Newton's third Law - action reaction pair

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Discussion Overview

The discussion centers around the concept of action-reaction pairs as described by Newton's third law, particularly in the context of a train engine pulling a buggy. Participants explore how mass and friction influence the forces involved and the resulting accelerations of the objects.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about whether the action-reaction pair depends on mass, questioning if the force applied by the buggy on the engine is equal to the force exerted by the engine.
  • Another participant agrees that the force the buggy applies on the train engine is also F, but notes that the accelerations of the two objects cannot be the same due to other forces acting on them.
  • A question is raised about the role of friction in this scenario, suggesting it may complicate the interaction.
  • Further clarification is provided that while the buggy and engine are connected, their accelerations can differ due to additional forces, such as friction and rolling resistance.
  • One participant emphasizes that Newton's second law is often mis-stated and should be understood in the context of the sum of forces acting on an object.
  • It is noted that in the train-buggy example, the interaction force can still obey Newton's third law while the objects accelerate differently due to other acting forces.

Areas of Agreement / Disagreement

Participants generally agree on the definition of action-reaction pairs but express differing views on how mass and friction affect the forces and accelerations involved. The discussion remains unresolved regarding the implications of these factors in the specific scenario presented.

Contextual Notes

Participants highlight the complexity introduced by additional forces such as friction and rolling resistance, which may not be fully accounted for in their initial considerations. There is also a mention of the need to consider the sum of forces acting on each object, which complicates the straightforward application of Newton's laws.

caligirl
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Hi,

I am a little confused with the concept of action reaction pair of forces. Does this depend on mass?

For example, if a train engine is pulling a buggy with force F, what would be the force applied by the buggy on the engine? The masses of the two are different and there is force of friction on the buggy.


I know that for 2 forces to be action reaction pair, they have to act on different objects. Does this mean that the force the buggy applies on the train engine is also F (in magnitude), but since mass is different, the acceleration would vary? I am not sure if I fully understand the concept.
 
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caligirl said:
Hi,

I know that for 2 forces to be action reaction pair, they have to act on different objects. Does this mean that the force the buggy applies on the train engine is also F (in magnitude), but since mass is different, the acceleration would vary?

The force that the buggy applies on the train engine is also F. Since the buggys is attached to the train their accelerations can't be the same. This can happen because the force F isn't the only force on the train or on the buggy.
 
Does friction play a role in this situation?
 
willem2 said:
Since the buggys is attached to the train their accelerations can't be the same.
I guess you mean "can't be different"?
 
caligirl said:
Does friction play a role in this situation?
Both the engine and buggy can have rolling resistance, but to simplify you can neglect that. The engine has also a propelling force which is static friction (traction). For both the engine and buggy the sum of all forces acting on them must produce the same acceleration.
 
caligirl said:
I know that for 2 forces to be action reaction pair, they have to act on different objects. Does this mean that the force the buggy applies on the train engine is also F (in magnitude), but since mass is different, the acceleration would vary? I am not sure if I fully understand the concept.
Yes, provided that is the only force acting. Remember, Newton's 2nd law is often mis-stated as f=ma, but it is actually ∑f=ma.

If you have only one force acting on an object and only the reaction force acting on the other object then the accelerations will necessarily be in opposite directions, and the accelerations will be different magnitudes if the masses differ. Consider, for example, the Earth and moon interacting through gravity.

In the train-buggy example, there are other forces acting on each object, so you can have their interaction force obey the third law (equal and opposite) while having them accelerate the same. In many cases you can use that fact to determine the other forces.
 

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