Source of Energy for the Rotation of Earth

[BasketDaN]

If the earth were to all of a sudden stop spinning, would it ever start up again? Basically, is the earth only spinning because it started that way,(and it being in space, no friction to slow it down), or is there any other contributing factor?

[Tyger]

To slow down the Earth has to shed both Energy and Angular momentum. The chief mechanism for that is the Tidal Effect which transfers angular momentum to the Moon and energy to the oceans. A lesser mechanism involves the fact that the Earth rotates in the Sun's magnetic field, transfering angular momentum to the sun and losing energy by eddy currents.

[BasketDaN]

I wish I could understand a word anybody said on this site lol.

[LURCH]

Originally posted by BasketDaN
I wish I could understand a word anybody said on this site lol.

LOL! The way you posed the question shows that you actually do understand the concepts to which Tyger is referring, even if you don't understand the exact terminology. You asked if the earth just "started out spinning", and there simply isn't enough friction in space to have stopped it. Tyger was basically answering "yes" to that question. The phenomenon to which you were referring (friction causing a spinning thing to slow down and stop) is a good example of what he was talking about when he referred to shedding the energy of angular momentum.

As you pointed out, in space there is not much friction; no "atmosphere" to brush against the planet's surface and absorb the angular momentum (spinning energy). However, there is the sun's magnetic field, and the moon's gravitational pull. In order to keep spinning, the earth has to disturb both of these "forces", and so each of them exerts some brakeing power on the planet.

Another force that might be leaching away the angular momentum is a gravitational effect known as "frame dragging", which is predicted by general relativity. We do not yet have empirical evidence of frame dragging, but NASA is scheduled to launch a probe in June that should collect that evidence.

[Phobos]

Originally posted by BasketDaN
I wish I could understand a word anybody said on this site lol.

[:)] Well, welcome to Physics Forums! Rest assured that you'll get the full story here. [:)]

[BasketDaN]

Hehe thanks for the help... Ya see I get all these stupid ideas,,, the one I was thinking about when I made this post was

"I wonder if the rotation of the earth could somehow be harnessed as an energy source? I have seen pendulums that perpetually swing with no energy added to them because of the rotation of the earth... hmmm but that'd be really bad if the earth could possibly be halted in its rotation!"

Lol yeahhhhhhhh [:D]

[LURCH]

Originally posted by BasketDaN
Hehe thanks for the help... Ya see I get all these stupid ideas,,, the one I was thinking about when I made this post was

"I wonder if the rotation of the earth could somehow be harnessed as an energy source? I have seen pendulums that perpetually swing with no energy added to them because of the rotation of the earth... hmmm but that'd be really bad if the earth could possibly be halted in its rotation!"

Lol yeahhhhhhhh [:D]

Egad, that really could get ugly! But I think you may be combining two different stories, here. AFAIK, there are no pendulums that swing perpetually because of the Earth's rotation.

However, I have seen a novelty item in the form of the pendulum that appears to continue swinging indefinitely on its own. In reality, there is a small magnet hidden in the base of the stand which continues to switch on and off giving the pendulum a "push" each time it swings. Also, there is a pendulum on display in a museum (I forget where) which is used to demonstrate the earth's rotation. The pendulum is so large and heavy and well-made that when museum curators start it swinging in the morning, it continues to swing all day.

BTW, if we ever did find a way to harness the kinetic energy of planetary movement to perform useful work, I don't think we would ever use enough to significantly affect the motion of the planet.

[Guybrush Threepwood]

Originally posted by LURCH
Also, there is a pendulum on display in a museum (I forget where) which is used to demonstrate the earth's rotation.

You must refer to Foucault's pendulum (http://visite.artsetmetiers.free.fr/site_anglais/pendulum_museum_a.html)
[t)]

[schwarzchildradius]

The source of the current rotation rate of the earth is most likely the impact that created the moon. Also, there was a lot of angular momentum stored when the pieces that made up the earth conglomerated.

[russ_watters]

Originally posted by schwarzchildradius
The source of the current rotation rate of the earth is most likely the impact that created the moon. Also, there was a lot of angular momentum stored when the pieces that made up the earth conglomerated. I would expect that its mostly the later.

[BasketDaN]

Well... in a frictionless environment (space), it is almost impossible for something to not be spinning, correct?

[schwarzchildradius]

Tend to agree, RW, because mars is an example of a planet without a significant moon (demios/phobos are small wrt planets mass) and mars rotates at a good what 23 hrs per day. However, most of the terrestrial planets rotate more slowly, tending to orbital resonance etc. It's not impossible that mars was struck with a massive object at some time in its past also.