Is action and reaction instantanious?

[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