# What causes precession or nutation in a spinning object?

1. What causes precession in a spinning object?

2. What causes nutation in a spinning object?

3. What causes a top, gyroscope, and the earth to wobble?

Just because it's a simple question, I'm not expecting a simple answer, but please do summarize whatever you say in laymen terms, thanks.

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Have you attempted to read this en.wikipedia.org/wiki/Precession? – joshphysics Feb 6 '13 at 0:36
The "wobbling" is called nutation. Precession is the "orbiting" motion. Which one did you mean? – Mark Eichenlaub Feb 6 '13 at 0:53
I thought the north pole moving in relation to the stars is called precession. I mean the wobble. – Arlen Beiler Feb 6 '13 at 1:13
– raindrop Feb 6 '13 at 4:11

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.

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The precession of an object can be linked to the concept of the conservation of angular momentum. When vector forces acting on a body are not perfectly aligned with axes of rotation, then a torque is induced that cause a secondary rotational motion. In planetary systems, most precession is caused by the other planets or internal dynamics which apply forces not in perfect alignment with principle axes of rotations. In fact, it is explaining of the anomalous precession of Mercury, which exceeded the influence of any accountable external body, that provided the earliest observational support for General Relativity by which can calculate the precession of Mercury precisely.

Update: Another discussion about precession and perturbation can be found here.

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