What makes the Earth keep spinning?
[Physics] What makes the Earth keep spinning
angular momentumconservation-lawsnewtonian-mechanicsrotational-kinematics
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There is an explanation of precession entirely in laymen terms in an earlier thread (answer by me, Cleonis, on december 26 of 2012) That earlier thread is What determines the direction of precession of a gyroscope?
Necessary and sufficient for an object to be in a state of gyroscopic precession: the object is spinning, and a torque is being exerted. In my explanation I show how that happens.
Nutation can be thought of as a form of overshooting. If you release a spinning top roughly then for sure there will be a nutation superimposed upon the precession. You can avoid nutation by releasing very gingerly. (In effect the careful release dampens extra motions.)
Generally the nutation motion of a spinning object will dampen out fairly quickly, the kinetic energy of the nutation motion dissipates relatively fast.
The nature of nutation:
I mentioned 'overshooting'.
Let's say you have a gyroscope set up with it's spin axis horizontal, so that the torque from gravity is maximal. Then you release the spinning top to just drop down.
- The center of mass of the spinning top then moves down. That downward motion induces precessing motion (see the linked to explanation)
- the precessing motion is so fast that the spinning top climbs again.
- The climbing motion reduces the precession rate.
- because of the reduced precession rate the center of mass drops down again.
So, the nutation is a cyclic energy conversion process:
- there is a stage of giving in to the torque that is being exerted.
- during that stage the torque is doing work and the kinetic energy of the spinning object increases, which goes to the precession rate.
- When the spinning object is climbing again its kinetic energy decreases again as it gains potential energy.
When nutation has dampened out then the rate of precession is precisely enough to keep gravity from pulling the center of mass of the spinning top lower.
Gyroscopic precession and nutation are intimately related. Nutation is inherently a perturbation of a state of precessing motion. That is, there is no such thing as nutation without gyroscopic precession.
Earth wobble
The Earth's spinning motion is very complicated. There is a torque from the Sun and a torque from the Moon, and the Moon's orbital plane is at an angle with the Earth orbit around the Sun. Probably those influences play a role in the Earth wobble.
General remark: It's obvious that the angular momentum of the spinning top is not conserved. You get gyroscopic precession when a torque is exerted. That torque causes change of angular momentum.
Still, stable gyroscopic precession is cyclic, so something is conserved there. The direction of the angular momentum is changing all the time; the thing that remains the same is the magnitude of the angular momentum.
Your intuition about spinning fluids is wrong for a couple reasons.
Angular momentum is conserved so an isolated system of any shape will keep on spinning unless it has a way to transfer that momentum elsewhere. If you spun in egg levitating in a vacuum it would spin forever.
The more bumps, flaws, or non-spherical features your container has the faster it can transfer the angular momentum of the fluid to the container, and then from the container to the environment. The Earth has these features, but they are very very tiny compared to the overall (spherical and smooth) size of the planet.
Most containers you've spun have probably caused your brain to over-estimate the amount of angular momentum they have when you spin them because you don't get all of the fluid spinning. When you twist a container the momentum gets transfered from the interface between the fluid and the container rather inefficiently. It takes a lot of revolutions to get everything "up to speed". The Earth, besides having been "up to speed" since the beginning, having been consolidated from an already-spinning volume of dust and gas, is also spinning in isolation, per #1 above.
The Exploratorium exhibit in San Francisco has a great demo of a fluid spinning in a spherical container called the Turbulent Orb:
Their description:
The Turbulent Orb is a large polycarbonate sphere full of special, colored, flow-visualization fluid. The sphere is mounted on top of a pedestal and can be spun in either direction and at different speeds. The fluid in the sphere shows swirls and waves of internal fluid motions produced by the actions of the visitors. The turbulence of the fluid in the sphere is reminiscent of the turbulent flows that occur in planetary atmospheres. This exhibit shows the complexities of fluid motion that can be produced by very simple circumstances.
My own experience playing with the exhibit is that you must spin the outer sphere around dozens of revolutions before all of the fluid in it is moving uniformly. Once you do that, the fluid inside of it continues to spin for quite some time. Because the fluid has a pearlescent additive you can even find evidence that the central portion of the fluid keeps spinning faster than the outer fluid (which slows faster due to friction with the stationary shell). If you first spin the orb in one direction and then let the outer fluid slow a bit, and then spin it in the opposite direction you'll see vortices form with axes tangential to the outer sphere. You do not get this effect if you start spinning it when the fluid is stationary.
Best Answer
I agree with the previous answer. Angular momentum, something the earth has because of its rotation about its axis, can only be changed when an external torque (twisting motion) is applied to the earth. As far as I know, there are two ways in which this can happen. If there was friction between the earth's surface and space, then that would slow down the earth. However, this is negligible because space is essentially a vacuum. The second way is through the gravitational/tidal force applied by the moon. This force creates the tidal bulge of the earth's oceans, exerting a torque. So, while the earth seems to be spinning at a constant rate, it is actually slowing down slightly over time because of these tidal forces.