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## Question about relativity - Physics Forum

### Question about relativity - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

#1
07-18-2008, 12:21 AM
 coilforth@gmail.com Guest Posts: n/a

Please pardon the simple question, but

if there is no hierarchy for frames of reference, how does one decide
what is moving and what is staying in place?

So, if I am walking along a path, am I moving forward or is my pushing
on the ground moving the earth (and the rest of the universe)
backward?

#2
07-18-2008, 01:46 AM
 N:dlzc D:aol T:com \(dlzc\) Guest Posts: n/a

Dear coilforth:

<[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

It depends on what you are trying to arrive at. If you are
inertial, it is simply more convenient to use your own frame.

Walking is actually controlled falling. The Earth pulls you down
and forward, and you rotate the Earth very slightly backwards.

The net momentum of the Earth-Moon system is unaffected by what
you (or anyone else) does. Until you start blowing propellants
out of the Earth-Moon's "gravitational pull", you are not
affecting the system (or the Universe).

David A. Smith

#3
07-18-2008, 01:54 AM
 Androcles Guest Posts: n/a

<[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...
| Please pardon the simple question, but
|
| if there is no hierarchy for frames of reference, how does one decide
| what is moving and what is staying in place?

No way of telling.

| So, if I am walking along a path, am I moving forward or is my pushing
| on the ground moving the earth (and the rest of the universe)
| backward?

Yes. The station comes to the train. Look out the window of a plane
and watch the ground move beneath you. It's even more fun when
you have no other point of reference such as you might experience
at sea. Is your ship moving or is the other ship moving?

[Only registered users see links. ]

#4
07-18-2008, 01:59 AM
 Androcles Guest Posts: n/a

"N:dlzc D:aol T:com (dlzc)" <[Only registered users see links. ]> wrote in message
news:FBSfk.22\$[Only registered users see links. ]...
| Dear coilforth:
|
| <[Only registered users see links. ]> wrote in message
| news:[Only registered users see links. ]...
| > Please pardon the simple question, but
| >
| > if there is no hierarchy for frames of reference,
| > how does one decide what is moving and what
| > is staying in place?
|
| It depends on what you are trying to arrive at. If you are
| inertial, it is simply more convenient to use your own frame.

That stupid blunder caused Ptolemy to say the Earth was the centre of
the Universe and for 1700 years the idiot Church believed him --
right up 'til Copernicus. "Inertial" my arse! You are a born cretin, Smiffy.

#5
07-18-2008, 03:28 AM
 coilforth@gmail.com Guest Posts: n/a

On Jul 17, 6:54*pm, "Androcles" <Headmas...@Hogwarts.physics> wrote:

Mr. Smith,

When you say: "you rotate the Earth very slightly backwards", doesn't
that conflict with your statement: "The net momentum of the Earth-Moon
system is unaffected by what you (or anyone else) does"? Please
explain.

does the gravitational pull end?

Thanks again.
#6
07-18-2008, 03:35 AM
 coilforth@gmail.com Guest Posts: n/a

On Jul 17, 6:54*pm, "Androcles" <Headmas...@Hogwarts.physics> wrote:

Maybe I'm confusing 2 concepts here:

(1) Sounds like all agree that the "station comes to the train" and we
can pick whichever frame of reference we need.
(2) But affecting the earth and universe is a different story.

I'm stuck on (2) because I always thought the earth was connected to
the universe with various forces, as if those (invisible) forces could
be substituted with giant steel poles, into one interconnected lattice-
work. If this lattice is true, wouldn't walking on the earth force the
whole shebang backward as we move forward?
#7
07-18-2008, 04:59 AM
 N:dlzc D:aol T:com \(dlzc\) Guest Posts: n/a

Dear coilforth:

<[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

You are part of the system too. If you are moving very slightly
differently "clockwise" (say), then the Earth will move very
slightly differently "counterclockwise". The Moon will see no
net change. The Earth-Moon system will express no net change.

Not so much "where", but "how fast". If you expell gas at
greater than 11.2 km/sec, and it doesn't happen to be directed
directly back at the Earth, it will continue on "forever". This
will (again very slightly) affect the momentum of the Earth-Moon
system.

David A. Smith

#8
07-18-2008, 08:42 AM
 Androcles Guest Posts: n/a

<[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...
On Jul 17, 6:54 pm, "Androcles" <Headmas...@Hogwarts.physics> wrote:

Maybe I'm confusing 2 concepts here:
================================================

You confused Smiffy with me, but that's ok.
Smiffy is not too bright.

================================================

(1) Sounds like all agree that the "station comes to the train" and we
can pick whichever frame of reference we need.
(2) But affecting the earth and universe is a different story.

I'm stuck on (2) because I always thought the earth was connected to
the universe with various forces, as if those (invisible) forces could
be substituted with giant steel poles, into one interconnected lattice-
work. If this lattice is true, wouldn't walking on the earth force the
whole shebang backward as we move forward?

==============================================

I see the Sun rise in the East, move across the sky, set in the West.
Obviously the Sun moves, I can see it does. It goes around the Earth.
If I could get on board Concorde (no longer in service) I could race the
Sun and win, the Sun would set in the East and look like it was going
backwards.
And I'd be reversing time, I could have breakfast in London and
breakfast in New York two hours later, it only takes three hours to
cross the Atlantic.

Modern science began when Copernicus saw that it was the Earth that
went around the Sun and not the Sun that went around the Earth, but he
had a difficult time of it because that darned Moon DOES go around
the Earth.
Then you come along and want to stick giant steel electricity pylons
into it, then just to confuse things further you suggest that might be
"true".

Let's just say that if you run fast enough you can stop the entire bangshe
Universe spinning around once a day.

BTW, this pencil is broken, I can see it is.
http://hyperphysics.phy-astr.gsu.edu...ic/brokpen.jpg

BTW, this star is being eclipsed... I can see it is.
[Only registered users see links. ]

Seeing is believing, and like Copernicus I'm having a hard time convincing
anyone that THEY are wrong, the idiots want to convince me that I am.
[Only registered users see links. ]

Truth is they are all sheep, when one bleats they all say "baa".

#9
07-18-2008, 06:05 PM
 coilforth@gmail.com Guest Posts: n/a

On Jul 17, 9:59*pm, "N:dlzc D:aol T:com \(dlzc\)" <[Only registered users see links. ]>
wrote:

When the propellant in this example affects the momentum of the Earth-
Moon system, does it also affect the momentum of the rest of the
universe?
#10
07-18-2008, 08:32 PM
 dlzc Guest Posts: n/a

Dear coilforth:

On Jul 18, 11:05*am, [Only registered users see links. ] wrote:
...

Ernst Mach would say so. Be really hard to measure... ;>)

David A. Smith

 Tags question , relativity

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