thrown off

Dear Mike Dworetsky:

On Friday, June 21, 2013 8:04:33 AM UTC-7, Mike Dworetsky wrote:
dlzc wrote:

Dear r_dela...:

On Thursday, June 20, 2013 7:28:24 PM UTC-7,

wrote:

How fast would the earth have to spin, so that stuff
started flying off?

Others have given you an answer based on the Earth
being a solid body with its current shape.

The Earth is like a creme filled chocolate, and if
you even tried to double its speed, it would lobe
up, and spin off another Moon. Even a 50% increase
would be pretty disastrous.

If by double its speed, you mean an equatorial
velocity of 0.93 km/s, it would have only a very
small effect (slightly more oblate) and in fact the
Earth probably had such a speed a very long time ago.

We had 16 hour days about 2.2 billion years ago (tidal rhytmites). This was when the Moon was much closer. Today, the Moon affects a 4m high lump in the Earth's crust. Spin it faster, and I think that lump will not have time to form, which will increase drag on the plates themselves. It will be really easy for the system to become unstable...

The idea that the Earth "spun off" the Moon is
very old and nowadays pretty well discredited.

I think it is inherent to a Theia collision, post collision, allowing for sorting of light elements into a lobe of sharper curvature... prior to the masses separating. Spinning off uncaused, yes I agree with you. Spinning off because the mergence had too much angular momentum, makes sense to me.

David A. Smith

On 21/06/2013 10:11 AM, dlzc wrote:
Dear r_dela...:

On Thursday, June 20, 2013 7:28:24 PM UTC-7,
wrote:
How fast would the earth have to spin, so that stuff started flying
off?

Others have given you an answer based on the Earth being a solid body
with its current shape.

The Earth is like a creme filled chocolate, and if you even tried to
double its speed, it would lobe up, and spin off another Moon. Even
a 50% increase would be pretty disastrous.

Ummm, creme-filled chocolate Moon.

Yousuf Khan

Dear Yousuf Khan:

On Friday, June 21, 2013 8:16:49 AM UTC-7, Yousuf Khan wrote:
....
Ummm, creme-filled chocolate Moon.

Good thing I am not that hungry...

David A. Smith

set the following eddies spiralling through the
space-time continuum:

How fast would the earth have to spin, so that stuff
started flying off?

--
Rich

Assuming the earth did not change shape at its new spin rate (it inevitably
would) then if it were to spin at one revolution in 84 minutes, that would
be enough for centrifugal force at the equator to balance earth's gravity.

It's no coincidence that 84 minutes is the orbital period of satellites in
low earth orbit.

In reality the earth would not hold together at that sort of spin rate. It
would flatten even more, which would reduce the critical spin rate even
more - positive feedback leading to earth breaking up.
--
ξ: ) Proud to be curly

Interchange the alphabetic letter groups to reply

In article ,
Curlytop wrote:

snip

It's no coincidence that 84 minutes is the orbital period of satellites in
low earth orbit.

In reality the earth would not hold together at that sort of spin rate. It
would flatten even more, which would reduce the critical spin rate even
more - positive feedback leading to earth breaking up.

How much flatter would the Earth have been in the Palaeozoic, when the
length of a day was, say, 20 hours (i.e. its rotational speed was 20%
greater than that of today)? Is there a Hooke's-Law-like idealized
relation between angular speed and oblateness for an elastic ball?

--
Odysseus

In article ,
Odysseus writes:
Is there a Hooke's-Law-like idealized
relation between angular speed and oblateness for an elastic ball?

The relation for liquids was worked out in the 18th(!) century. See
http://farside.ph.utexas.edu/teachin...ml/node38.html

Looking up "Maclaurin spheroids" and "Jacobi ellipsoids" will provide
more information. I think these are all based on constant density,
but there must be similar solutions for any specified density law.
The tricky bit is that for fast enough rotations, the body tends to
become non-axisymmetric. I never took the course that studied all
this stuff, though, so don't ask me for details.

--
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