Is action and reaction instantanious?

[pallidin]

Simultaneous, yes; instantaneous, no.

Sorry, I cannot extract meaning from those words.

OK, I'll try to express this as best I can...

1) Action MUST precede reaction otherwise there is a presumption of time-reversal.

2) However, initialization of the A/R event, being FIRST a potential, requires simultaneity and instantaneousness. Why? The potential of an eventual expression includes all aspects of it's original identity. What's important here is that this event is very brief; seemingly sudden and dissociated with the result.

3) The notion of "instantaneous" is hard to grasp. Much like the notion of "infinity"
We all expect each to be real and intuitively sound, but somehow we have difficulty embracing the concepts. A photon "ejected" from a bound-electron goes from zero to C in how much time? NONE. From this we have solid evidence of instantaneousness.

4) Now that one's neurons have been ripped-apart, I'll return to this:
Action MUST precede reaction otherwise there is a presumption of time-reversal.


As always, just my humble opinion... thanks.

 

[Vanadium 50]

Action MUST precede reaction otherwise there is a presumption of time-reversal.

This sounds awfully philosophical. I also don't quite understand the argument, or for that matter why "presumption of time-reversal" is a bad thing.

Earlier in the thread the very valuable notion of "pairs" was introduced. Momentum is conserved, so forces occur in pairs* that balance each other. Which one of the pair you call "action" and which one "reaction" is entirely convention. Switching them may sound funny, but the physics doesn't change.

This symmetry prevents you from saying "this one comes first". Just remember that forces are produced in pairs, and it's much clearer.



* I know that Newton's Third Law is more general than this, but taking the simplest case makes the argument easiest to follow.

 

[russ_watters]

3) The notion of "instantaneous" is hard to grasp. Much like the notion of "infinity"
We all expect each to be real and intuitively sound, but somehow we have difficulty embracing the concepts. A photon "ejected" from a bound-electron goes from zero to C in how much time? NONE. From this we have solid evidence of instantaneousness.

This is a simple misunderstanding of the definition of insstantaneous. Instantaneous is not typically regarded as zero time, it is a very small but finite amount of time.

an infinitesimal or very short space of time.

http://dictionary.reference.com/browse/instant

 

[Bad Monkey]

I think the OP's query is fairly simply stated.

The answer is that action and reaction occur simultaneously and thus yes the reaction is "instantaneous", although it is not really correct to think like that.

However the reaction might take some time to propagate.

Make sense?

I.e., a shock wave moving through matter (speed of sound?), or relativistic limits. So, for the former, an example is the long solid rod which when pushed reacts simultaneously, but the opposite end of the rod does not begin to translate until the shock has propagated along its length. For the latter, the sun disappearing example where Earth, ~8 light minutes away, does not 'react' until the relativistic programming of the universe gets around to enforcing its rules.

 

[Dale]

There is a lot of sloppy use of physics terms in this thread.

When physicists speak of "action" and "reaction" they are talking about Newton's third law. The simplified phrase "every action has an equal and opposite reaction" means that whenever an object A exerts a force f on another object B, B simultaneously exerts a force -f on A. These two forces are called an "action/reaction pair" or more correctly a "third-law pair". They always act simultaneously with no delay; otherwise momentum would not be conserved. In physics terms action does not come before reaction, they are two halves of the same interaction.

Everyone in this thread who is saying that action preceeds reaction is wrong (or at least not using the physics terms). What you can say is that cause preceeds effect.

Here is a concrete example illustrating the difference between action/reaction and cause-and-effect. A cue ball hits the 8-ball. The cue ball exerts a force (action) on the 8-ball and simultaneously the 8-ball exerts an equal and opposite force (reaction) on the cue ball. Due to these forces the cue ball decelerates and the 8-ball accelerates. A short time later the 8-ball goes into the corner pocket. The collision with the cue ball (cause) resulted in the 8-ball later falling into the pocket (effect).

Hope that helps.

 

[schroder]

OK, I'll try to express this as best I can...

1) Action MUST precede reaction otherwise there is a presumption of time-reversal.

2) However, initialization of the A/R event, being FIRST a potential, requires simultaneity and instantaneousness. Why? The potential of an eventual expression includes all aspects of it's original identity. What's important here is that this event is very brief; seemingly sudden and dissociated with the result.

3) The notion of "instantaneous" is hard to grasp. Much like the notion of "infinity"
We all expect each to be real and intuitively sound, but somehow we have difficulty embracing the concepts. A photon "ejected" from a bound-electron goes from zero to C in how much time? NONE. From this we have solid evidence of instantaneousness.

4) Now that one's neurons have been ripped-apart, I'll return to this:
Action MUST precede reaction otherwise there is a presumption of time-reversal.


As always, just my humble opinion... thanks.

1). I don’t see the necessity for time reversal in the case of simultaneity. Time can only elapse between two events. If two objects are acting simultaneously to some force, that is considered as one event, in the context of Newton’s third law.

2). I’m not sure what you mean by “initialization”? I hope you are not talking about intent, as that definitely goes beyond physics. The action/reaction is initialized as soon as the simultaneous event begins. Anything previous to that is potential and is not covered by the third law.

3). A photon is not ejected from an electron, at least not in the normal usage of the language. The photon is “created” by the energy given up by the electron. As soon as it is created it has the property of moving at the velocity c without any acceleration of the photon. The deceleration of the electron, and creation of the photon, does take a finite amount of time; it is not instantaneous.

 

[atyy]

So all action and reaction in the entire universe is within an instant.

And action is reaction.

Which is not even action, since it is zero in an instant.

Is this logically flawed?

And are we within the bounds of physics?

Action and reaction are not so much about cause and effect as about interaction. We think that we can push something without it pushing us back, but action and reaction says that's impossible. All actions are a part of interaction. When you jump up from the ground, did the ground push you up, or did you push the ground down? Some may prefer to say the ground pushed you up, because the effect of the ground on you is much more evident than your effect on the ground, but as everyone knows, you can't jump up without pushing on the ground.

Action and reaction makes sense only locally. Charges that interact at a distance do so through electromagnetic waves, in which case the action and reaction is not between the charges, but between each charge and the wave in its immediate vicinity. If the charges are close enough that we can approximate the speed of the wave to be infinite, then we can say that there is action and reaction between spatially separated charges.

 

[pallidin]

By default, action MUST precede reaction else there will be a system failure due to failure of system initialization.
What's so hard to understand about that?

 

[pallidin]

In furtherance, I hold considerable disregard to theories absent of experimental evidence or responsible conjecture.
As such, show to me a reaction that can exist apart from an prescriptive action.

 

[Dale]

By default, action MUST precede reaction else there will be a system failure due to failure of system initialization.
What's so hard to understand about that?

No, cause must precede effect. "Action" and "reaction" (3rd law pairs) are simultaneous.

In furtherance, I hold considerable disregard to theories absent of experimental evidence or responsible conjecture.

Me too. Newton's laws have over 300 years of accumulated scientific evidence supporting them. In particular, any experiment where momentum is conserved validates the third law. Here is an http://sdsu-physics.org/physics_lab/p182A_labs/indi_labs/Momentum.pdf" you can do yourself.

On the other hand, your assertion that one force in a third law pair (the "action") can exist without the other (the "reaction") has no experimental support. Such a situation would violate the conservation of momentum, so it should be easy to detect. Please give your experimental evidence supporting a violation of Newton's 3rd law.

 

[lightarrow]

It was said that action and reaction are simultaneous. Don't understand why; if two spatially separated events are simultaneous in a frame of reference, they are not in another moving with respect to the first.

 

[atyy]

It was said that action and reaction are simultaneous. Don't understand why; if two spatially separated events are simultaneous in a frame of reference, they are not in another moving with respect to the first.

In relativity, action and reaction only work for things interacting at the same spacetime point. A charge interacts with the electromagnetic field, not with another charge directly. Coulomb's law in which spatially separated charges interact directly is not relativistic.

 

[Dale]

Hi lightarrow, atyy's comments are correct. In relativity Newton's 3rd law remains intact with the understanding that all interactions are local so issues of simultaneity do not arise. In the example atyy provided, each object interacts only with the local EM field (photons). Any momentum leaving one object is carried by the photons to interact with the other object locally.

 

[peleus]

This is my understanding -

Action / Reaction are instant, this is Newton's third law. (Now referred to A/R). A/R is an instant transaction of forces if you like, don't forget though forces take time to take effect, we only see the effect of forces, not the transaction of them which takes place. An example of this is acceleration, we apply force to an item, and it gets faster, we maintain that force and it gets faster and faster and faster, we are seeing the time line of force being applied, the effect isn't instant.

Cause / Effect is what we actually see occur (Now referred to as C/E). C/E is what we actually perceive to be happening, the popular cue ball hitting the 8 ball example, the force applied to the 8 ball happens instantaneously when hit, however the 8 ball doesn't move for a very small amount of time until it accelerates.

Think of Newton's cradle. Although it looks as though the end balls move instantly, we know that its actually a very small time in between the first ball and end ball moving. The real thing which is happening (lets say there are 5 balls) is ball 1 is hitting ball 2, instantly the forces are transferred but it takes a fraction of a second for ball 2 to hit ball 3, repeats for ball 4 and ball 5.

The forces were transferred instantly between each ball as they hit, however the action on those forces take an amount of time, in this case a very small amount. This is why the action 1 -> action 2 and action 2 -> action 3 isn't the same as action 1 -> 3 as mentioned before.

Also it seems there is some confusion with which happens first, the action / reaction. In terms of forces they both happen at exactly the same time, because it is the forces being transferred, not the result action (C/E). This can get confusing when its something with motion hitting something static (i.e. Hand hitting a wall), because we always perceive the hand (action) to be hitting the wall (reaction), this isn't the case, the movements (arm / wall) aren't the subject of Newton's 3rd law, their contact is.

Let me ask you this, two vehicles are traveling head on at equal speed along a road. They crash together. Which one hits first? Which is the action and which is the reaction?

 

[Dale]

Let me ask you this, two vehicles are traveling head on at equal speed along a road. They crash together. Which one hits first? Which is the action and which is the reaction?

That is a good example, it highlights the arbitraryness of the designations "action" and "reaction".

 

[granpa]

think of gravity acting on a mass. gravity is the action. inertia is the reaction. both occur at exactly the same time and are perfectly balanced yet the mass moves because inertia is proportional to rate of change of velocity.

 

[Vanadium 50]

By default, action MUST precede reaction else there will be a system failure due to failure of system initialization.
What's so hard to understand about that?

Quite a lot. To be honest, I don't know what you are talking about. You use terminology that is very non-standard, such as

"system failure due to failure of system initialization"

and (from a previous message)

"presumption of time reversal"

 

[Vanadium 50]

think of gravity acting on a mass. gravity is the action. inertia is the reaction

I'm afraid that's not correct. If you have a rock on the ground, the downward force on the rock due to gravity is one element of the action-reaction pair, and the upward force provided by the ground keeping the rock from falling farther in is the other element.

 

[granpa]

I'm afraid that's not correct. If you have a rock on the ground, the downward force on the rock due to gravity is one element of the action-reaction pair, and the upward force provided by the ground keeping the rock from falling farther in is the other element.

who said it was on the ground?

 

[Vanadium 50]

Inertia is mass. It is not a "reaction" to anything.

 

[Dale]

Inertia is not a force, so it is not part of a 3rd law pair.

If an object is in free fall there is one 3rd-law pair, the gravity of the Earth pulling down on the object and the equal and opposite gravity of the object pulling up on the earth.

If an object is at rest on the surface there are two 3rd-law pairs, the one mentioned above plus the contact force pushing up on the object and the contact force pushing down on the ground.

 

[granpa]

well let's say then that an astronaut begins pulling in a satellite with a rope. the astronauts muscles are producing a force which is the action. the inertia of the satellite is the reaction which exactly equals it.

inertia is equivalent to indiction in electronics.

 

[Doc Al]

well let's say then that an astronaut begins pulling in a satellite with a rope. the astronauts muscles are producing a force which is the action.

If the astronaut pulling on the rope is the action, the reaction is the rope pulling back on the astronaut. (True, the astronaut wouldn't be able to exert a force on the rope/satellite if they had no mass, but that's not the same thing as saying "inertia" is the reaction force.)

 

[granpa]

If the astronaut pulling on the rope is the action, the reaction is the rope pulling back on the astronaut. (True, the astronaut wouldn't be able to exert a force on the rope/satellite if they had no mass, but that's not the same thing as saying "inertia" is the reaction force.)

semantics.

what about induction? do inductors not create a force (that is proportional to the change in velocity of the electrons) that opposes the applied voltige?

 

[Vanadium 50]

As was pointed out, I crossed two pairs in my example.

Nevertheless, you can't have "inertia" as one member of the pair, and "force" as the other. They have to be the same kind of thing, and these don't even have the same units. It's more than semantics.

 

[granpa]

obviously by 'inertia' I meant the 'force' associated with inertia. most people would have been able to figure that out by themselves.

 

[jtbell]

If you have a rock on the ground, the downward force on the rock due to gravity is one element of the action-reaction pair, and the upward force provided by the ground keeping the rock from falling farther in is the other element.

That's not an action-reaction pair. An action-reaction pair can always be described as "the force that A exerts on B" and "the force that B exerts on A."

In your example, your downward force is "the (gravitational) force that the Earth exerts on the rock" and your upward force is "the (contact) force that the Earth exerts on the rock."

In this example, there are actually two action-reaction pairs:

1. The (gravitational) force that the Earth exerts on the rock (downward), and the (gravitational) force that the rock exerts on the Earth (upward).

2. The (contact) force that the Earth exerts on the rock (upward), and the (contact) force that the rock exerts on the Earth (downward).

 

[Vanadium 50]

Yes, I know. That's what I meant when I said "I crossed two pairs" in my example.

 

[Type_R]

The confusion started when he said Third law was
"For every action there's an opposite and equal reaction"


no No nonoNOnono


"For every force there's an opposite and equal force"

 

[Dale]

obviously by 'inertia' I meant the 'force' associated with inertia. most people would have been able to figure that out by themselves.

What force is associated with inertia? Are you talking about "ficticious" forces in non-inertial reference frames (e.g. the Coriolis force in a rotating reference frame). I don't know of any other forces associated with inertia, and these "fictitious" inertial forces don't obey Newton's 3rd law.

 

[granpa]

the force that is equal and opposite to the force you apply to a mass. same as an inductance resisting an applied voltage. there is a force proportional to change in current (speed of electrons) just as the force associated with inertia is proportional to change in velocity of mass.
you can call it fictitious if you want. but its a fact that all forces must always be balanced by an equal and opposite force.

 

[Dale]

the force that is equal and opposite to the force you apply to a mass. ... you can call it fictitious if you want. but its a fact that all forces must always be balanced by an equal and opposite force.

No, that is definitely not a ficticious force. That is just a normal third-law force.

Sorry about the confusion. I just have never heard the third-law force described as associated with inertia before. Usually inertia is a term that describes Newton's first law, not the third law, and forces aren't introduced until the second law.

 

[Andrew Mason]

the force that is equal and opposite to the force you apply to a mass. same as an inductance resisting an applied voltage. there is a force proportional to change in current (speed of electrons) just as the force associated with inertia is proportional to change in velocity of mass.
you can call it fictitious if you want. but its a fact that all forces must always be balanced by an equal and opposite force.

The force that is equal and opposite to the force of gravity of a mass M on mass m is the force of gravity of mass m on mass M. They are equal and opposite and act simultaneously.

Newton's third law is equivalent to the law of conservation of momentum. In other words, if action and reaction pairs were not equal, opposite and simultaneous, momentum would not be conserved. Conservation of momentum has been proven to apply without exception, even at the highest relativistic speeds.

If the two forces (the action and reaction pairs) did not act simultaneously, they would not be equal and opposite in all interial frames of reference. If they were not equal and opposite in all inertial frames of reference, momentum would not be conserved at relativistic speeds.

AM

 

[granpa]

The force that is equal and opposite to the force of gravity of a mass M on mass m is the force of gravity of mass m on mass M. They are equal and opposite and act simultaneously.

Newton's third law is equivalent to the law of conservation of momentum. In other words, if action and reaction pairs were not equal, opposite and simultaneous, momentum would not be conserved. Conservation of momentum has been proven to apply without exception, even at the highest relativistic speeds.

If the two forces (the action and reaction pairs) did not act simultaneously, they would not be equal and opposite in all interial frames of reference. If they were not equal and opposite in all inertial frames of reference, momentum would not be conserved at relativistic speeds.

AM

yes that true. but i wasnt referring to gravity. just force.

 

[Andrew Mason]

yes that true. but i wasnt referring to gravity. just force.

There is an important difference between gravity and all other forces, in that there is no inertial effect with a gravitational force.

If all forces were "balanced" by an equal and opposite force there would be no "net force" so there would be no acceleration. Inertia is not a force. It is resistance to change in motion. It only appears to be a force (inertial effect) in the frame of reference of the accelerating body (which is not an inertial frame) when an unbalanced force (other than gravity) is applied to the body. It does not appear to be a force in the inertial frame.

AM