# Is action and reaction instantanious?

by throng
Tags: action, instantanious, reaction
P: 2,258
 Quote by Andrew Mason 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??
Mentor
P: 17,293
 Quote by granpa 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 tē + 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. Here 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.
Mentor
P: 17,293
 Quote by WarPhalange 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.
Mentor
P: 17,293
 Quote by Andrew Mason 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").
Mentor
P: 17,293
 Quote by granpa 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 any one 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.
P: 8,613
 Quote by granpa 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.
HW Helper
P: 6,678
 Quote by granpa Kirchoffs law has nothing to do with inductance??
You may be thinking of Lenz's law.

AM
 P: 2,258 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 possess some kind of symmetry. possibly related to conservation laws and Noethers theorem. I'm just guessing at this point.
P: 8,613
 Quote by 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.
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/teachi...x/chap11-6.pdf
P: 2,258
 Quote by atyy 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/teachi...x/chap11-6.pdf

this hypothetical massless charged particle isnt a point change. it has finite diameter. and no I dont know whits holding it together. it doesnt matter. maybe superglue.
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P: 6,678
 Quote by 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.
There is, of course, no such thing as a massless charged particle. A massless particle travels at the speed of light relative to all inertial frames, so it cannot accelerate.

A particle with charge q in an electric field $\vec E$ experiences a force $\vec F$ = q$\vec E$. The only thing that affects its acceleration is its mass. $\vec F$ = q$\vec E$ = ma. There is no other force.

AM
P: 2,258
 Quote by Andrew Mason There is, of course, no such thing as a massless charged particle. A massless particle travels at the speed of light relative to all inertial frames, so it cannot accelerate. A particle with charge q in an electric field $\vec E$ experiences a force $\vec F$ = q$\vec E$. The only thing that affects its acceleration is its mass. $\vec F$ = q$\vec E$ = ma. There is no other force. AM

whoa. a moving charge has energy in its magnetic field. this energy must be supplied by the external force. it would not move at the speed of light.
P: 8,613
 Quote by granpa this hypothetical massless charged particle isnt a point change. it has finite diameter. and no I dont know whits holding it together. it doesnt matter. maybe superglue.
Xiaochao Zheng's notes that I linked to above also treated the non-point charge case.

Jammer, Concepts of Mass, Chapter 11

Acceleration-dependent self-interaction effects as a possible mechanism of inertia
Vesselin Petkov
http://arxiv.org/abs/physics/9909019
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P: 6,678
 Quote by granpa whoa. a moving charge has energy in its magnetic field. this energy must be supplied by the external force. it would not move at the speed of light.
?? A moving charge has energy in its magnetic field only relative to a charge in a different intertial frame of reference. The magnetic field disappears for a charge in the same intertial frame of reference.

A charged particle cannot move at the speed of light because all particles with charge have mass. Light will always travel at the speed of light, c, relative to a particle with mass.

The only inertia, or resistance to change in speed, of a charged particle in an electric field is due to the mass of the charged particle, as explained in my previous post. (At relativistic speeds, the apparent inertia increases due to relativistic effects).

AM
P: 8,613
 Quote by atyy Xiaochao Zheng's notes that I linked to above also treated the non-point charge case. Jammer, Concepts of Mass, Chapter 11 http://books.google.com/books?hl=en&...sult#PPA136,M1 Acceleration-dependent self-interaction effects as a possible mechanism of inertia Vesselin Petkov http://arxiv.org/abs/physics/9909019
Apparently a massless charged particle is not classically possible even if it is not a point charge. All discussions use F=dp/dt, ie. no change in momentum unless there is a net force. granpa: your superglue is referred to by Jackson (Classical Electrodynamics, Wiley 1998) as Poincare stresses.

References that seem to be regarded as standard, but not available for free are:
A.D. Yagjian, Relativistic Dynamics of a Charged Sphere, 2nd ed. Springer 2006
F. Rohrlich, Classical Charged Particles, 3rd ed. (World Scientific 2007)

On arXiv:
Radiation reaction of a classical quasi-rigid extended particle
Rodrigo Medina
http://arxiv.org/abs/physics/0508031
 P: 2,258 it doesnt need to be possible. its hypothetical. I'm not suggesting that mass really is due to self induction. I'm saying that one can look at it in the same way. lorentz spent a good deal of energy looking for a way to make mass be due to self induction. so apparently he didnt think it was an impossible idea.
P: 8,613
 Quote by granpa it doesnt need to be possible. its hypothetical. I'm not suggesting that mass really is due to self induction. I'm saying that one can look at it in the same way. lorentz spent a good deal of energy looking for a way to make mass be due to self induction. so apparently he didnt think it was an impossible idea.
All the links I gave above are referring to that idea of Lorentz and others, which they consider very seriously. I think you should study them before thinking I'm talking about something different.
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P: 6,678
 Quote by granpa it doesnt need to be possible. its hypothetical. I'm not suggesting that mass really is due to self induction. I'm saying that one can look at it in the same way. lorentz spent a good deal of energy looking for a way to make mass be due to self induction. so apparently he didnt think it was an impossible idea.
Perhaps you could give us a cite for the last statement.

Self induction is a phenomenon that occurs in a coil due to current flow. The expanding magnetic field from increasing current in one coil cuts across an adjacent coil and induces a current that opposes the increasing current.

Electrons do not experience self induction. You seem to be suggesting that there is something other than mass that causes a charged particle to resist changes in its motion.

AM

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