How much force would be needed to get a rotationally locked planet to rotate

[Graham2001]

I've seen the concept a few times in SF but I've always wondered just how much force would be needed to get a rotationally locked world rotating.

For simplicities sake lets say an earth-sized world orbting a red-dwarf. Target rotation is between 24-30 hours.

 

[Jarvitä]

Depends on how long you want it to continue to rotate. Remember, even after you "get it started" again, you have to keep applying some force, otherwise its primary will tend to tidally lock it again over time.

 

[Moach]

well - the force here is really torque....

to get something to spin, you need to torque it against its moment of inertia (which for a planet is quite a figure) - once you get that going, the tidal forces that would "unspin" it are still there, so that's just another thing to add to accounts


i think the force you'd need to begin with would be just anything larger than the tidal force - once you clear that out, you're just applying torque to the planet, so slowly-but-surely, it would begin to accelerate into rotation

once you reach the desired RPM - you still need to keep countering the tidal forces torque, lest it'll "unspin" again




but then again.... how does one ever "torque a planet"? :blink:

 

[agentgonzo]

Moment of inertia is force multiplied by distance. You could get a planet rotating with a single newton of force if you applied it at a far enough distance from the centre of mass of the planet (you may need a long stick on the end of which you need to apply the force)

 

[Moach]

Moment of inertia is force multiplied by distance. You could get a planet rotating with a single newton of force if you applied it at a far enough distance from the centre of mass of the planet (you may need a long stick on the end of which you need to apply the force)

correct - that why i first said "torque" - which is also force X distance

but then i confusingly mixed it up with saying "force" a number of times... ok, my bad

 

[garyw]

You wouldn't want to apply all that torque in one go though. If you did the results would be a bit messy.

 

[tori]

You're thinking of angular (rotational) momentum. Torque you do apply "in one go" (just start the engine or whatever you have at the end of the stick), angular momentum builds up slowly and gently.

Applying angular momentum in one go (e.g. by setting large roll rate in the Scenario Editor of this universe) would be the messy part. With bits of cities flying about and everything.

 

[Artlav]

Applying angular momentum in one go...

Reminds me of a book by H.G.Wells called something like "A man who could work miracles".
In it, the man in question wished for the Earth to stop spinning (so he could have more night time to fix all the drunks), which was granted literally - Earth stopped spinning, but air, water, buildings, people, and so on kept on going, making one hell of a mess.
Fortunately, he had the presence of mind to wish an undo button.