Is action and reaction instantanious?

[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

 

[granpa]

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

there is inertia with gravity. its just that the force (or rather the field) is proportional to the mass.

yes there could be, would be, and is acceleration even though the net forces are zero. that's exactly what I'm saying. if what you say was true then kirchhoffs law would mean that no current could ever flow.

 

[granpa]

imagine a hypothetical massless charged particle. if acted on by an external electric field it would begin to accelerate. us it did so it would create a magnetic field around itself. this changing magnetic field would produce a force upon the particle that would exactly balance the force from the external electric field. I certainly hope you wouldn't say that that force wasnt a real force.

the behavior of this 'self inductance' is almost identical to the behavior of mass. indeed it was once hypothesized that all mass was the result of self inductance.

so the force of gravity acting on one mass due to another must equal the force of gravity acting on the second due to the first. but the sum of all forces, including those due to inertia, acting on a single object must also be zero. both statements are true and complement one another.further, while what has been said about gravity isn't exactly wrong nevertheless there is no reason to see gravity as being any different from other forces. replace mass with change and reverse the direction of the resulting force and you have the electric field. (excluding relativistic effects like time dilation of course.)

 

[Dale]

yes there could be, would be, and is acceleration even though the net forces are zero.

Newton disagrees. If f=0 then a=0 by the second law.

 

[granpa]

and the massless changed particle?

 

[granpa]

no disagreement at all

f=ma (ma=inertial force)
f=f
action=reaction
f-f=0

http://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws
The directed sum of the electrical potential differences around any closed circuit must be zero.

 

[WarPhalange]

Newton disagrees. If f=0 then a=0 by the second law.

So when two masses are attracted to each other via gravity, there is a net force? Because obviously a =/= 0, but I can't figure out how there can be a net force.

 

[granpa]

there is a net force acting on each individual mass but only if you disregard the force due to inertia.

 

[Andrew Mason]

there is a net force acting on each individual mass but only if you disregard the force due to inertia.

In physics, force is defined as dp/dt. You will need a new definition of force if you wish to call inertia a force.

Consider a system that experiences no external force. Masses within that system can exert forces on each other and cause momentum of the individual masses to change with time. All Newton's third law says is that the sum of those forces ie. \sum dp_i/dt must be 0. If you add interia as a force, they would not sum to 0.

AM

 

[granpa]

In physics, force is defined as dp/dt. You will need a new definition of force if you wish to call inertia a force.

Consider a system that experiences no external force. Masses within that system can exert forces on each other and cause momentum of the individual masses to change with time. All Newton's third law says is that the sum of those forces ie. \sum dp_i/dt must be 0. If you add interia as a force, they would not sum to 0.

AM

other forces might be defined that way. but that doesn't contradict what i said.

obviously you can always define all the other forces in terms of any single force.

maybe it would be better to think of force as being defined in terms of energy.

they would not sum to zero? they are zero everywhere so why would the sum not be zero over the whole?

 

[Andrew Mason]

there is inertia with gravity. its just that the force (or rather the field) is proportional to the mass.

I did not say there was no inertia. I said there is no inertial effect. The orbiting astronaut feels no centrifugal "force".

AM

 

[Andrew Mason]

and the massless charged particle?

I am not aware of any massless charged particle. Can you give us an example?

AM

 

[granpa]

I did not say there was no inertia. I said there is no inertial effect. The orbiting astronaut feels no centrifugal "force".

AM

people don't 'feel' the force of air pressure either but its there.

 

[granpa]

read post 72

notice the third word

 

[Doc Al]

so the force of gravity acting on one mass due to another must equal the force of gravity acting on the second due to the first. but the sum of all forces, including those due to inertia, acting on a single object must also be zero.

Viewed from the non-inertial frame of the accelerating mass, one must introduce fictitious inertial forces to make use of Newton's laws. Is that what you mean by "forces due to inertia"?

Such "forces" are just artifacts of viewing things from a non-inertial frame.

 

[granpa]

I can't understand why so many people have difficulty with this. It seems pretty obvious to me. simply think of mass the way you would self inductance.

no relativity has nothing to do with it.

 

[Andrew Mason]

other forces might be defined that way. but that doesn't contradict what i said.

maybe it would be better to think of force as being defined in terms of energy.

It may be more useful to use energy rather than force in some situations. In nuclear physics that is exactly what is done: we speak of binding energies and collision energies rather than forces.

they would not sum to zero? they are zero everywhere so why would the sum not be zero over the whole?

I don't follow you here. Kinetic energies are always greater than zero. So a system experiencing no external force may still have significant energy. Think of a star that undegoes a supernova.

AM

 

[granpa]

I don't follow you here. Kinetic energies are always greater than zero. So a system experiencing no external force may still have significant energy. Think of a star that undegoes a supernova.

AM

I don't even know how to begin to answer that. Its clear to me that you are making it all much much more complicated than it really is. stop trying so hard and maybe you will get what I'm saying

are you familiar at all with Kirchoffs law?

 

[Andrew Mason]

people don't 'feel' the force of air pressure either but its there.

This is not equivalent. A person on the surface of the Earth can measure atmospheric pressure by doing a local experiment. An orbiting astronaut cannot detect gravity by doing a local experiment.

AM

 

[granpa]

This is not equivalent. A person on the surface of the Earth can measure atmospheric pressure by doing a local experiment. An orbiting astronaut cannot detect gravity by doing a local experiment.

AM

once again, I don't even know how to begin to answer. I see no relevance and no contradiction to anything I've said. I think you think I'm saying something complicated when all I'm saying is really very simple.

 

[Andrew Mason]

I don't even know how to begin to answer that. Its clear to me that you are making it all much much more complicated than it really is. stop trying so hard and maybe you will get what I'm saying

are you familiar at all with Kirchoffs law?

I am not sure how you are using it. Kirchoff's law does not have anything to do with inertia or its electrical equivalent.

AM

 

[granpa]

I am not sure how you are using it. Kirchoff's law does not have anything to do with inertia or its electrical equivalent.

AM

Kirchoffs law has nothing to do with inductance??

 

[Dale]

f=ma (ma=inertial force)
f=f
action=reaction
f-f=0

OK, first the action=reaction part is simply wrong. Action and reaction refer to Newton's 3rd law and action-reaction pairs act on different bodies. F=ma refers to Newton's 2nd law and acts on the same body. So even if you want to call ma a force it is not a reaction force to f since it is acting on the same body that f is acting on.

Now, the remainder of what you did is mathematically correct and leads to the trivially true assertion 0=0. This algebraic manipulation can be done for any formula.

W = f.d
W = W
0 = 0

x(t) = 1/2 a t² + v0 t + x0
x(t) = x(t)
0 = 0

Yes you can always do it, but in doing so you completely lose the meaning of the original expression.

It looks like you are using the D'Alembert approach which can be useful in certain circumstances, but you need to understand what it is doing. https://www.physicsforums.com/showthread.php?t=219929" is a thread on the subject. It should generally be avoided because of the conceptual confusion it causes, and it should only be applied when the specific problem demands it.

In the end, if you are talking about inertial reference frames then ma is not a force and the body accelerates. If you are talking about the non-inertial rest frame of an accelerating body (as D'Alembert does) then there is a fictitious inertial force (like the Coriolis force) of magnitude ma and the body does not accelerate. This force that exists only in the non-inertial frame does not follow Newton's 3rd law because its source is the non-inertial reference frame and not an interaction with another object.

 

[Dale]

So when two masses are attracted to each other via gravity, there is a net force? Because obviously a =/= 0, but I can't figure out how there can be a net force.

There is a net force on each object (Newtonian gravity). So the Earth pulls the moon "down" and the moon pulls the Earth "up". The force on the moon is equal and opposite to the force on the earth, per Newton's 3rd law.

 

[Dale]

In physics, force is defined as dp/dt. You will need a new definition of force if you wish to call inertia a force.

Consider a system that experiences no external force. Masses within that system can exert forces on each other and cause momentum of the individual masses to change with time. All Newton's third law says is that the sum of those forces ie. \sum dp_i/dt must be 0. If you add interia as a force, they would not sum to 0.

This is correct. Inertia is only a force in non-inertial reference frames where it appears as a so-called ficticious force. In non-inertial reference frames the momentum of an isolated system is not conserved (hence the designation "non-inertial").

 

[Dale]

they would not sum to zero? they are zero everywhere so why would the sum not be zero over the whole?

Because they are not zero everywhere. You can choose a non-inertial frame where anyone object is at rest, but in that reference frame an object which is not experiencing any real forces will still experience the inertial force and will therefore accelerate. The net effect is a violation of the conservation of momentum for an isolated system, or equivalently, a violation of Newton's 3rd law. This is simply what happens in non-inertial frames.

By the way, none of this has anything to do with relativity, this is all just Newtonian mechanics.

 

[atyy]

f=ma (ma=inertial force)
f=f
action=reaction
f-f=0

I have no idea what inertia is (and don't wish to find out), so I am not addressing that. But I think some subscripts are missing on the equations.

Consider a two particle system:
m₁, v₁, a₁: mass, velocity and acceleration of particle 1
m₂, v₂, a₂: mass, velocity and acceleration of particle 2
F₁₂: force on particle 1 due to the effect of particle 2
F₂₁: force on particle 2 due to the effect of particle 1

F₁₂=m₁a₁ [E1: Newton's 2nd law for particle 1]
F₂₁=m₂a₂ [E2: Newton's 2nd law for particle 2]
F₁₂=-F₂₁ [E3: Newton's 3rd law, action and reaction are equal and opposite]

We can rearraange E3 as you did: F₁₂+F₂₁=0 [E3b]

From E3 or E3b, neither F₁₂ nor F₂₁ is necessarily zero, only equal and opposite. From E1 and E2, assuming m₁ and m₂ positive and constant, F₁₂ and F₂₁ must be zero only if a₁ or a₂ are zero.

But we can combine E3b with E1 and E2:
F₁₂+F₂₁=m₁a₁+m₂a₂=d(m₁v₁+m₂v₂)/dt=0,
which says that there is no net force on both particles considered together (not separately) and that total momentum does not change over time, ie. momentum is conserved.

 

[Andrew Mason]

Kirchoffs law has nothing to do with inductance??

You may be thinking of Lenz's law.

AM

 

[granpa]

nobody has addressed my very simple question. what about a hypothetical massless charged particle? it has self inductance so it would accelerate under the influence of an external field in exactly the same way that a massive particle would. the force due to self inductance exactly balancing the force due to the external field. net force is zero yet it still accelerates.

thats all I'm saying. I'm comparing the behavior of mass to the behavior of self inductance.

as for defining force/mass, I can imagine a video game like universe in which time distance and velocity are all well defined but possessing nothing that we would recognize as force or mass. so I would guess that both arise simultaneously if the system possesses some kind of symmetry. possibly related to conservation laws and Noethers theorem.

I'm just guessing at this point.

 

[atyy]

nobody has addressed my very simple question. what about a hypothetical massless charged particle? it has self inductance so it would accelerate under the influence of an external field in exactly the same way that a massive particle would.

I think this is not quite what you had in mind, but is it close?
Xiaochao Zheng's notes on "Radiation reaction and electron's self energy -- an unsolved problem" http://www.jlab.org/~xiaochao/teaching/PHYS343/tex/chap11-6.pdf

 

[granpa]

I think this is not quite what you had in mind, but is it close?
Xiaochao Zheng's notes on "Radiation reaction and electron's self energy -- an unsolved problem" http://www.jlab.org/~xiaochao/teaching/PHYS343/tex/chap11-6.pdf

this hypothetical massless charged particle isn't a point change. it has finite diameter. and no I don't know whits holding it together. it doesn't matter. maybe superglue.