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