Another rotation question

Reading the postings about the earth's rotation has led me to a
question. Suppose a spherical cloud of dust is rotating about a star
and that this cloud has no inherent spin. i.e. the "innermost"
particles of the cloud, for the moment, always face the star. When
this cloud collapses into a planet, will the planet spin on an axis?
If so which way? I've always been intrigued by this question, but
sadly my knowledge of math and physical is rather limited.

"BigKhat" wrote in message
om...
Reading the postings about the earth's rotation has led me to a
question. Suppose a spherical cloud of dust is rotating about a star
and that this cloud has no inherent spin. i.e. the "innermost"
particles of the cloud, for the moment, always face the star. When
this cloud collapses into a planet, will the planet spin on an axis?
If so which way? I've always been intrigued by this question, but
sadly my knowledge of math and physical is rather limited.

Let's start off by being clear about the initial conditions.

Do you posit a spherical cloud surrounding the star, or a
spherical cloud orbiting about the star as a unit (kind of
like a very diffuse planet)?

Greg Neill wrote
"BigKhat" wrote in message
. com...
Reading the postings about the earth's rotation has led me to a
question. Suppose a spherical cloud of dust is rotating about a star
and that this cloud has no inherent spin. i.e. the "innermost"
particles of the cloud, for the moment, always face the star. When
this cloud collapses into a planet, will the planet spin on an axis?
If so which way? I've always been intrigued by this question, but
sadly my knowledge of math and physical is rather limited.

Let's start off by being clear about the initial conditions.

Do you posit a spherical cloud surrounding the star, or a
spherical cloud orbiting about the star as a unit (kind of
like a very diffuse planet)?


Good question, bearing in mind the second option is an impossible
situation, and as such, incapable of a realistic solution.

Denis
--
DT
Replace nospam with the antithesis of hills

"Greg Neill" wrote in message ...
"BigKhat" wrote in message
om...
Reading the postings about the earth's rotation has led me to a
question. Suppose a spherical cloud of dust is rotating about a star
and that this cloud has no inherent spin. i.e. the "innermost"
particles of the cloud, for the moment, always face the star. When
this cloud collapses into a planet, will the planet spin on an axis?
If so which way? I've always been intrigued by this question, but
sadly my knowledge of math and physical is rather limited.

Let's start off by being clear about the initial conditions.

Do you posit a spherical cloud surrounding the star, or a
spherical cloud orbiting about the star as a unit (kind of
like a very diffuse planet)?

Sorry, I was vague. I mean the latter (diffuse planet) rather than the former.

Thanks!

"BigKhat" wrote in message
m...
"Greg Neill" wrote in message
...
"BigKhat" wrote in message
om...
Reading the postings about the earth's rotation has led me to a
question. Suppose a spherical cloud of dust is rotating about a star
and that this cloud has no inherent spin. i.e. the "innermost"
particles of the cloud, for the moment, always face the star. When
this cloud collapses into a planet, will the planet spin on an axis?
If so which way? I've always been intrigued by this question, but
sadly my knowledge of math and physical is rather limited.

Let's start off by being clear about the initial conditions.

Do you posit a spherical cloud surrounding the star, or a
spherical cloud orbiting about the star as a unit (kind of
like a very diffuse planet)?

Sorry, I was vague. I mean the latter (diffuse planet) rather than the
former.

Thanks!

Planets do not actually agglomerate that way, but if there was a
self-gravitating cloud of small objects such as you describe, it would of
course be spinning initially with period of rotation = period of orbital
revolution (it would definitely not be "not spinning"). As it condenses
into a smaller planet of the same mass as the cloud, its rotation must speed
up because it conserves its angular momentum. Angular momentum = moment of
inertia (of a sphere in this case) times angular spin rate (radians per
second). The moment of inertia is 2/5 m r^2 (2/5 times mass times radius
squared) so, if the initial cloud was 10 times larger in diameter than the
planet it coalesced to become, the spin rate would make the day 1/100 of the
orbital year.

This explanation ignores tidal effects, which might be important if the
protoplanet was close to the star.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)