# Are acceleration and deceleration the only ways we feel motion?

1. Mar 26, 2013

### Seminole Boy

And if that is the case, why does our reference frame feel the motion, if acceleration is relative?

I hope I'm explaining this the right way. I'm not saying that its being relative acceleration should shelve the notion of feeling (physical awareness of motion). But it doesn't make entire sense.

My golden retriever and I are on a train. The conductor applies the brakes. The pup and I jerk forward. However--and Einstein explains this in his book--this could also be seen as my pup and I were at rest and the backwards speed of the earth passing by was slowed down.

So, if that's the case, and regardless of reference frame, how did I really feel anything? Or is this feeling of motion not being defined the right way? Or am I just a dodo bird?

Also, what role does space play in our feeling of this motion (if we really do)? Space seems to have an unexplained role in this physical feeling of (encounter with) motion.

Last edited: Mar 26, 2013
2. Mar 26, 2013

### 1977ub

Acceleration is not relative the way velocity is.

3. Mar 26, 2013

### Turion

Can't we also feel jerks, the derivative of acceleration?

4. Mar 26, 2013

### Seminole Boy

Please do explain. I'm currently reading Einstein's book, and I just started the GR section, but I haven't noticed how the relativity of acceleration is any more complicated than the relativity of uniform motion. What am I missing?

5. Mar 26, 2013

### Passionflower

This is incorrect.

With respect to you and the pup the front of the train simply starts accelerating towards you.

6. Mar 26, 2013

### Staff: Mentor

You left out a key part of Einstein's explanation: slowed down by what?

The answer is, you could view this as your pup and you remaining at rest while the application of the brakes created a gravitational field that caused the Earth to accelerate relative to you (because the Earth is in free fall in the field while you are being held at rest in the field by the train). Since the Earth is traveling "upward" relative to you, and the field pulls it "downward", it appears to slow down and finally come to a stop, like a ball thrown upward.

Because you were being held at rest in a gravitational field. Objects held at rest in a gravitational field feel a force (which we usually call "weight"), just like you do standing on the Earth's surface. To be weightless and not feel anything, you would have to free-fall in the field.

How so? Look at my explanation above; where does "space" play a role?

7. Mar 26, 2013

### Seminole Boy

Peter:

Would you please explain this in a different way? Your quote: "The answer is, you could view this as your pup and you remaining at rest while the application of the brakes created a gravitational field that caused the Earth to accelerate relative to you (because the Earth is in free fall in the field while you are being held at rest in the field by the train). Since the Earth is traveling "upward" relative to you, and the field pulls it "downward", it appears to slow down and finally come to a stop, like a ball thrown upward."

The basic concept of general relativity seems to be stitched into all that, but I cannot entirely grasp what you're saying here.

8. Mar 26, 2013

### 1977ub

In terms of spacial coordinates, that's absolutely true. You could set your spacial coordinates so that the train is moving north and slows down, or that the landscape was moving south and then slowed down. From a spacial coordinate system based upon you, the brakes have changed the relative motion of the Earth. The fact that you experience the acceleration and not somebody the train is passing is due to inertia. There is currently no satisfactory explanation for it. You just have to measure it. You can make this example simpler by placing it in outer space and use rocket brakes. This removes gravity and friction, but also removes the teency effect by which the Earth is immeasurably slowed by the application of the brakes.

Search results for "Acceleration Not Relative" http://bit.ly/11Irp9o

9. Mar 26, 2013

### Staff: Mentor

How about this: when the brakes are applied, a gravitational field comes into being (from your viewpoint at rest inside the train) such that the direction towards the front of the train is "down" and towards the back of the train is "up".

You and your pup are held at rest in this field: you don't move relative to the train. That means you feel "weight"; you feel a force pushing you "upwards" (towards the back of the train) that holds you at rest in the field.

The Earth, however, is not held at rest: it "falls" freely in the field. Since it starts out moving "upwards" (towards the back of the train), the field causes it to decelerate, i.e., to move "upwards" more and more slowly until it comes to rest. At this point the brakes are released and the field disappears, so the Earth stays at rest relative to the train.

Does that help?

10. Mar 26, 2013

### Seminole Boy

Yes, you are always helpful. I'm struggling with the "ups" and "downs" because I'm not convinced direction has any meaning in motion. Going up is the same as going down, so how can direction be used to explain the physical feeling of motion?

There has to be some other interaction occurring that allows for motion, or our physical understanding of motion.

How does my weight push me upwards (or to the back of the train)? Why not towards the front of the train?

11. Mar 26, 2013

### nitsuj

Direction has meaning...of course.

isotropy [isotropic] is maybe a word you would like to replace "Direction has no meaning in "motion" / Up & Down are the "same".

12. Mar 26, 2013

### Passionflower

To compare acceleration with a gravitational field I think is rather far fetched (yes I know of the equivalence principle, but "equivalent" does not mean "identical") and worse it may completely confuse those who want to get a better understanding.

Acceleration is absolute so it is always unambiguous what accelerates and it has absolutely nothing to do with gravity. However the direction of acceleration (e.g. acceleration v.s. deceleration) is not absolute and observer dependent.

13. Mar 26, 2013

### Staff: Mentor

There are certainly limitations to the comparison (which have been discussed *many* times here on PF ), but the OP specifically referred to Einstein's book and that comparison is what is in the book. For the limited purpose for which Einstein was trying to use it (at least in that particular case in the book), it does serve to illustrate how one might interpret events in a non-inertial frame.

Proper acceleration is, yes. And that principle holds in the train scenario: who feels weight, i.e., who experiences proper acceleration, is invariant--the train occupant and his pup do.

Perhaps that's a good point to add: you can't feel "motion" per se, but you can feel weight, and feeling weight is absolute--it has to be, since it's a direct observable. So proper acceleration, which is just the mathematical property that corresponds to feeling weight, is also absolute.

But feeling weight does not have to correspond to coordinate acceleration--you can feel weight while you are at rest with respect to some reference frame. (It won't be an inertial frame, though.) This is one way of saying why "motion" (meaning velocity) has to be relative while proper acceleration can be absolute.

14. Mar 26, 2013

### Seminole Boy

Peter:

What do you mean by "proper acceleration"?

15. Mar 26, 2013

### Staff: Mentor

Really? Climbing up a flight of stairs is just as easy as going down? You might want to stop and think for a bit before stating these "obvious" things that are actually false.

What is true is that *in empty space*, any direction is the same as any other. But here on the surface of the Earth, you are not in empty space; there is a large gravitating mass whose center is in a particular direction. That changes things: it picks out "up" and "down" directions that *are* physically different, not because the laws of physics have a preferred direction, but because the particular *solution* of those laws that you are in--the one with the Earth in it--has a preferred direction.

Because the application of the brakes does not act equally in all directions; it picks out a particular direction, just as the presence of the Earth in my discussion above does.

16. Mar 26, 2013

### Staff: Mentor

As I said, it's the mathematical way to model "feeling weight", or "feeling a force", in relativity. If you are feeling a force, you have a nonzero proper acceleration. The Wikipedia page has a fairly good discussion:

http://en.wikipedia.org/wiki/Proper_acceleration

17. Mar 26, 2013

### WannabeNewton

In SR it is just $a = (a^{b}a_{b})^{1/2}$ where $a^{b} = \frac{\mathrm{d} u^{b}}{\mathrm{d} \tau }$ is the 4 - acceleration; here $u^{b}$ is the 4 - velocity of the massive particle and $\tau$ is the proper time along it's world - line. In GR it is a bit different equation wise but I don't think there's a need to go there unless you are interested.

18. Mar 27, 2013

### HallsofIvy

Staff Emeritus
What we feel is FORCE. And F= ma so we "feel" acceleration.

19. Mar 28, 2013