What puts particles in motion?

In summary, the conversation discusses the concept of action and reaction as stated by Newton. It is pointed out that the labels for these forces are arbitrary and can be interchanged without changing the physics. The conversation also touches on the finite speed of causality, the violation of Newton's third law in certain cases involving electromagnetism, and the role of the electromagnetic field in conserving momentum. The conversation concludes with a reminder to keep discussions on topic and examples of how the concept of action and reaction can be problematic in certain scenarios.
  • #1
DrAupo1
6
0
My grandson asked me this question:
If in fact,as Newton said,all actions are accompanied by an equal and opposite reaction, then there can be no single actions in the universe;all actions must therefor be part of a reaction between two opposite and equal actions,which are in fact also reactions.
Any thoughts on this are welcome.
 
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  • #2
Ok...so, what does that do for us? Is that physics or philosophy?
 
  • #3
Yes, it is well known that the labels "action" and "reaction" are completely arbitrary and can be equally arbitrarily swapped without any change in the physics. Most experts just identify a pair of forces as a "third law pair" and never even bother to identify one or the other as the "reaction".

If you search you should find many threads where this is discussed already.
 
  • #4
Just remember that Einstein showed us that there's a finite speed of causality...there's no such thing as instantaneous action at a distance. Push the Sun and it'll take a while for the Earth to notice. Also, if I remember correctly, Newton's action/reaction force pairings are violated in certain cases involving electromagnetism.
 
  • #5
Please give examples of an electromagnetic violation of action/reaction.
 
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  • #6
"Please give examples of an electromagnetic violation of action/reaction."

As for electromagnetism, imagine two charged particles moving at one another. The electric force will be equal but opposite, but the magnetic force won't be in opposite directions. This seemingly violates conservation of momentum (which is pretty much what Newton's third law is all about), but it's salvaged by allowing the electromagnetic field itself to possesses momentum.

Edit: I shouldn't have said moving directly at one another, the magnetic force would be zero in that case. A better example would be two positively charged particles, one moving on the x-axis one moving on the y-axis, both moving toward the origin. The electric force will be equal but opposite, the magnetic forces will be in the positive y and positive x directions.
 
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  • #7
Closed pending moderation.

Edit: the thread is reopened. Several posts have been edited or deleted to remove off topic content. Discussions about entanglement belong in the Quantum Mechanics forum and discussions about the big bang belong in the Cosmology forum. Neither belong in a thread on Newton's third law.
 
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  • #8
Megaquark said:
"Please give examples of an electromagnetic violation of action/reaction."

As for electromagnetism, imagine two charged particles moving at one another. The electric force will be equal but opposite, but the magnetic force won't be in opposite directions. This seemingly violates conservation of momentum (which is pretty much what Newton's third law is all about), but it's salvaged by allowing the electromagnetic field itself to possesses momentum.

Edit: I shouldn't have said moving directly at one another, the magnetic force would be zero in that case. A better example would be two positively charged particles, one moving on the x-axis one moving on the y-axis, both moving toward the origin. The electric force will be equal but opposite, the magnetic forces will be in the positive y and positive x directions.
Imagine the two particles at rest.What action puts them in motion?
 
  • #9
DrAupo1 said:
Imagine the two particles at rest.What action puts them in motion?
This is precisely why this terminology is discarded. A gravitational force could put them in motion, but there is no reason to assign one or the other as the "action". Similarly with other forces.
 

What does the phrase "All actions are reactions" mean?

The phrase "All actions are reactions" means that every action taken by an object or organism will result in a corresponding reaction. This is also known as Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction.

Is "All actions are reactions" a scientific law?

Yes, "All actions are reactions" is a scientific law, specifically Newton's Third Law of Motion. This law has been tested and proven through numerous experiments and observations, and it is a fundamental principle in the field of physics.

How is "All actions are reactions" related to conservation of momentum?

"All actions are reactions" is closely related to the principle of conservation of momentum, which states that the total momentum in a closed system will remain constant. When an action is taken, the resulting reaction will have an equal and opposite momentum, thus conserving the overall momentum of the system.

Can "All actions are reactions" be applied to non-physical interactions?

Yes, "All actions are reactions" can be applied to non-physical interactions, such as social interactions or psychological reactions. In these cases, the action may not have a physical force, but there will still be a corresponding reaction or consequence.

Are there any exceptions to "All actions are reactions"?

There are no known exceptions to "All actions are reactions" in the physical world. However, in certain scenarios, the reaction may not be readily apparent or may not occur immediately. For example, the reaction of an object to being pushed may not be visible until it reaches a solid surface or encounters another force.

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