A Space & astronomy forum. SpaceBanter.com

Go Back   Home » SpaceBanter.com forum » Others » Solar
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

Concept of Gravity



 
 
Thread Tools Display Modes
  #31  
Old November 17th 06, 08:26 AM posted to alt.astronomy,alt.astronomy.solar
John Santos
external usenet poster
 
Posts: 4
Default Concept of Gravity

In article , bobofficers@
127.0.0.7 says...
On 12 Nov 2006 14:59:39 -0800, in alt.astronomy, "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


Centripetal force... Not centrifugal force.


Actually, in the situation described, it *is* centrifugal force!
Centripetal forces are forces acting toward the center, in this case
gravity, which acts towards the center of the Sun. Centrifugal force
is a fictitious force acting away from the center, I.e. in the opposite
direction or up.

Just because it is a fictitious force doesn't mean it's not real! :-)

Centrifugal force arises because the problem is being viewed from the
perspective of someone on the surface of the Sun, which is a rotating
frame of reference. When viewed from a "normal", inertial frame, there
is no centrifugal force because the only forces involved are gravity
and the upwards pressure on the bottom of the person's feet, which is
what keeps him from falling to the center of the Sun. If the person
is standing on the equator of a rotating sun, he isn't stationary, but
is moving in a circle and therefor is being accelerated. For this to
happen, the forces have to be out of balance. (If they were completely
balanced, he wouldn't be moving, or would be moving in a straight line
at constant velocity.) To get constant circular motion, gravity pulls
at right angles to the direction of motion (I.e. toward the center of
the sun, since the direction of motion at any instant is horizontal,
toward the East.) The surface exerts an almost equal force upwards,
which the person would feel as his weight, but this force is slightly
less than the force of gravity. So the net force is inwards, toward
the center of the Sun, at right angles to the direction of motion.
(The result of a constant force at right angles to the direction of
motion is circular motion, since the sideways (or in this case,
downward) deflection in any small unit of time is constant.)
Since the upward pressure is slightly less than the downward gravity,
the person feels like he weighs less at the equator.

Why does the upward force behave like this, allowing the slight
difference from gravity that results in circular motion? Well,
actually it is a given in the problem, since the problem states
that the person is standing "stationary" on the surface of the Sun,
neither rising nor falling nor flying off into space! Fortunately,
it is perfectly possible to satisfy this condition, just by standing
there and moving at the same horizontal speed as the surface of the
Sun is rotating.

All this discussion is from the view point of an inertial frame of
reference (one that is stationary or moving at a constant velocity),
where the math is usually easiest.

However, when viewed from a rotating frame of reference (I.e. the
frame of the person standing on the rotating Sun), the forces have
to be transformed, which is where the fictitious centrifugal force
arises. Normally the math is much harder in a non-inertial frame, but
in the special case of uniform circular motion, the math actually
gets simpler, you just have to add in the constant centrifugal force,
which is the fictitious force necessary to keep a stationary object
from flying off, since it is actually moving.

All this applies to an object held by gravity to the surface of any
rotating body, not necessarily the Sun or the Earth, but it applies
equally well to the Moon or Jupiter or an asteroid or another star.

Disclaimer: it's been 35 years since I took freshman physics, so this
could be horribly garbled, but I think it is essentially correct.



less gravitational force in the direction of the spin? I read that on


The gravitational force is *not* in the "direction of the spin".
Actually, "direction of the spin" can mean two different things.
What I am taking you to mean is the direction that the surface
(and the person standing on it) are moving at any given instant.
(It is very important to note that this direction is constantly
changing!)

"Direction of spin" could also mean the direction of the spin vector.
(Imagine looking down on the object from above one of its poles. The
equator would appear as a circle at the outside edge of the object.
From above one pole, the object would appear to be spinning clockwise,
and from above the other pole, it would appear to be spinning
counterclockwise. The spin vector points in the direction of the
counterclockwise pole (IIRC) and its magnitude depends on how fast the
object is spinning.) However, that's way beyond this discussion.

The gravitational force is towards the center of the Sun, which is
neither of these two directions.

earth you weigh slightly less at the equator than at the poles due to
centrifugal force. Would this be true of the sun also?


Yes. However the Sun doesn't rotate very fast (about once a month), so
the effect is fairly small. On Jupiter, which rotates in about 10
hours, the effect is strong enough to make the planet slightly but
visibly oblate.

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.


Probably, but this could also be due to some general relativity effect.
(I didn't get far enough in physics to study GR, but there are lots of
strange and un-intuitive effects.)


Doug




--
John
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Gravity 1A - Back to the Drawing Board Golden Boar Misc 59 January 13th 06 12:18 AM
Putting relativity to the test, NASA's Gravity Probe B experimentis one step away from revealing if Einstein was right (Forwarded) Andrew Yee Astronomy Misc 0 October 7th 05 05:09 AM
Can't get out of the universe "My crew will blow it up"!!!!!!!!!!! zetasum History 0 February 5th 05 12:06 AM
Can't get out of the universe "My crew will blow it up"!!!!!!!!!!! zetasum Policy 0 February 5th 05 12:06 AM
Gravity as Falling Space Henry Haapalainen Science 1 September 4th 04 04:08 PM


All times are GMT +1. The time now is 02:44 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 SpaceBanter.com.
The comments are property of their posters.