Observer 1 & 2 View Force on Subject A: Analysis & Resolution

[Andrew Mason]

In a free-falling NIRF (in a uniform gravitational field) a free-falling object has a=0. Therefore, by Newton's 2nd law the net force on the object must be 0.

In other words, we are assuming that the NIRF is an IRF and we are trying to make Newton's 2nd law work.

In Newtonian mechanics it is acted on only by a real gravitational force of -mg.

This is where I see a problem. I don't see how you can detect that gravitational force except by reference to a genuine IRF. There is no way to detect it by reference only to the NIRF. In the free-falling NIRF a uniform gravitational field is undetectable.

If it is undetectable I am having difficulty understanding why you need a pseudo force to counter-act it or explain it. Otherwise, what you are saying is perfectly reasonable and rather obvious.

AM

 

[A.T.]

In other words, we are assuming that the NIRF is an IRF and we are trying to make Newton's 2nd law work.

I told you that in post #18

In the free-falling NIRF a uniform gravitational field is undetectable.

Gravity is still a real force that exits in every frame. That is a nice simple law, easy to remember. You don't have to worry about detectability.

If it is undetectable I am having difficulty understanding why you need a pseudo force to counter-act it or explain it.

Every accelerated frame has an inertial force -ma. That is a nice simple law, easy to remember. You don't have to worry about what real forces act.

If all legal laws were so simple without special cases, then lawyers would be superfluous.

 

[Dale]

In other words, we are assuming that the NIRF is an IRF and we are trying to make Newton's 2nd law work.

What we are trying to do is to correctly describe physics in the NIRF. This requires modifications to the equations from what you would have if the frame were inertial. Those modifications are the pseudo forces.

This is where I see a problem. I don't see how you can detect that gravitational force except by reference to a genuine IRF. There is no way to detect it by reference only to the NIRF. In the free-falling NIRF a uniform gravitational field is undetectable.

If it is undetectable I am having difficulty understanding why you need a pseudo force to counter-act it or explain it. Otherwise, what you are saying is perfectly reasonable and rather obvious.

I understand the distinction you are making. It isn't really a distinction between an IRF and a NIRF, but rather a distinction between the Newtonian and relativistic definition of "inertial". My comments above were in the context of the Newtonian definition of "inertial", but personally I agree with you in my preference for the relativistic definition.

 

[DrStupid]

In other words, we are assuming that the NIRF is an IRF and we are trying to make Newton's 2nd law work.

We are simply using Newton's 2nd law.

I don't see how you can detect that gravitational force

You only have to detect the bodies including their mass. Than the gravitational forces are given by Newton's Law of gravitation.

except by reference to a genuine IRF.

Of course this works too. Just choose a frame of reference where all forces cancel each other out. All remaining forces are also present in every other frame of reference.

 

[Zula110100100]

I think the problem is whether or not to "forget" that the NIRF is accelerating by a certain amount(g for free-fall) if you do not forget, then gravity is what keeps you in the frame, but I think as described in Newtonian physics, you do forget, so an anti-gravity is required to counteract the known gravity, also with the acceleration of the frame forgotten, a force is required to move the Earth up to meet me.

The question It seems is why can't I just remember I am using an accelerated frame K relative to an inertial frame k and add the requirement that to appear at rest in this frame there must be a real force giving acceleration equal to the acceleration of K?