# Inertial Frame Of Reference and free fall.

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And Einstein realized in developing GR that freefall is an inertial frame

As seen from someone standing on the ground, freefall means that the falling astronaut is accelerating towards the Earth.

It seems obvious that the one on the ground and the falling astronaut are not in the same FOR, since one is accelerated towards the other.

So I went looking back at the basics and found those old excellent videos

In the second video, take attention after timestamp 4.00.

In which it is stated that

1. The Earth IS an Inertial Frame Of Reference

2. An accelerated FOR relative to the Earth IS NOT an Inertial Frame Of Reference.

Isn't an astronaut in free fall accelerated towards the Earth?

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So I went looking back at the basics and found those old excellent videos

Old excellent videos on the subject of kinematics.

In the second video, take attention after timestamp 4.00.

In which it is stated that

1. The Earth IS an Inertial Frame Of Reference

2. An accelerated FOR relative to the Earth IS NOT an Inertial Frame Of Reference.

What's the timestamp for where he explains that his discussion applies to relativity? Context matters.

For the purposes of the experiment shown, the earth is an inertial frame. It's a reasonable approximation. That is, the experiment in question does not have sufficient precision to show that it is not. A Foucault pendulum will, over a slightly longer period of time, demonstrate that the earth is not actually an inertial frame. But that's not really relevant for the discussion of relativistic effects.

http://www.physics.fsu.edu/courses/spring98/ast3033/relativity/generalrelativity.htm

Einstein summarized the results of his reasoning in his Principle of Equivalence, which can be stated thus:

• All experiments will give the same results in a local frame of reference in free fall and in a local frame of reference far removed from gravitational influences.
That is, there is no experiment we can perform that will tell us whether we are in a free falling reference frame (like the elevator above) or in a reference frame far away in space. The consequences of this profound hypothesis are quite remarkable.

That is Einstein's insight, and what he meant.

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Old excellent videos on the subject of kinematics.

What's the timestamp for where he explains that his discussion applies to relativity? Context matters.

The discussion is not about relativity.

At timestamp 3.26 he explains that "the cart will be accelerated relative to the Earth's frame" (3.38)

At time stamp 5.48 he says

"This is a strange Frame Of Reference"

For the purposes of the experiment shown, the earth is an inertial frame. It's a reasonable approximation. That is, the experiment in question does not have sufficient precision to show that it is not. A Foucault pendulum will, over a slightly longer period of time, demonstrate that the earth is not actually an inertial frame. But that's not really relevant for the discussion of relativistic effects.

Yes I understand that.

What boggles my mind is that here on Earth, we are indeed "in a strange Frame Of Reference".

Let me explain my POV.

Look at 4.45

What do you see?

I see a ball moving sideways.

I understand that the force that makes the ball deviates from the vertical is a fictitious force.

I understand also (please correct me) that in the first place, the force that makes the ball fall vertically can also be a fictitious force.

I understand that we call this force "gravity" but nonetheless this force shares the same caracteristics with the sideways fictitious force.

Not to say that the cameraman in accelerated motion will feel the sideways fictitious force exactly in the same manner he feels gravity on Earth.

So, IMHO it is wrong to state that there is no fictitious force on Earth. There is a fictitious force and we call it gravity.

The only difference is in the orientation of the 2 forces.

The one is pointed sideways, in the opposite direction of accelerated motion.

The other one (gravity) is pointed towards the centre of the Earth.

IOW, if both forces are fictitious, as I claim, then gravity must also be pointed in the opposite direction of accelerated motion.

Which means that we are in an expanding Frame Of reference, and not in an Inertial Frame Of Reference.

A "strange Frame Of Reference".

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Not to be misunderstood.

I do not claim that the Earth is actually expanding at an accelerating rate 9.81 m/s^2.

I claim that our FOR is expanding at an accelerating rate. I mean: everything that has mass.

Edited by michel123456