Science > Physics > What's the difference between a rock, and an asteroid that's stopped?
| Topic: |
Science > Physics |
| User: |
"Jonathan" |
| Date: |
21 Jul 2003 09:43:34 PM |
| Object: |
What's the difference between a rock, and an asteroid that's stopped? |
Hi,
Could anyone from the brains trust out there explain to me the
difference between:
A) An object with a certain velocity travelling in a certain
direction, and
B) The same object, when its not in motion.
Is there any difference physically between the two?
Ie. If we were to freeze time and take a snapshot of all the bits and
pieces that constitute the object in motion (let's say its an
asteroid), is there any way of distinguishing between that and a
similar "snapshot" of a physically identical asteroid which has no
velocity?
If not, then where is the information about an object's velocity
"stored"?
In other words, how does an object in motion know that it should keep
moving, while a stationary object knows it should keep standing still.
Or am I just confused?
Thanks,
Jon.
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| User: "The Ghost In The Machine" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
22 Jul 2003 06:01:25 AM |
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In sci.physics, Jonathan
<johnathan.may@zurich.com.au>
wrote
on 21 Jul 2003 19:43:34 -0700
<5443d086.0307211843.639ba972@posting.google.com>:
Hi,
Could anyone from the brains trust out there explain to me the
difference between:
A) An object with a certain velocity travelling in a certain
direction, and
B) The same object, when its not in motion.
Is there any difference physically between the two?
Yes, your reference frame. :-) If it's the exact same
object, and it now has no velocity -- then you have its
velocity and are moving with or parallel to that object,
and would probably notice the Earth is now moving.
Ie. If we were to freeze time and take a snapshot of all the bits and
pieces that constitute the object in motion (let's say its an
asteroid), is there any way of distinguishing between that and a
similar "snapshot" of a physically identical asteroid which has no
velocity?
If the object is tumbling one might see magnitude variations.
There might be a possibility of doing something with the
reflected light from the object, as it should exhibit
a red or blue shift. I'm not sure if that shift is detectable
although I don't see why not; the motion of the object should
be around 10^-4 c. Of course that's assuming we see the object
as an actual object, as opposed to a faint star (an object
going to hit us will not shift position in the sky, at least
during the final moments; it would shift prior thereto as
it curves in orbit, though, and we should be able to
compute its position based on the magnitude and sky position
(declination and right ascension, IIRC).)
One one can attempt to ping the object using
a radiotelescope array as a giant radar antenna.
This probably won't work very well. :-) (Even shooting
lasers at the Moon through a telescope we only get a few
photons back, about 1 1/4 second later, and that's with a
specially designed passive cornerreflector device sitting
on the Moon.)
If not, then where is the information about an object's velocity
"stored"?
In other words, how does an object in motion know that it should keep
moving, while a stationary object knows it should keep standing still.
Or am I just confused?
You are confused. :-) Without external forces, an object
at rest will remain at rest; an object in motion will
remain in motion. F = ma and all that. No intelligence
is required on the part of the object: it could be a
spacecraft, a probe, a giant dormant mutant plant spore,
or just a hunk o' rock. (The last is by far the most likely. :-) )
External forces at the planetary scale are primarily the
result of the Sun or the Earth's gravity, although Jupiter
might get involved if the cometary bits get near it.
Comets introduce outgassing but I don't know offhand how
much of an issue (ahem) that is.
Thanks,
Jon.
--
#191,
It's still legal to go .sigless.
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| User: "Old Man" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
22 Jul 2003 02:11:39 AM |
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Jonathan <johnathan.may@zurich.com.au> wrote in message
news:5443d086.0307211843.639ba972@posting.google.com...
Hi,
Could anyone from the brains trust out there explain to me the
difference between:
A) An object with a certain velocity travelling in a certain
direction, and
B) The same object, when its not in motion.
Is there any difference physically between the two?
Ie. If we were to freeze time and take a snapshot of all the bits and
pieces that constitute the object in motion (let's say its an
asteroid), is there any way of distinguishing between that and a
similar "snapshot" of a physically identical asteroid which has no
velocity?
If not, then where is the information about an object's velocity
"stored"?
In other words, how does an object in motion know that it should keep
moving, while a stationary object knows it should keep standing still.
Or am I just confused?
As confused as an ancient Greek. No such beast as absolute
motion exists. There is no universal frame of reference. Space
has no handles. All observations of uniform motion are made
relative to the location of other massive objects. [Old Man]
Thanks,
Jon.
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| User: "Uncle Al" |
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| Title: Re: What's the difference between a rock, and an asteroid that'sstopped? |
22 Jul 2003 10:45:38 AM |
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Jonathan wrote:
Hi,
Could anyone from the brains trust out there explain to me the
difference between:
A) An object with a certain velocity travelling in a certain
direction, and
B) The same object, when its not in motion.
Is there any difference physically between the two?
The observer. Velocity is a relative observation. You can dial in
any measurement you wish as long as the object is *not* both massed
and traveling at lightspeed in vacuum, or not massed and not traveling
at lightspeed in vacuum.
Ie. If we were to freeze time and take a snapshot of all the bits and
pieces that constitute the object in motion (let's say its an
asteroid), is there any way of distinguishing between that and a
similar "snapshot" of a physically identical asteroid which has no
velocity?
If not, then where is the information about an object's velocity
"stored"?
Velocity does not exist as an independently measurable quantity.
Velocity is an arbitrary consequence of observation. Stand on the
Equator doing 1000 mph relative to the fixed stars. Travel east in a
plane and do 1600 mph likewise. Try doing 1600 mph tangential veocity
in the NASA human centrifuge - you will note a difference.
In other words, how does an object in motion know that it should keep
moving, while a stationary object knows it should keep standing still.
Or am I just confused?
You are ignorant. Read a physics text.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
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| User: "Spencer" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 02:11:08 PM |
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(Jonathan) wrote in message news:<5443d086.0307222122.daa7e15@posting.google.com>...
If rocks X and Y are physically the same, then WHAT IS IT about rock X
that causes it to keep moving?
If you wanted to assign a name to the property that an object at rest
will remain at rest, or an object in motion will remain in motion
(unless acted upon by an external force), then use the word inertia.
But that doesn't really tell you anything more. There's no "thing"
that keeps rock X moving, there's simply nothing to stop it.
Suppose that you used your spacesuit rocket pack to accelerate you to
the same velocity as rock X. Now you look at rock X and it's
motionless relative to you. Rock Y now appears to be moving away from
you. For both cases, you've done nothing to either rock, neither
added nor taken away any "thing" from them, yet you see them
differently. Your co-pilot, of course, sees rock Y as motionless, and
he sees you and rock X drifting away. Perhaps he's secretly pleased,
because he drained most of the fuel from your rocket pack and knows
you'll never return. He can thus continue on with his affair with
your wife with impunity. Or maybe not. Regardless, you both observe
rock X and rock Y differently.
Spencer
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| User: "Jim" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 06:13:38 AM |
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(Jonathan) wrote:
Thanks to all who posted responses, I'll just respond to the last one
I read...
Velocity does not exist as an independently measurable quantity.
Velocity is an arbitrary consequence of observation.
I can accept this. Let me re-phrase my question:
I'm floating in space outside my spaceship. There are 2 identical
space-rocks (X and Y) floating in front of me.
Of my spaceship, myself, and the 2 rocks, none of us are moving
relative to each other, therefore to the observer (me) nothing is
moving.
I then give rock X a kick with my space-boot and send it zooming off
into the distance (my space-suit's retro-thrusters activate
automatically to keep me stationary relative to the remaining rock and
my spaceship).
So now I'm looking at rock X, which appears to be zooming off away
from me, and rock Y which appears to be stationary. There is clearly
some difference between these two rocks.
If rocks X and Y are physically the same, then WHAT IS IT about rock X
that causes it to keep moving?
You kicked it.
Jim
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| User: "The Ghost In The Machine" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 09:21:06 AM |
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In sci.physics, Jonathan
<johnathan.may@zurich.com.au>
wrote
on 22 Jul 2003 23:05:13 -0700
<5443d086.0307222122.daa7e15@posting.google.com>:
Thanks to all who posted responses, I'll just respond to the last one
I read...
Velocity does not exist as an independently measurable quantity.
Velocity is an arbitrary consequence of observation.
I can accept this. Let me re-phrase my question:
I'm floating in space outside my spaceship. There are 2 identical
space-rocks (X and Y) floating in front of me.
Of my spaceship, myself, and the 2 rocks, none of us are moving
relative to each other, therefore to the observer (me) nothing is
moving.
I then give rock X a kick with my space-boot and send it zooming off
into the distance (my space-suit's retro-thrusters activate
automatically to keep me stationary relative to the remaining rock and
my spaceship).
So now I'm looking at rock X, which appears to be zooming off away
from me, and rock Y which appears to be stationary. There is clearly
some difference between these two rocks.
If rocks X and Y are physically the same, then WHAT IS IT about rock X
that causes it to keep moving?
Assuming the rock is 10 kg and you gave it a kick to
move relative to you 10 m/s, you gave it 50 J of energy
with that kick. So technically the rocks aren't the
same anymore.
At some point the rock and your spacecraft will come back
together -- fortunately the rock will still be moving only
10 m/s relative to you (assuming there's no frictional
effects and/or other such differences) so all it might
do is klunk the spacecraft (or your suit) as opposed to
holing it. (The orbit mechanics are mildly interesting;
if one kicks the rock along the direction of orbit the
rock will be *slowed* in period, arriving some time later
at the same point in subsequent orbits than the craft.
The rock's orbit will also be slightly elliptical.)
[rest snipped]
--
#191,
It's still legal to go .sigless.
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| User: "Jonathan" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 08:16:38 PM |
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Thanks Ghost,
But at the risk of flogging a long-dead horse, my question is still
unanswered...
Assuming the rock is 10 kg and you gave it a kick to
move relative to you 10 m/s, you gave it 50 J of energy
with that kick. So technically the rocks aren't the same anymore.
Okay, so apart from the fact that one is now moving, HOW ARE THEY
PHYSICALLY DIFFERENT? I mean, are the molecules in the energised rock
a bit bigger ,or has their arrangement changed or something to let us
know this rock has energy?
What I can't grasp is this...
If one of the rocks now has 50 J of energy, then where and/or in what
form is that energy associated with the rock's physical makeup? If
the physicality of the 2 rocks is no different, then how does one of
them "know" that it has energy, and should therefore keep moving?
Thanks,
Jon
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| User: "Jim" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 08:30:59 PM |
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(Jonathan) wrote:
Thanks Ghost,
But at the risk of flogging a long-dead horse, my question is still
unanswered...
Assuming the rock is 10 kg and you gave it a kick to
move relative to you 10 m/s, you gave it 50 J of energy
with that kick. So technically the rocks aren't the same anymore.
Okay, so apart from the fact that one is now moving, HOW ARE THEY
PHYSICALLY DIFFERENT? I mean, are the molecules in the energised rock
a bit bigger ,or has their arrangement changed or something to let us
know this rock has energy?
What I can't grasp is this...
If one of the rocks now has 50 J of energy, then where and/or in what
form is that energy associated with the rock's physical makeup? If
the physicality of the 2 rocks is no different, then how does one of
them "know" that it has energy, and should therefore keep moving?
Thanks,
Jon
It takes energy to keep moving?
In ten words of less; why?
Jim
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| User: "Laurel Amberdine" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 08:54:29 PM |
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On Thu, 24 Jul 2003 01:30:59 GMT, Jim <lose30lbs@workfromhome.com> wrote:
johnathan.may@zurich.com.au (Jonathan) wrote:
Thanks Ghost,
But at the risk of flogging a long-dead horse, my question is still
unanswered...
Assuming the rock is 10 kg and you gave it a kick to
move relative to you 10 m/s, you gave it 50 J of energy
with that kick. So technically the rocks aren't the same anymore.
Okay, so apart from the fact that one is now moving, HOW ARE THEY
PHYSICALLY DIFFERENT? I mean, are the molecules in the energised rock
a bit bigger ,or has their arrangement changed or something to let us
know this rock has energy?
What I can't grasp is this...
If one of the rocks now has 50 J of energy, then where and/or in what
form is that energy associated with the rock's physical makeup? If
the physicality of the 2 rocks is no different, then how does one of
them "know" that it has energy, and should therefore keep moving?
FWIW, I think this is a spiffy question, Jon.
It takes energy to keep moving?
In ten words of less; why?
It does??
-Laurel
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| User: "Jim" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 09:44:47 PM |
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Laurel Amberdine <circle@mtco.com> wrote:
On Thu, 24 Jul 2003 01:30:59 GMT, Jim <lose30lbs@workfromhome.com> wrote:
johnathan.may@zurich.com.au (Jonathan) wrote:
Thanks Ghost,
But at the risk of flogging a long-dead horse, my question is still
unanswered...
Assuming the rock is 10 kg and you gave it a kick to
move relative to you 10 m/s, you gave it 50 J of energy
with that kick. So technically the rocks aren't the same anymore.
Okay, so apart from the fact that one is now moving, HOW ARE THEY
PHYSICALLY DIFFERENT? I mean, are the molecules in the energised rock
a bit bigger ,or has their arrangement changed or something to let us
know this rock has energy?
What I can't grasp is this...
If one of the rocks now has 50 J of energy, then where and/or in what
form is that energy associated with the rock's physical makeup? If
the physicality of the 2 rocks is no different, then how does one of
them "know" that it has energy, and should therefore keep moving?
FWIW, I think this is a spiffy question, Jon.
It takes energy to keep moving?
In ten words of less; why?
It does??
-Laurel
No it doesn't.
Hence the question mark after my... ah... question. :)
Jim
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| User: "Laurel Amberdine" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 10:04:56 PM |
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On Thu, 24 Jul 2003 02:44:47 GMT, Jim <lose30lbs@workfromhome.com> wrote:
Laurel Amberdine <circle@mtco.com> wrote:
On Thu, 24 Jul 2003 01:30:59 GMT, Jim <lose30lbs@workfromhome.com> wrote:
<snip>
It takes energy to keep moving?
In ten words of less; why?
It does??
No it doesn't.
Hence the question mark after my... ah... question. :)
Oh good. :) I am feeling rather confused lately!
-Laurel
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| User: "The Ghost In The Machine" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
24 Jul 2003 12:10:13 AM |
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In sci.physics, Laurel Amberdine
<circle@mtco.com>
wrote
on 24 Jul 2003 03:04:56 GMT
<bfnicn$govsg$1@ID-45790.news.uni-berlin.de>:
On Thu, 24 Jul 2003 02:44:47 GMT, Jim <lose30lbs@workfromhome.com> wrote:
Laurel Amberdine <circle@mtco.com> wrote:
On Thu, 24 Jul 2003 01:30:59 GMT, Jim <lose30lbs@workfromhome.com> wrote:
<snip>
It takes energy to keep moving?
In ten words of less; why?
It does??
No it doesn't.
Hence the question mark after my... ah... question. :)
Oh good. :) I am feeling rather confused lately!
It does take energy to keep moving, if one has friction.
Even in space, satellites cannot stay in orbit, as the
atmosphere of Earth never really ends, but gradually peters
out until it merges, very high in the ionosphere, with
the solar wind, comet gas (whatever it is; I'm inclined to
think water vapor but there are a lot of possibilities),
and other such stuff that comes in between planets.
The magnetic field complicates things as well, leading
to the auroras.
So the satellite hits air molecules; the air molecules
gain momentum, but the satellite loses some. Since the
satellite is far larger the speed of the satellite
shrinks by a very small amount -- but over time, those
amounts add up. Uncompensated for, they add up enough
to have the satellite hit thicker air, slow down more
quickly, transfer its blazing speed (8 m/s or so) to
the air, causing it to glow in fiery reentry. If
the satellite is properly designed, it might be able
to land. If not ... well, try not to get underneath it. :-)
To compensate, a satellite might expend energy and mass
using a small rocket or gas vent (I'd have to look but some
mixtures are very convenient as they jet once they mix; no
heating or other manipulation needed -- I think the term is
"hypergolic" and it's very useful for maneuvering jets).
Satellite momentum restored (more or less).
Of course the density of this interplanetary dust is so
thin that the Earth will still be orbiting the Sun in much
the same orbit even to the point where the Sun bloats
up into a gigantic, cool (well, relatively speaking) red
ball. The astronomists aren't too clear on what happens
afterward; the best scenario is a nice, blackened Earth.
Even the oceans may be blown away.
If the Sun gets big enough the Earth will of course vaporize.
Fortunately for us we have about a billion years before
the Earth becomes uninhabitable (stars increase insolation
with age until they run out of hydrogen fuel) and another
4 billion years before the Earth becomes extra crispy.
I don't remember if the Sun will explode once the
helium runs out, or not. (That's one of the reasons
the Sun is large and cool; it's "burning" a different
type of fuel. That fuel will ultimately run out, too.)
Plenty of time to discover an interstellar drive (if
that's at all possible) or just build a slow "Ark"-type
spacecraft to places more to our liking. :-) If we plan
well enough at that point, anyway.
-Laurel
--
#191,
It's still legal to go .sigless.
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| User: "Jim" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 10:51:39 PM |
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Laurel Amberdine <circle@mtco.com> wrote:
On Thu, 24 Jul 2003 02:44:47 GMT, Jim <lose30lbs@workfromhome.com> wrote:
Laurel Amberdine <circle@mtco.com> wrote:
On Thu, 24 Jul 2003 01:30:59 GMT, Jim <lose30lbs@workfromhome.com> wrote:
<snip>
It takes energy to keep moving?
In ten words of less; why?
It does??
No it doesn't.
Hence the question mark after my... ah... question. :)
Oh good. :) I am feeling rather confused lately!
-Laurel
Thinking about using energy to accelerate bodies
in space, is cool. I especially like gravity/orbits.
Others here are much more educated on these subjects
then I am. Doesn't hinder my enjoyment, however. :)
Jim
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| User: "tj Frazir" |
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| Title: Re: What's the difference between a rock, and an asteroid that'ss... |
23 Jul 2003 11:51:30 PM |
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The big hole in the ground .
earth stops them boath
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| User: "" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
24 Jul 2003 12:01:50 AM |
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In article <5443d086.0307231716.a057039@posting.google.com>, (Jonathan) writes:
Thanks Ghost,
But at the risk of flogging a long-dead horse, my question is still
unanswered...
Assuming the rock is 10 kg and you gave it a kick to
move relative to you 10 m/s, you gave it 50 J of energy
with that kick. So technically the rocks aren't the same anymore.
Okay, so apart from the fact that one is now moving, HOW ARE THEY
PHYSICALLY DIFFERENT? I mean, are the molecules in the energised rock
a bit bigger ,or has their arrangement changed or something to let us
know this rock has energy?
The rock doesn't "have energy". This energy characterizes its
relationship to you, there is nothing inherent about it.
What I can't grasp is this...
If one of the rocks now has 50 J of energy, then where and/or in what
form is that energy associated with the rock's physical makeup?
See above.
If the physicality of the 2 rocks is no different, then how does one of
them "know" that it has energy, and should therefore keep moving?
Sigh. Motion is relative. Right now, as you're sittingin your chair,
you're moving at some speed relative tot he sun, at another relative
to the moon, at yet another relative to Mars. You're moving at nearly
the speed of light northwards relative to a cosmic ray electron
impinging on Earth from the North, and also at nearly the speed of
light southward relativ to another cosmic ray electron, arriving from
the South. You've simultaneously an infinite number of different
velocities (and kinetic energies, as well) relative to an infinity of
reference frames.
Got it? It is relationships we're talking about, not inherent
properties.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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| User: "The Ghost In The Machine" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
24 Jul 2003 12:10:12 AM |
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In sci.physics, Jonathan
<johnathan.may@zurich.com.au>
wrote
on 23 Jul 2003 18:16:38 -0700
<5443d086.0307231716.a057039@posting.google.com>:
Thanks Ghost,
But at the risk of flogging a long-dead horse, my question is still
unanswered...
Assuming the rock is 10 kg and you gave it a kick to
move relative to you 10 m/s, you gave it 50 J of energy
with that kick. So technically the rocks aren't the same anymore.
Okay, so apart from the fact that one is now moving,
HOW ARE THEY PHYSICALLY DIFFERENT? I mean, are the
molecules in the energised rock a bit bigger ,or has
their arrangement changed or something to let us
know this rock has energy?
Now that's a bit tricky to answer. Of course you conveniently
waved away part of the problem by assuming reactive retrojets
on your spacesuit.
Let's take those away...and now we have you imparting 50J
of energy to the rock with that kick. However, there's the
problem of conservation of momentum; the momentum of the
rock is now 100 kg m/s. Since the momentum of the system
is 0 (in our local reference frame; I won't bore you with
computations from the Earth's), momentum must be conserved
(at least at the Newtonian level; I'm not sure regarding
relativistic but at these velocities the difference is
miniscule). Assuming you + suit weigh 100 kg (to make
the math easy) you would now be moving at 1 m/s backward.
If we put back the reactive jets we can assume that they
puffed 1 kg of gas at a velocity of 100 m/s, to compensate.
You remain motionless. 10kg rock is moving at 10 m/s.
1 kg of gas is moving at 100 m/s. Total momentum: zero.
Total energy expended: 5,050 J: 50 for the initial kick,
5,000 for the retrojets. (This suggests that the designer
of the retrojets didn't design them quite right. :-)
Since I'm the designer in that case ... um ... well... :-) )
The 50 J came from the motion of your leg, and is
fed by chemical reactions therein. These reactions
will utilize food and generate waste, which you'll
eventually have to urinate out. (I'm not sure if feces
absorbs anything.) The 5,000 J probably came from
the pressure jet's storage tanks, although other
possibilities include small heaters and pressurized
liquid carbon dioxide, oxygen, xenon, or any of a
number of liquids or gases.
So the answer to your question is yes, the rock molecules
are now in a slightly different configuration: they're now
all moving at 10 m/s, still in a cohesive mass (50 J of
mechanical energy is hardly enough to bust up a good solid
rock, although kicking a small ball of sand in microgravity
would yield some mildly interesting results). If your
kick was not aimed at the mass center you've imparted a
tumble as well, which shows up as angular momentum and
angular energy. There may have been some elastic and even
inelastic deformation, depending on the material of the
rock (and your shoe). A small amount of heating may have
ensued as well.
What I can't grasp is this...
If one of the rocks now has 50 J of energy, then where and/or in what
form is that energy associated with the rock's physical makeup?
Velocity, of course. The rock is moving. Any movement of
an initially motionless object requires energy, at least
in free space (dropping things in a gravity field takes
advantage of curved space). Stopping a moving object also
requires energy.
One can look at it from the difference of a car peacefully
sitting parked near a tree and the same car rushing towards
the same tree at 60 mph. How does the car in the latter case
"know" to crumple up and knock down the tree, causing
massive damage to its grillwork, and probably deployment
of the air bag and such? (The car, admittedly, is designed
to absorb some of the energy of the crash using crumple
zones, the aforementioned airbag which is basically triggered
by special sensors, and the seat belts which keep the occupants
from flying through the air, windshield, or both -- the bodies
don't "know" that they're supposed to stop. :-) In fact,
in high speed crashes internal organs can hit the skeleton.
A simple calculation: a 2,000 kg car moving at 60 mph = 26.8 m/s
will contain about 719,000 J of energy. That same amount
of energy in a different form can power one's "boom box"
for many hours, if not days. [A D-cell contains about 89 kJ.])
If
the physicality of the 2 rocks is no different, then how does one of
them "know" that it has energy, and should therefore keep moving?
How does a light ray "know" how to get from point A to point B?
How does a sunflower "know" when to open its head?
How does rain "know" to go down instead of up?
How does a particle of dirt "know" to get sucked up by a vacuum cleaner?
At some level I'm not at all sure one can answer such questions,
although the last three are at least answerable: the
sunflower is essentially pre-programmed (through very long
evolutionary adaptation), the rain is heavier than the air
once it condenses in the freezing cloud, the particle of
dirt is pushed by the ambient air into the partial vacuum
of the vacuum cleaner's vacuum chamber, and, if it's a newer
design, maybe spun around a bit. (In older designs it
will probably hit the side of a porous bag; the air goes
through but the dirt doesn't -- at least in theory.)
I think your main block to understanding -- insofar as I can
figure it out -- is that you're somehow assuming the rocks
are still the same, even after you've kicked the one.
They are not, as the one is now moving.
I'm not sure I have a better answer than that, right now. :-)
Thanks,
Jon
--
#191,
It's still legal to go .sigless.
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| User: "Jim" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
23 Jul 2003 06:34:30 PM |
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On 22 Jul 2003 23:05:13 -0700, (Jonathan)
wrote:
Thanks to all who posted responses, I'll just respond to the last one
I read...
Velocity does not exist as an independently measurable quantity.
Velocity is an arbitrary consequence of observation.
I can accept this. Let me re-phrase my question:
I'm floating in space outside my spaceship. There are 2 identical
space-rocks (X and Y) floating in front of me.
Of my spaceship, myself, and the 2 rocks, none of us are moving
relative to each other, therefore to the observer (me) nothing is
moving.
I then give rock X a kick with my space-boot and send it zooming off
into the distance (my space-suit's retro-thrusters activate
automatically to keep me stationary relative to the remaining rock and
my spaceship).
So now I'm looking at rock X, which appears to be zooming off away
from me, and rock Y which appears to be stationary. There is clearly
some difference between these two rocks.
No there isn't.
How can you prove that *you* are not observing any difference?
Using your spaceship, catch up to the rock you kicked,
Now you will observe that it is not moving. But the one you left
behind is moving away from you, your spaceship, and the rock you
kicked.
Is it different from the rock you kicked? Or are you just observing it
differently?
If rocks X and Y are physically the same, then WHAT IS IT about rock X
that causes it to keep moving?
Futhermore... if I return to my spaceship, and send my co-pilot
outside in my place, he would be able to tell me which one of the
rocks X and Y it was that I kicked (because one appears to be in
motion, the other appears stationary).
This information can be gained for observing the 2 rocks; so if
they're physically identical, where was this information "stored"?
So I guess my question is, really, WHY does an object continue to move
when acted upon by a force (in this case my Space-boot)?
You almost answered your own question. "Acted upon by a force".
OK, you forced it to move. What is forcing it to stop? You aren't.
I'm not, nothing is. Right? So why should it NOT, keep moving?
Jim
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| User: "Jonathan" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
24 Jul 2003 01:58:27 AM |
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Jim <lose30lb@workfromhome.com> wrote in message news:<qa6uhvgka86aem5ce2tcgg98lib5k90hpg@4ax.com>...
So now I'm looking at rock X, which appears to be zooming off away
from me, and rock Y which appears to be stationary. There is clearly
some difference between these two rocks.
No there isn't.
How can you prove that *you* are not observing any difference?
Using your spaceship, catch up to the rock you kicked,
Now you will observe that it is not moving. But the one you left
behind is moving away from you, your spaceship, and the rock you
kicked.
Indeed,
Okay,
I can see there is no difference between Rocks X and Y in that
relative to itself, each rock is motionless (Rock X is not moving
relative to itself, nor it Rock Y), and is physically identical. I
can appreciate this.
Furthermore, I would observe no difference in either Rock X or Rock Y,
provided I was moving along at the same speed as the rock I am
observing: The rock would have a certain physical makeup, and it
would also appear motionless in relation to myself. So I understand
your statement above: "No there isn't [any difference between X and
Y]". This makes sense to me, but does not answer my original question
to my satisfaction because...
....when my co-pilot comes out of the spaceship, he would be able to
tell me which rock I kicked (because he would see one of them zooming
off).
Isn't the fact that he is able to tell me which one was kicked proof
that there IS A DIFFERENCE BETWEEN THEM?
I mean, my co-pilot is able to distinguish between X and Y. He is
percieving INFORMATION about X which is different to that of Y. WHERE
IS THIS INFORMATION STORED??? If there was really absolutely no
difference between them, my co-pilot should not be able to tell me
which rock I kicked? No?
Also, any person who comes out of my spaceship and observes rocks X
and Y at ANY TIME FROM NOW UNTIL THE END OF TIME, will be able to
distinguish between X and Y, because X is moving away from them. So,
even though (as most people in this discussion have told me) there is
no difference between X and Y, the tiny act of me kicking it has been
indelibly associated with Rock X from here to eternity. Rock X now
"carries" this information. Where is it stored if there is no
physical difference between them?
So, according to the responses above:
1. There is no difference between X and Y.
2. An observer can tell the difference between X and Y.
Hmmm.
You almost answered your own question. "Acted upon by a force".
OK, you forced it to move. What is forcing it to stop? You aren't.
I'm not, nothing is. Right? So why should it NOT, keep moving?
Good question -
Let me put a slightly different slant on what I'm trying to get at.
Rock X will keep moving until acted upon by another (opposing) force.
Great.
Presumably, the direction, speed and mass of my space-boot when I
kicked the rock will determine in which direction the rock zooms away,
and how fast.
Let's say I also now give Rock Y a kick (but not as hard). So now, X
and Y are moving off in the same direction, but X is travelling twice
as fast as Y. Fine.
After 1 second, rock X is 10M away from me, rock Y is 5M away from me.
At this time, I use my space-time-freeze-camera, to take a snapshot of
the physical composition of both rocks.
The snapshot of each rock looks identical (they're physically
identical). But *I* know that after another second, rock X will be 20M
away, and Rock Y will be 10M away. THIS IS A DIFFERENCE!! (20M vs
10M). So, where in my time-freeze photo of each rock is this
difference apparent? Where is an object's inertia "stored"???
Jon.
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| User: "Spencer" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
24 Jul 2003 07:41:38 PM |
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(Jonathan) wrote in message news:<5443d086.0307232258.32f0fed2@posting.google.com>...
<snippety snip>
Let me put a slightly different slant on what I'm trying to get at.
Rock X will keep moving until acted upon by another (opposing) force.
Great.
Presumably, the direction, speed and mass of my space-boot when I
kicked the rock will determine in which direction the rock zooms away,
and how fast.
Let's say I also now give Rock Y a kick (but not as hard). So now, X
and Y are moving off in the same direction, but X is travelling twice
as fast as Y. Fine.
After 1 second, rock X is 10M away from me, rock Y is 5M away from me.
At this time, I use my space-time-freeze-camera, to take a snapshot of
the physical composition of both rocks.
The snapshot of each rock looks identical (they're physically
identical). But *I* know that after another second, rock X will be 20M
away, and Rock Y will be 10M away. THIS IS A DIFFERENCE!! (20M vs
10M). So, where in my time-freeze photo of each rock is this
difference apparent? Where is an object's inertia "stored"???
Jon.
Hi Jonathan,
Okay, try this:
You've kicked rock X so that it has a velocity of 10m/s in the
positive x-direction relative to a reference frame with you at the
origin, at some time T1. Rock Y is still sitting there near you,
motionless relative to you, and you take all the snapshots you want
until your camera is out of memory. You get in your spaceship, glare
suspiciously at your co-pilot, but you both go home.
At some time T2, Zaphod shows up in his space ship and he happens to
be on a course in which he is traveling at 5m/s second in the positive
x-direction of your old reference frame. Suddenly, he notices rock X
ahead of him, moving away from him in the positive x-direction at
5m/s. He also notices rock Y moving away from him in the negative
x-direction at 5m/s. He too takes snapshots of the rocks. Apparently
it's all the rage in this part of the galaxy. He carefully studies
the photographs that have infintite detail about all aspects of the
rocks.
"Gee," Zaphod concludes. "Some guy must have kicked rocks X and Y in
opposite directions at +/-5m/s."
Or, of course, Zaphod could have been on the same course at +10m/s and
concluded that some joker had kicked rock Y at -10m/s.
You see, there is no inherent velocity, there is no universal
reference frame, there is no information stored or property of rock X
that can forever be used to determine that it was kicked.
Spencer
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| User: "Jim" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
24 Jul 2003 10:10:36 AM |
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On 23 Jul 2003 23:58:27 -0700, (Jonathan)
wrote:
<snip>
Y]". This makes sense to me, but does not answer my original question
to my satisfaction because...
...when my co-pilot comes out of the spaceship, he would be able to
tell me which rock I kicked (because he would see one of them zooming
off).
Isn't the fact that he is able to tell me which one was kicked proof
that there IS A DIFFERENCE BETWEEN THEM?
It only proves that he now shares your vantage point.
I mean, my co-pilot is able to distinguish between X and Y. He is
percieving INFORMATION about X which is different to that of Y. WHERE
IS THIS INFORMATION STORED??? If there was really absolutely no
difference between them, my co-pilot should not be able to tell me
which rock I kicked? No?
From your, now identical, vantage points, one is moving, one is not.
Also, any person who comes out of my spaceship and observes rocks X
and Y at ANY TIME FROM NOW UNTIL THE END OF TIME, will be able to
distinguish between X and Y, because X is moving away from them. So,
even though (as most people in this discussion have told me) there is
no difference between X and Y, the tiny act of me kicking it has been
indelibly associated with Rock X from here to eternity. Rock X now
"carries" this information. Where is it stored if there is no
No, you carry the information.
Another observer somewhere in eternity, will carry different
information about Rock X. (Perhaps moving towards him)
physical difference between them?
Movement is a physical difference.
So, according to the responses above:
1. There is no difference between X and Y.
2. An observer can tell the difference between X and Y.
The difference is movement.
Hmmm.
Hmmmm, Indeed.
Suppose you *are* able to impart some physical change in Rock X.
Change its color, size, weight, etc., with your kick.
Now try and explain how that can happen. :)
Giving something movement, using force, is a *lot* easier to explain.
You push, it moves.
Nothing else pushes it, it keeps moving.
Jim
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| User: "Laurel Amberdine" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
24 Jul 2003 10:17:58 AM |
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On 23 Jul 2003 23:58:27 -0700, Jonathan <johnathan.may@zurich.com.au> wrote:
Jim <lose30lb@workfromhome.com> wrote in message news:<qa6uhvgka86aem5ce2tcgg98lib5k90hpg@4ax.com>...
So now I'm looking at rock X, which appears to be zooming off away
from me, and rock Y which appears to be stationary. There is clearly
some difference between these two rocks.
No there isn't.
How can you prove that *you* are not observing any difference?
Using your spaceship, catch up to the rock you kicked,
Now you will observe that it is not moving. But the one you left
behind is moving away from you, your spaceship, and the rock you
kicked.
Indeed,
Okay,
I can see there is no difference between Rocks X and Y in that
relative to itself, each rock is motionless (Rock X is not moving
relative to itself, nor it Rock Y), and is physically identical. I
can appreciate this.
Furthermore, I would observe no difference in either Rock X or Rock Y,
provided I was moving along at the same speed as the rock I am
observing: The rock would have a certain physical makeup, and it
would also appear motionless in relation to myself. So I understand
your statement above: "No there isn't [any difference between X and
Y]". This makes sense to me, but does not answer my original question
to my satisfaction because...
...when my co-pilot comes out of the spaceship, he would be able to
tell me which rock I kicked (because he would see one of them zooming
off).
Isn't the fact that he is able to tell me which one was kicked proof
that there IS A DIFFERENCE BETWEEN THEM?
I mean, my co-pilot is able to distinguish between X and Y. He is
percieving INFORMATION about X which is different to that of Y. WHERE
IS THIS INFORMATION STORED??? If there was really absolutely no
difference between them, my co-pilot should not be able to tell me
which rock I kicked? No?
I believe the "information" (which is kind of an unusal way of putting it)
is in the co-pilot/rock *system*, which takes into account all elements
and the relationship between them.
Also, any person who comes out of my spaceship and observes rocks X
and Y at ANY TIME FROM NOW UNTIL THE END OF TIME, will be able to
distinguish between X and Y, because X is moving away from them. So,
even though (as most people in this discussion have told me) there is
no difference between X and Y, the tiny act of me kicking it has been
indelibly associated with Rock X from here to eternity. Rock X now
"carries" this information. Where is it stored if there is no
physical difference between them?
So, according to the responses above:
1. There is no difference between X and Y.
If you can change reference frames.
2. An observer can tell the difference between X and Y.
In the observer's fixed reference frame.
Hmmm.
:)
BTW, if you can get to a library or bookstore, Feynman has an essay on
"Symmetry in Physical Laws" which explores similar questions to yours.
It's in _Six Not So Easy Pieces_ and probably in _The Feynman Lectures_
too. ("Symmetry" in this context means that you changed something, but you
can't tell that it's been changed.)
-Laurel
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| User: "Jim" |
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| Title: Re: What's the difference between a rock, and an asteroid that's stopped? |
25 Jul 2003 08:40:21 PM |
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On 24 Jul 2003 15:35:20 -0700, (Jonathan)
wrote:
Laurel Amberdine <circle@mtco.com> wrote in message news:<bfotb4$fsmij$1@ID-45790.news.uni-berlin.de>...
BTW, if you can get to a library or bookstore, Feynman has an essay on
"Symmetry in Physical Laws" which explores similar questions to yours.
It's in _Six Not So Easy Pieces_ and probably in _The Feynman Lectures_
too. ("Symmetry" in this context means that you changed something, but you
can't tell that it's been changed.)
-Laurel
Thanks Laurel,
I'll look into it - this sounds fascinating.
Thanks also to all who posted responses to my query (although I'm more
confused now than when I first posted!)
Jon :)
Re-read the replies.
Think about it awhile.
Ask some more questions.
Read some of the other threads in this group.
You'll see an _ocean_ of patience in some of 'em. :)
Sure some here are kinda short tempered. But that makes
the reading all the more interesting. Indeed _some_ of the
best answers I've seen here were SHOUTED. :)
I've asked some pretty stupid questions, and offered some pretty
useless answers here, and I'm still standing. Don't give up.
Jim
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| User: "vonquark" |
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| Title: Re: What's the difference between a rock, and an asteroid that'sstopped? |
21 Jul 2003 10:32:17 PM |
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Jonathan wrote:
Hi,
Could anyone from the brains trust out there explain to me the
difference between:
A) An object with a certain velocity travelling in a certain
direction, and
B) The same object, when its not in motion.
Is there any difference physically between the two?
Ie. If we were to freeze time and take a snapshot of all the bits and
pieces that constitute the object in motion (let's say its an
asteroid), is there any way of distinguishing between that and a
similar "snapshot" of a physically identical asteroid which has no
velocity?
If not, then where is the information about an object's velocity
"stored"?
In other words, how does an object in motion know that it should keep
moving, while a stationary object knows it should keep standing still.
Or am I just confused?
Thanks,
Jon.
I think you need to re read an article
on Special Relativity where the
same questions are asked, but
usually stated in regard to a train in
Motion.
THE SHORT RELATIVISTIC ANSWER.
You must define your point of reference
before you make your calculations.
You can say the asteroid is moving towards
the earth or, equally valid, the earth is moving
towards the asteroid. If no reference point
is stated, use common sense. In most cases,
for ex.: in the case of an asteroid, it is the earth
or the sun which is the assumeed reference
point of stasis.
A LONGER NON-RELATIVISTIC
ANSWER
You have to define your inertial frame of
reference and any point is valid. So the question
is moot as no informations needs to be stored
at all at the point which is the defined frame of
reference.
There are mathematical transforms to take
such things into consideration.
For ex: If we have an asteroid heading
towards the earth [from our frame of reference]
but we wanted to use the asteroid as the
frame of reference, we would use the proper
transform. The velocities however would be
adjusted, however, to the center of gravity [COG]
of the two body system The earth would
move slower to that center of gravity than
the asteroid would, under the transform, and
so the energy equations would work out
the same.
What is usually never explained. and they should
explain it, is that in order to simply the equations
the more massive object is assumed to be static.
Since an asteroid/earth mass ratio would be
1:1,000,000 or more, this is a very good
assumption. So the equations can be simplified
and it is a good enough to assume the asteroid
is moving to a static earth. But in actuallity
the asteroid and the earth are both moving
to their center of gravity [COG], the earth is just
moving at about one-millionth the acceleration
so all but the most anal-retentive of astronomers
can ignore it.
But if we assume a static asteroid, the equations
would be adjusted for the center of gravity
and here the massive earth would have a low
acceleration towards the COG. So the high
mass would be countered by a low earth
velocity towards the COG, even if it appears
to be a high velocity towards the asteroid.
the energy equations would be stated from
COG tranforms and energy would not
change from either frame of reference. It is
easier to assume a static earth and to assume
the COG is the center of the earth. Then
the equations are easier; but not entirely
accurate. Even if we assume a static asteroid,
we still have to account for the COG to
asteroid movement.
Your error results from this not being explained.
The center of gravity of a two body system
is the static point of the system, technically.
In practice the earth is assumed to be the
static [COG] so that certain variables can
be ignored and the equations made palatable
to liberal arts majors.
This is an excellent approximation if the
asssumed static point is >>>> more massive
than the assumed solitary moving point.
THE GENERAL RELATIVISTIC ANSWER:
Reread Einstein's opening comments
on a man falling from a building.
Here gravitional and inertial mass
are demonstrated to be equivalent
and so there is no need for information.
I DO HOPE I GOT THAT RIGHT!
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