will determine its acceleration. The forces it imparts on other objects have no direct effect.

So what I am missed in my logic?
 
Ajit Kumar said:
So what I am missed in my logic?
You stated that you are pushing on an object with a 10N force.

As everyone here has tried to point out, the equal and opposite reaction is the 10N force of the object on you. But that is a force on you, not a force on the object. The object is still subject to the 10N force and can still move as a result of that force.
 
jbriggs444 said:
But that is a force on you, not a force on the object. The object is still subject to the 10N force and can still move as a result of that force.
...or not, if there is also friction. The point you are missing, Ajit, is that the friction interaction is a second force pair. This is a very common misconception.
 
Ajit Kumar said:
So what I am missed in my logic?
There is a massive difference between conducting the experiment in deep space, where you can say there is only the 10N you apply and when the object is on a sticky table, where the 10N of friction force also is introduced.
The Third Law applies in both cases, though. The hand providing the 10N force will 'feel' a 10N force pushing back at it in both cases. That is the 'reaction force' that people talk about. Of course, the hand, out in space will be accelerating the object so you will have to chase after it in order to maintain your 10N force on it (and its reaction).
 

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