# What is Precession: Definition and 258 Discussions

Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called nutation. In physics, there are two types of precession: torque-free and torque-induced.
In astronomy, precession refers to any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the Earth, known as the precession of the equinoxes.

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1. ### Minimal rotational kinetic energy for a gyroscope to precess

TL;DR Summary: I cannot find any information on how to calculate min. KErot at which the gyroscope does not fall over I am doing a project for school in which I investigate energy loss in a gyroscope. I apply a torque on a gyroscope to initiate its rotation and then measure the time it takes...
2. ### I Why do bullets precess in the opposite way from gyroscope diagrams?

1) Bullet spin causes a bullet to become a gyroscope. Specifically, bullets have their center of pressure in front of their center of mass. Therefore, when pressed, gyroscopic forces cause a bullet to spin 90 degrees instead of tumble. See this diagram: 2)The precession does not cause the...
3. ### I General Relativity and the precession of the perihelion of Mercury

Hi, as test of GR I'm aware of there is the "anomalous" precession of the perihelion of Mercury. My question is: in which coordinate system are the previsions of GR verified concerning the above ? Thanks.
4. ### Why is there are precession frequency here?

so I need to use the eq.: omega_pr = ##mgr / L_s## with r = 6cm. But from how I imagine/see it, the rod pierces the disk through the middle so how does this create precession if the weight will not cause a torque then and thus a hortizontal change in angular momentum? Thanks in advance!
5. ### I Gyroscopic precession of a spin-stabilized bullet

Hi! I am trying to understand the physics behind the gyroscopic phenomenon called spin drift. Spin drift occurs to bullets that are spin-stabilized over the course of their flight. Spin drift starts with an induced rotation in a spin stabilized bullet. As a bullet flies through the air...
6. ### A Corresponding case of steady precession but for Tait-Bryan angles

(This has continued to bother me. I tried asking, and no response. May I please try again?) Using Euler angles, we rotate about an axis (often, axis three of a gyroscope frame), then a second (axis one of the gimbal frame), then return to the same axis as the first one (back to axis 3, but of...
7. ### A Euler vs. Tait (steady precession vs... what?)

Good Morning When one studies body rotations, there are two general approaches one uses: Euler Angles vs. Tait-Bryan Angles. The significant difference is that: Tait–Bryan angles represent rotations about three distinct axes (e.g. x-y-z, or x-y′-z″): Yaw, Pitch, Roll Euler angles use the same...
8. ### A Does the invariable plane of the Solar system have axial precession?

As the title asks, does the invariable plane of the Solar system have axial precession? And if so, how much and at what rate? I have tried to find an answer to these questions for a while now, but still haven't found any. I recently asked on reddit too, which pointed me to some speculation...
9. ### I Orbital Precession Calculation: Unit Explained

Hi, I've just calculated the orbital precession for the earth using the sigma formula of general relativity. $$\sigma=\frac{24 \pi^{3} R^{2}}{T^{2} c^{2}\left(1-e^{2}\right)}=\frac{24\pi^3×1.5×10^{11}}{3×10^7×3×10^8(1-0.0034^2)}=0.012$$ What is the unit of the result? Degrees per century or...
10. ### A Computing Thomas Precession: Part 2 | Francesco

Dear PFer's, I am reproducing the steps to derive the expression for the Thomas precession, as described in Goldstein/Poole/Safko "Classical Mechanics". Hereafter an excerpt from the book describing the step I am currently working on. Based on the text, the transformation S_3 -> S_1 should be...
11. ### A Computing the Thomas Precession: Help from PFers Needed!

Dear PFer's, I am reproducing the steps to derive the expression for the Thomas precession, as described in Goldstein/Poole/Safko "Classical Mechanics". Hereafter an excerpt from the book describing the step I am currently working on. I have been able to compute the matrix L'' (eqn. 7.18)...
12. ### B Nodal Precession: Satellites Orbiting Earth Explained

Hello, the artificial satellites that orbit the Earth with an orbital inclination greater than 0 are all subject to the nodal precession caused by the equatorial bulge of the Earth at the equator. But what I don't really understand is that at the equator the gravity is a little less strong than...
13. ### I Torque required to prevent precession

A torque is applied to a spinning gyro, and a second torque is applied to prevent precession. How to quantify the second torque? Example, a gyro spinning about the z axis is connected to a frame that can only rotate about the x axis. A torque about the x-axis is applied to the frame. What is...
14. ### I What causes the precession of the equinox?

I am reading an introductory undergraduate textbook problem in rigid body dynamics, about the precession of the equinox. I will quote the beginning of the problem: All the other planets and the sun, exert a combined average moment, M, on the earth. This moment causes the axis of the Earth to...
15. ### A Venus' non-relativistic contribution to Mercury's precession

Mercury's orbit is rather eccentric, its perigee brings it much nearer to the sun than its apogee. Venus by contrast has practically a circular orbit. When Mercury is approaching apogee, it can get close to venus if it is in the similar part of its orbit. At this point, Mercury's precession...
16. ### I Apparent Initial Acceleration due to Precession of Wheel Gyroscope?

The center of mass of a system is defined as a point that acts as if all the mass is concentrated on that one point whereby all forces may act on it. With a wheel gyroscope as an example where the center of mass is in the middle of the wheel when the wheel is spun and let go, the wheel undergoes...
17. ### Torque imposed by the Moon and Sun on the Earth given its precession

From the givens: Approximate Earth as a sphere: ##I_e = \frac{2}{5}MR^2 = \frac{2}{5}(5.97x10^{24})(6.371x10^6)^2 = 9.69x10^{37} kg*m^2## ##\omega_e = 7.29x10^{-5} \frac{rad}{s}## To calculate the rate of precession of the disk the Earth precesses around (1 revolution every 26,000 years)...
18. ### I Precession of a gyroscope in freefall

Imagine I have the following gyroscope: This gyroscope has been spun up with significant angular momentum and rests on a platform below it. It's offset at an angle and so I've drawn the R vector from the axis of rotation. So the only force being applied right now is gravitational force...
19. ### I Order of rotations: precession, nutation, spin

Hello I attach a picture of a problem from a dynamics textbook. The axle rotates about the axis AB WHILE (and the "while" here is a significant word to my question) it does that, the disk spins about an axis through C, but perpendicular to the face of the disk. As the textbooks solve...
20. ### I What does weight actually do in gyroscopic precession?

I 'get' how the gyroscope works, but I only understand it mathematically. I understand that because Torque = r x F, so the direction of the torque is such that it can change the direction of the Angular momentum, resulting in the gyroscope precessing. However, I just cannot visualise how this...
21. ### A Relationship between Larmor precession and energy eigenstates

Quantum mechanically, a spin 1/2 particle in a uniform magnetic field has two energy eigenstates ##\ket{up}## and ##\ket{down}## and rotational degrees of freedom (distinct from the energy eigenstates) about the axis of the magnetic field. this can be derived from the Pauli matrix commonly...
22. ### I Larmor precession of an electron in a magnetic field?

In QFT where the electromagnetic field is mediated by virtual photons, is it possible to describe the larmor precession of an electron as a series of emission and absorption of virtual photons? how does the spin angular momentum "evolve" over a series of events? This feels like a challenging...
23. E

### I Precession of a spherical top in orbit around a rotating star

Looking at L&L's solution to problem four of section §106. Lagrangian for a system of particles:\begin{align*} L = &\sum_a \frac{m_a' v_a^2}{2} \left( 1 + 3\sum_{b}' \frac{km_b}{c^2 r_{ab}} \right) + \sum_a \frac{m_a v_a^4}{8c^2} + \sum_a \sum_b' \frac{km_a m_b}{2r_{ab}} \\ &- \sum_a \sum_b'...
24. ### A What happens first: precession or nutation?

Good Morning Suppose, for the sake of this question, the following Euler rotations for a gyroscope) A precession about the vertical 3-axis (like with a top, going around a vertical) Then, a nutation (a leaning over) about the 1-axis Then, back to the spin itself of the top body about the...
25. ### I Can Precession Explain the Stability of High-Speed Ice Skaters?

does it take wheels for precession to happen? can precession be applied to ice skates, for example?
26. ### I Precession of spin in a magnetic field

In this lecture Lenny Susskind describes a spin in a magnetic field precesses around the axis of the direction of the magnetic field. This description is also frequently found in NMR theory which is a semi-classical theory. Lenny says if the magnetic field ##B_o## is applied in the ##z##...
27. ### Finding the precession of a gyro using Euler's equations of rotation

The rate of precession of this gyro ##\Omega## can be found by solving ##\tau_1=DMg=I_s\omega_s\Omega##. But when I apply Euler's equations to this problem, it fails. I first set the frame in the way shown in the diagram above. Then I wrote the first equation...
28. E

### B Clashing approximations for this precession problem

I have a bit of a strange puzzle I can't work out. Let's say, we have a thin cylindrical disk of mass ##m## and radius ##r## connected on one side to a light axle of length ##d## through its centre. The axle is itself connected to a light string of length ##l##, the other end of which is...
29. ### What is the minimum gyroscope speed to yield force precession?

Does anyone know what the minimum speed of a gyro to make it so a force at 0deg will yield a movement at 90deg... or even better, what the angle is wrt speed... see http://www.copters.com/aero/gyro.html for pictures. -thx, rich
30. ### Rotating birefringent calcite crystals: precession of image?

I have been doing some reading about birefringence in order to understand colors observed in different birefringent crystals when I came across the following page in connection with calcite crystal birefringence. https://www.microscopyu.com/tutorials/birefringence-in-calcite-crystals I think I...
31. ### I Precession of Mercury: Adaptation for Observers

For the observation of the gravitational redshift one needs an adaptation of the GTR of the object (related to the Sun´s gravity) and the observer (related to the Earth’s gravity). I assume that the situation is similar for the observation of the precession of Mercury, another experiment...
32. ### I Gyroscope, Gimbals Frame, lack of precession

Hello, A simple gyroscope, which is nothing but a spinning top, will exhibit precessional motion if a) it spins and b) a nonzero gravitational torque is applied to it. The tip of the gyroscope rotates while the tip rotates on itself. If the gyroscope is mounted on gimbals, which are rings, the...
33. ### I Mercury Perihelion Precession: Analytic Derivation

HI, I'm curios about the analytic derivation of Mercury perihelion precession starting from EFE - Einstein Field Equation (or simply just from Schwarzschild solution of the EFE). Can you advise me about some source or online material to learn it ? Thanks.
34. ### Engineering Gyroscopic Precession - Mass of wheel & Angular Momentum

Hello, I have this i am learning. I have been trying to find information online but have struggled to find anything which helps me. YouTube usually has good videos, but doesn't seem to on this. This is one topic i have never learned before. But keen to. I was hoping someone could help me...
35. ### I Steady precession of a gyroscope

Hi I am trying to understand the concept of why a gyroscope that is not spinning drops down but when it is spinning then it precesses. I have looked in "University Physics" by Young + Freedman and Kleppner , Morin and Gregory but i am not getting anywhere. Does anyone know of a relatively simple...
36. ### A Proton Precession in Hydrogen-Rich Liquids

When a hydogen rich is subjected to a strong magnetic field , the protons ( which has a magnetic moment) lines up along the applied field in a parallel or anti parallel state. My doubt is that- protons ( Hydrogen ion) in a hydrogen rich liquid cannot exist as an ion in a liquid because it...
37. ### B The precession of Mercury's orbit

Hi, I was reading about the general relativity to get some basic understanding and it was said that the proper answer to problem of precession of Mercury was provided by the general relativity. Then, I started reading about the precession of Mercury orbit. "Mercury deviates from the precession...
38. ### I Magnetic momentum alignment and precession

In my lecture notes on paramagnetism and diamagnetism is stated that "according to van vleck critical considering an electron in motion around the nucleus diamagnetic and paramagnetic effect compensate (classical)" The proof following takes the intrinsic momentum related to L as the momentum...
39. ### Units for Precession Rate (Ω)?

The precession rate is given as ##Ω = \frac{Mgr}{Iω}##. What are the units here?: I'm calculating it as ##\frac{1}{rad . s}##. Am I supposed to interpret this as revolutions per second, sort of like frequency, and ignore the ##rad##? Also, period is calculated as: ##T = \frac{2π}{Ω}##. So...
40. ### Resistance and Precession Torque of a Gyroscope

Hi, Say there's a gyroscope with moments of inertia Ix, Iy and Iz spinning around a vertical z-axis (see attached illustration) with a given angular velocity ωz. Notice that the gyroscope is floating in space as in that there's no gravity acting on the gyroscope. Now I apply a torque τx...
41. ### Larmor Precession and a bar magnet

As stated in the link below, when a magnet bar (or a current loop) is placed in an external magnetic field it (its magnetic moment) becomes aligned with the magnetic field while an orbiting electron would have precession with Larmor frequency. What is the reason for these different behaviors...
42. ### Precession in a 45 m high metalic antenna tower

1- the antenna + motor weights about 400 Kg (the helix weights about 170 Kg and has adiameter of 6.28 m, and the motor of the antenna weights 230Kgs), there is an excentricity between the centre of gravity of the antenna + motor and the centre of rotation of the anetnna which is 42 mm in the yy...

44. ### Larmor precession in a 2D B-field

I just tried to find the eigenvalues (for the energy), obtaining E = ±(γħ.√(Bo² + Γo²))/2 and the corresponding eigenvectors for the H matrix. But I don't know what to do to create de state vector χ.
45. ### Solving a Cipher: Celestial coordinates and precession

You guys were really helpful last time I came to you. Let's hope you can do it again. I have a sort of weird question, in a weird context. It's pretty complex, which is why I'm asking for help from experts. Let me explain. (Better grab a beverage, this will take awhile.) . I am not even an...
46. ### I Time period of precession of Sx about B

Consider an electron for which l=0 is kept in a uniform magnetic field B. For which the hamiltonian matrix is {μBB,0,0,-μBB} now if the electron is in the state 1/√2{1,1}(e.g in the eigenstates of Sx eigenvalue ħ/2} If this state is time evolved 1/√2{1,0}exp(-iEt/ħ)+1/√2{0,1}exp(iEt/ħ) where...
47. ### I About spin precession in a magnetic field

So I was trying to see what the result for the spin precession would be if the magnetic field pointed in the y-direction instead of z. I feel like either something with what I'm doing is wrong or, I'm just overlooking something because I keep getting complex energy eigenvalues. So what I'm doing...
48. ### Determining the precession radius

Homework Statement A stick of length 2L and of mass M spins with angular velocity ω around the axis perpendicular to the COM of the stick.It is rotating with making an angle Θ,with the axis parallel to Z axis.The COM has an initial velocity v=υ(to the y axis).Find it's precession radius R...
49. ### Torque-Free Precession and Wobble direction

In Torque-Free Precession of a plate, can the direction of the Wobble be either in the spin direction or retrograde to it? For instance if you tossed the plate upward vs downward with the same spin direction. Would the wobble direction and frequency be inverse?
50. ### Estimating Displacement of Uranus Due to Neptune's Gravity

I've been scratching my head over this for a while now. Seems pretty simple, but I just can't get anywhere! So, this is a problem out of pater & Lissauer's Planetary Sciences book (prob 2.10). Starts off by describing how Neptune was discovered by observing anomalies in Uranus's orbit...