# Freefall isn't acceleration?

1. Jun 20, 2015

### inertiaforce

According to this video, freefall isn't accelerated motion and is actually stationary. In other words, you aren't moving in freefall:

2. Jun 20, 2015

### Orodruin

Staff Emeritus
No, this is the wrong interpretation. You are not accelerating in free fall. (We are here talking about what is called proper acceleration, which is what an accelerometer measures.) Movement is relative.

3. Jun 20, 2015

### Staff: Mentor

When you free fall you are in an inertial frame of reference.

Consider the Vomit Comet where the plane is in feefall for a few seconds. Inside you feel weight-less and move around without any feeling of gravity. If you kick off from the wall you will travel at constant velocity toward the other wall.

Last edited by a moderator: Jun 21, 2015
4. Jun 20, 2015

### inertiaforce

You are correct Orodruin. I meant to say that you are not "accelerating" in freefall. Thank you for catching the mistake.

5. Jun 20, 2015

### inertiaforce

An "inertial frame of reference" is one in which you aren't experiencing accelerated motion, correct?

6. Jun 20, 2015

### Orodruin

Staff Emeritus
Also observe that this does not mean that "gravity is an illusion" as stated in the video title. All it means is that you need proper acceleration to remain stationary.

7. Jun 20, 2015

### inertiaforce

Lol. "You need proper acceleration to remain stationary." I'm sure that's going to go over well with the general public.....

8. Jun 20, 2015

### Orodruin

Staff Emeritus
What is the point of this post? Do you understand what stationary and proper acceleration means?

9. Jun 20, 2015

### inertiaforce

No not exactly lol. I'm a layperson who has studied gravity on his own lol.

But I have come to realize that free fall is not an accelerated frame of reference. Prior to today, I was always told that freefall was an accelerated frame of reference. It is only today that I started to realize that freefall is not an accelerated frame of reference. It is an inertial (nonaccelerated) frame of reference because there are no net forces acting on a person in freefall. Therefore, it is not an accelerated frame of reference. This is so nonintuitive for the layperson. It has taken me a lot of research on my own just to come to this understanding. Is this understanding correct?

I am not familiar with the term "proper acceleration." Please educate me.

Last edited: Jun 20, 2015
10. Jun 20, 2015

### inertiaforce

It seems like I may have inadvertently insulted you with my comment about not going over well with the general public. That wasn't my intention. I did not intend it to be an insult. I intended it to mean that this stuff is complicated for the average person to understand. It was a joke about the average person not being able to understand the terms "acceleration to remain stationary." The terms "acceleration to remain stationary" will cause confusion for the general public.

11. Jun 21, 2015

### atyy

Both are right.

In Newtonian gravity, one usually talks about 3-acceleration, which is not zero in free fall.

In general relativity, one usually talks about 4-acceleration, which is zero in free fall.

The proper acceleration is the acceleration read by an accelerometer, and it is more closely related to the 4-acceleration.

12. Jun 21, 2015

### DrGreg

Like almost everything else in relativity, acceleration is relative. Proper acceleration means "acceleration relative to a freefalling object that you are momentarily at rest with". Practically, it is acceleration as measured by an accelerometer. Other "non-proper" sorts of accleration are possible, usually referred to as "coordinate acceleration", which depends which coordinates you choose to use.

When someone talks about acceleration, you need to be clear whether they mean proper acceleration or coordinate acceleration. Usually, in relativity, they mean proper accleration, but they might not.

13. Jun 21, 2015

### A.T.

14. Jun 22, 2015

### PWiz

Strictly speaking, free-fall is an inertial frame of reference locally only. This distinction becomes important when we're talking about frames in regions of space close to a massive object.

Last edited: Jun 22, 2015
15. Jun 22, 2015

### harrylin

I don't think that that video is an acceptable source for discussion on this forum.

Apart of that, motion is defined wrt your chosen reference system. Consequently, freefall is accelerated motion wrt to a Newtonian (or "Galilean") reference system, but it's stationary wrt another free falling reference system. In principle it's as simple as that, and it's the consideration of free falling reference systems that led the way to GR.

Note however: falling reference systems behave locally just like Galilean reference systems (and in absence of gravitation there is no difference). As Galilean reference systems are called "inertial frames", such free falling frames are also called "local inertial frames".

16. Jun 22, 2015

### bahamagreen

Video starts off without even pausing to think about anything, continues without follow-up or follow-through, just steady non-stop hipster blather like a cleaning product sales pitch.

One might wonder that if it were the Earth that accelerates to the apple at rest, the apple must observe that the whole universe must instantaneously and universally accelerate at 10m/S^2 wrt the resting apple in the apple's "up" direction for the duration of the Earth's fall, and this acceleration of the whole universe will stop suddenly, instantaneously, and universally when the Earth is stopped when it contacts the apple.

How did the distant regions of the universe conspire to coordinate their differential sudden onset of acceleration and subsequent cessation of motion over billions of years in order to appear to be instantaneous and universal to the apple just when the stem that held the Earth broke?

How much energy does it take to present the appearance to the apple that the whole universe has accelerated, and then stopped suddenly, and what stops the universe since it is only the Earth that is stopped when it contacts the resting apple?

I don't disagree that gravitation was ready for rethinking, but the video has nary a single real thought throughout the whole high speed irritating un-shaven narrative.

17. Jun 22, 2015

### Orodruin

Staff Emeritus
You are trying to take a global view, which is not as straightforward as it seems in GR. The ground has a proper acceleration of ca 10 m/s^2. This does not mean that it is not stationary. The Earth is (essentially) stationary as well. The apple is not stationary, but at rest in a local inertial frame.

18. Jun 22, 2015

### stevendaryl

Staff Emeritus
I don't think your characterization is at all accurate. There is nothing "hipster" about it. He pretty much defines all the terms that he uses. But I agree that it's too fast to do anyone any good unless they already know this stuff.

19. Jun 23, 2015

### harrylin

So hip that it's simply wrong:

"according to Einstein there is no such thing as a gravitational force, instead it's more appropriate to think of the apple as stationary and the ground [..] as accelerating upward"" - video (emphasis mine)

"The general theory of relativity renders it likely that the electrical masses of an electron are held together by gravitational forces."
"I must warn the reader against a misconception [..] we might easily suppose that the existence of a gravitational field is always only an apparent one. [..] This is by no means true for all gravitational fields, but only for those of quite special form. It is, for instance, impossible to choose a body of reference such that, as judged from it, the gravitational field of the earth (in its entirety) vanishes - Einstein 1916, Relativity: The Special and General Theory (emphasis mine)

However:
Regretfully Einstein's original GR did correspond to somewhat similar views, see:
- https://en.wikisource.org/wiki/The_...ain_the_extension_of_the_relativity-postulate.
- https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity

Last edited: Jun 23, 2015
20. Jun 23, 2015

### A.T.

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