Concept of Gravity

Doug wrote:
skddlbyp wrote:
I don't really understand the astro-physics of it. I'm not sure if the
sun spins, or, if it does, it exerts a gravitational force in the direction
of the spin.

I may be wrong, but if the sun spins, wouldn't centrigugal force cause
less gravitational force in the direction of the spin? I read that on
earth you weigh slightly less at the equator than at the poles due to
centrifugal force. Would this be true of the sun also?


If you were on the Sun, and had a solid place to stand. If you weren't
spinning with the Sun, you would feel the same gravity as without the
spin. It is the tidal force that can transfer momentum from a spinning
body to its satellite, causing it to move into a higher orbit. It's
the Moon's pull on the oceans, and the oceans pull on the Moon that are
causing the Earth to gradually slow in its rotation, and the Moon to
move further away. The Sun rotates once in from 25 to 30 days,
depending on at what latitude one the Sun you measure its rotation.
But the tidal interaction between the Sun and Earth that could move it
to a higher orbit is small.

Double-A


In another
thread in this forum, a discussion of black holes, it was implied that
black holes have stronger gravity at their poles than at their equators
due to their very rapid spin.

Doug

Sun does spin.


--
The Lone Sidewalk Astronomer of Rosamond

Telescope Buyers FAQ
http://home.inreach.com/starlord
Sidewalk Astronomy
www.sidewalkastronomy.info
The Church of Eternity
http://home.inreach.com/starlord/church/Eternity.html


"Doug" wrote in message
ups.com...

skddlbyp wrote:
I don't really understand the astro-physics of it. I'm not sure if
the
sun spins, or, if it does, it exerts a gravitational force in the
direction
of the spin.

I may be wrong, but if the sun spins, wouldn't centrigugal force cause
less gravitational force in the direction of the spin?

In article ,
"skddlbyp" wrote:

snip

I don't really understand the astro-physics of it. I'm not sure if the
sun spins, or, if it does, it exerts a gravitational force in the direction
of the spin. May be confusing a gravitational field with an
electro-magnetic field.

Gravity is not affected by spin; it's simply proportional to a body's
mass regardless of what that mass is doing. Now it is true that objects
on the surface of a spinning body seem to experience centrifugal forces
that work against gravity, strongest near the equator, but this is due
to the object's motion as it rotates with the body. A free object that
hovers over the surface without 'chasing' it will not experience any
centrifugal force; from such a position there'd be no way to detect the
rotation of the body without looking -- at least not from measuring its
gravity.

--
Odysseus

"Odysseus" wrote in message
news
In article ,
"skddlbyp" wrote:

snip

I don't really understand the astro-physics of it. I'm not sure if
the
sun spins, or, if it does, it exerts a gravitational force in the
direction
of the spin. May be confusing a gravitational field with an
electro-magnetic field.

Gravity is not affected by spin; it's simply proportional to a body's
mass regardless of what that mass is doing. Now it is true that objects
on the surface of a spinning body seem to experience centrifugal forces
that work against gravity, strongest near the equator, but this is due
to the object's motion as it rotates with the body. A free object that
hovers over the surface without 'chasing' it will not experience any
centrifugal force; from such a position there'd be no way to detect the
rotation of the body without looking -- at least not from measuring its
gravity.
Possibly not quite true.
The effect is _tiny_, if it exists, but it is part of current theory, that
spin does have an effect on gravity. Do a search on the 'Lense-Thirring
effect'. There was an space probe planned to measure it, and while this
has not happened, some scientists believe they have measured the effect
from long term measurements made on some other probes. The 'jury is out'
on this...

Best Wishes

I found this very interesting article during a web search:

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

To my mind, It brings up the question again abot black holes and the
concept of "space flowing" into them. While the Lense-Thirring effect
is very small with objects that are not black holes and spin slowly,
like planets, what would the effect be with objects like black holes
that spin fast?

Doug

I found this very interesting article during a web search:

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

To my mind, It brings up the question again abot black holes and the
concept of "space flowing" into them. While the Lense-Thirring effect
is very small with objects that are not black holes and spin slowly,
like planets, what would the effect be with objects like black holes
that spin fast?

Doug

Roger Hamlett wrote:

Possibly not quite true.
The effect is _tiny_, if it exists, but it is part of current theory, that
spin does have an effect on gravity. Do a search on the 'Lense-Thirring
effect'. There was an space probe planned to measure it, and while this
has not happened,

GRAVITY PROBE B
http://einstein.stanford.edu/

Gravity Probe B
http://www.gravityprobeb.com/index.html


some scientists believe they have measured the effect
from long term measurements made on some other probes.
The 'jury is out'
on this...

Best Wishes

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gravitational theme

ISBN 1844264068

Released Friday 10th November 2006 512 pages

Now available online from Amazon.co.uk, WH Smith, Tesco, etc.

"Odysseus" wrote in message
news
In article ,
"skddlbyp" wrote:

"Doug" wrote in message
oups.com...
During a web search I found this page on the spin of black holes:

http://www.space.com/scienceastronom...ay_031124.html

Doug

Thanks! Nice article.

H.J.

During a web search I found this page on the spin of black holes:

http://www.space.com/scienceastronom...ay_031124.html

Doug