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53 votes
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What did general relativity clarify about Mercury?

I frequently hear that Kepler, using his equations of orbital motion, could predict the orbits of all the planets to a high degree of accuracy -- except Mercury. I've heard that mercury's motion ...
Carson Myers's user avatar
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34 votes
5 answers
11k views

How do you calculate the anomalous precession of Mercury?

One of the three classic tests of general relativity is the calculation of the precession of the perihelion of Mercury's orbit. This precession rate had been precisely measured using data collected ...
David Z's user avatar
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33 votes
7 answers
19k views

Why don't spinning tops fall over?

One topic which was covered in university, but which I never understood, is how a spinning top "magically" resists the force of gravity. The conservation of energy explanations make sense, but I don't ...
Casebash's user avatar
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22 votes
5 answers
7k views

Why does a spinning top get back upright when knocked?

I have observed that, a spinning top, when toppled by knocking it with finger, becomes almost upright again (with negligible precessional motion), instead of undergoing precession while being heavily ...
Archisman Panigrahi's user avatar
13 votes
1 answer
1k views

Details of Newtonian Prediction for Mercury's Precession

Could anyone point me to a book or outline the methods used to actually calculate the 532 arcseconds per century that Newtonian theory apparently predicts for Mercury's precession. I am completely ...
13 votes
1 answer
3k views

Feynman's wobbling plate

Let us consider a symmetric top, i.e., a body whose mass distribution has axial symmetry (a cylinder, a disk, a cone, etc) free of any external torque. The Euler equations for this body are $$I_1\dot\...
Diracology's user avatar
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13 votes
4 answers
6k views

What is the physics of a spinning coin?

When we spin a coin on a table, we observe 2 things: It slows down and stops after sometime. It does not stay at just one point on the table but its point of contact with table changes with time. I ...
Sahil Chadha's user avatar
  • 2,763
11 votes
2 answers
4k views

What happens to orbits at small radii in general relativity?

I know that (most) elliptic orbits precess due to the math of general relativity, like this: source: http://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity I also know that something is ...
Alan Rominger's user avatar
11 votes
3 answers
977 views

What's wrong with this application of Thomas Precession to circular motion velocity measurements?

If you happen to have the Third Edition of Classical Electrodynamics by John David Jackson, turn to section 11.8, as that's where I'm getting all this from. If not, you should still be able to follow ...
Izzhov's user avatar
  • 1,222
10 votes
2 answers
520 views

Intuitive explanation of why orbits are closed?

Yesterday my brother asked me how orbits work. Suppose for the sake of the question that you are trying to put a rocket in orbit around the Earth. I explained that orbiting is essentially being in ...
Javier's user avatar
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7 votes
1 answer
1k views

Without using rotational mechanics, why does a gyroscope precess the direction it does?

When a top is spun, it will precess in some direction, either clockwise or counterclockwise. It's possible to find out which way using $\boldsymbol{\tau} = d\mathbf{L}/dt$ and $\boldsymbol{\tau} = \...
knzhou's user avatar
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7 votes
3 answers
520 views

Weird rotational motion of a heavy thick disc

I and my friend noticed a strange behaviour of a rotating disc (or a cylinder) and we don't know how is that happening. This is the video. Actually what is happening in the video is that the disc ...
Ankit's user avatar
  • 8,196
7 votes
2 answers
641 views

Feynman's disk paradox used as a gyroscope?

I have recently learned about Feynman's disk paradox, where angular momentum is stored in the properties of the electric field. One example would be a coil with current going through whose axis is ...
uniquehorn's user avatar
6 votes
2 answers
2k views

Mercury's Orbital Precession in Special Relativity

I am researching Mercury's orbital precession. I have considered most perturbations and general relativity. I am still not satisfied. I need your help. I need a solution to Exercise 13, Chapter 6, in ...
Raghu Singh's user avatar
6 votes
1 answer
689 views

Does motorcycle counter-steering work by precessional motion?

When a motorcycle is going over a particular speed ($\approx 20$ m/h or 30 km/h) you learn in motorcycle class the best way to steer is using the so-called "counter steering"; I can't find ...
nammerkage's user avatar
6 votes
3 answers
2k views

Why objects with magnetic dipole moment sometimes orient towards external B and other times they do precession?

Given the magnetic dipole moment in external uniform magnetic field $\vec{B}$, I am trying to understand why sometimes $\vec{\mu}$ simply aligns with the $\vec{B}$ and stays that way, while other ...
matori82's user avatar
  • 893
6 votes
1 answer
431 views

Do 2-body elliptic orbits precess in special relativity?

Einstein famously explained the anomalous precession of Mercury by showing that in general relativity elliptic orbits precess even in the 2-body problem. But apparently in the early days of quantum ...
Conifold's user avatar
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5 votes
4 answers
1k views

Are elliptical orbits really elliptical?

I have wondered for a long time how elliptical orbits can work. It seems awkward for a freely-moving object to come very close to a source of gravity and then return to the exact point where it ...
Wutaz's user avatar
  • 394
5 votes
2 answers
2k views

What gives energy for precessional motion of gyroscope?

Let us suppose we have an ideal friction-less gyroscope and it is suddenly given an angular momentum, and left as in the figure. It will start precessional motion. Since there is no friction, the ...
Archisman Panigrahi's user avatar
5 votes
2 answers
899 views

$LS$-coupling vector model precession?

In $LS$-coupling it is said (e.g. here, link to Google Books1) that in the $LS$-coupling scheme the individual $\vec l_i$ precess around $\vec L$ and the individual $\vec s_i$ precess around $\vec S$. ...
Quantum spaghettification's user avatar
5 votes
1 answer
466 views

Why can you assume that the angular momentum vector of a top will always track its axis of rotation?

My favorite physics 101 textbook (Giancoli) explains precession in terms of a spinning top whose axis is tilted from the vertical. The way the book sets things up, $L$ (angular momentum) points along ...
tel's user avatar
  • 255
5 votes
2 answers
617 views

Infinite Larmor precession

Say an isolated electron (meaning it's not part of an atom) is at rest and we turn on a homogeneous magnetic field around it. The electrons' spin undergoes the Larmor precession (except in the case ...
ritter's user avatar
  • 263
5 votes
1 answer
277 views

What is the definition of precession (in the context of Spinors)?

What is the definition of "precession"? How is it applicable to abstract objects such as Spinors? I understand the mathematics, but don't understand what one means by "precession angle" etc when it ...
Antillar Maximus's user avatar
5 votes
3 answers
2k views

When an electron orbits in a magnetic field, how exactly does its spin precess?

In the case of a cyclotron, with a constant magnetic field $B$ in the vertical direction, a moving electron circles in a horizontal orbit. The cyclotron frequency is $\omega = eB/m$. At the same ...
Giulia Tozzi's user avatar
5 votes
4 answers
5k views

Precession and alignment in a magnetic field

I am very confused about the concept of alignment in a magnetic field. Perhaps I am also confusing two different phenomena and that may be the issue. My classical understanding of a dipole says that ...
jaslibra's user avatar
  • 199
5 votes
1 answer
2k views

Relation between Zeeman effect and Larmor precession

I'm wondering if there's some intrinsic reason that the precessional frequency, $\omega$, of a spin about an external magnetic field matches the frequency of a photon emitted (or absorbed) in ...
bigbadpiano's user avatar
5 votes
1 answer
4k views

How to show circular orbit as a precessing ellipse

A uniform distribution of dust in the solar system adds to the gravitational attraction of the sun on the planet an additional force linear in $r$ where m is the mass of the planet, k is a constant (...
user avatar
5 votes
0 answers
239 views

MRI and precession

A lot of explanations of the quantum mechanics of MRI discuss the precession of a proton in an external magnetic field, for example here: http://www.physicscentral.com/explore/action/mri.cfm Doing ...
Watw's user avatar
  • 1,209
4 votes
2 answers
18k views

How, exactly, does a reaction wheel work?

Reaction wheels, mounted on spacecrafts and satellites, are used for precision attitude control. It is not clear to me how they can do this, though. My best guess is that when a motor accelerates a ...
CoilKid's user avatar
  • 1,332
4 votes
1 answer
2k views

Understanding the operation of Thomas precession

How can we physically understand the operation of Thomas Precession? Thomas Precession is an effect that modifies the effective energy of coupling between the spin and the orbital angular momentum ...
SRS's user avatar
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4 votes
1 answer
2k views

Newtonian contribution to perihelion precession of Mercury

We have seen that the orbits of the planets around the sun can be found, to a good approximation, by only considering a two-body system (a planet and the sun) interacting through a central force. ...
PG1995's user avatar
  • 717
4 votes
1 answer
2k views

How to tell if an orbit is closed given the potential?

I'm working on a homework question in which I am supposed to discuss the types of motion that orbits may have under a certain potential. What I've done so far is draw the energy diagram for such a ...
Germ's user avatar
  • 297
4 votes
3 answers
2k views

Does the length of the sidereal day vary systematically?

I'm confused about some properties of the sidereal day, in particular whether its duration varies systematically over the course of the year.1 It seems to me that that must be the case, but the ...
orome's user avatar
  • 5,125
4 votes
2 answers
672 views

How does one calculate the full perihelion shift of Mercury, including perturbations from other planets?

I'm talking about the full calculation, including perturbations from other planets. I've seen the general relativistic correction done a half dozen times before, but I can't say that I've seen the ...
Ryan Unger's user avatar
  • 8,843
4 votes
2 answers
14k views

What causes precession or nutation in a spinning object?

What causes precession in a spinning object? What causes nutation in a spinning object? What causes a top, gyroscope, and the earth to wobble? Just because it's a simple question, I'm not expecting ...
Arlen Beiler's user avatar
4 votes
2 answers
388 views

Dual gyro system: will it resist a turning force?

Let's say there are 2 gyroscopes. Both are connected to the same frame (orange). Both are spinning at the same speed, but in opposite directions (red). If I spin the frame (green arrow), the gyros ...
Botspot's user avatar
  • 95
4 votes
4 answers
1k views

How do I calculate Earth's axial precession in an idealized Sun-Earth system?

I am experimenting with an idealized two-body system based on the Sun-Earth system. I want to calculate how much Earth's axis precesses and nutates given the simplified setup. In my system, the orbit ...
Lawton's user avatar
  • 327
4 votes
1 answer
104 views

What physical phenomenon causes racing bike to lift automatically after it fall down?

What physical phenomenon causes racing bike to lift automatically after it fall down? What causes it to travel further distance after it fall down? Please help me. Animation: YouTube link :https://...
Satya's user avatar
  • 141
4 votes
2 answers
664 views

Orbital period and nodal precession & apsidal precession

In classical orbital mechanics as per Newton/Kepler, the time taken to complete one orbit around a body of mass $M_e$ is: $$ T = 2 \pi \cdot \sqrt { \frac {a^3} {G \cdot M_e} } $$ But also, for ...
Avijit's user avatar
  • 625
4 votes
1 answer
3k views

Minimum speed needed for a gyroscope to precess

I was reading about gyroscopes and their precession. Based on the text, the angular speed of precession is: $\omega_{precession}=\dfrac{\tau}{L}$ But intuitively, if the wheel of the gyroscope is ...
Soroush khoubyarian's user avatar
4 votes
2 answers
2k views

Why do gyroscopes not maintain their stability as seen in this video?

What exactly causes gyroscopes to oscillate as seen in this video. http://www.youtube.com/watch?v=gdAmEEAiJWo Even when my toy gyroscope spins on Earth, I get oscillations. Is this a result of ...
Monte Carlo's user avatar
4 votes
1 answer
609 views

Does the Moon's orbital inclination follow the wobble in the Earth's rotation?

As the Earth wobbles during rotation, does the higher gravity at the equator tend to pull the moon toward an equatorial orbit even as the earth does that thousands of years wobble cycle? It would ...
Ike Higgins's user avatar
4 votes
1 answer
255 views

Spinning Wheel subjected to Gravitational Force

have a question about the dynamics of a suspended horizontally rotating wheel subjected to gravitational force like in following picture I took from here: At the beginning the wheel has an angular ...
user267839's user avatar
  • 1,375
4 votes
0 answers
68 views

What kind of torques cause an object to precess? [closed]

In studying precession, my textbook (Taylor's Classical Mechanics) makes the assumption that a top spinning about its symmetric axis, but tipped at an angle $\theta$, will precess nicely so long as ...
Nick Chapman's user avatar
4 votes
2 answers
2k views

Will a precessing spinning wheel fall down if there is no friction at all?

If there where no friction at all, would a spinning wheel held up by one end of the axis spin precess forever without falling down? I just asked another question about the same problem: Direction ...
martin's user avatar
  • 1,161
4 votes
3 answers
3k views

Anti-gravity wheel?

So I was just watching some YouTube videos on a spinning wheel that seemed to "defy" gravity. The creator made two videos on it, the first showing the wheel, and the second with an attempt ...
User3141's user avatar
  • 783
3 votes
5 answers
16k views

Direction of torque precession of a spinning wheel

Consider a spinning wheel, which is held up by one end of it's axis like this: To explain why the change of angular momentum is directed as shown in the figure above, one usually says that there is ...
martin's user avatar
  • 1,161
3 votes
2 answers
471 views

Mercury's orbit

Mercury has a weird orbit. It's elliptical orbit rotates around an axis, but for no readily apparent reason. My question is why? What is causing this?
Nicholas Pipitone's user avatar
3 votes
3 answers
596 views

Can precession happen with no external forces?

I wanted to ask the following question: Can a body that experiences no forces whatsoever precess? Let's say I have a body in space - no gravity or anything - can I make it precess without applying ...
GSofer's user avatar
  • 343
3 votes
3 answers
396 views

Is the torque on a gyroscope a conventional virtual force perpendicular to rotation?

So I am having some difficulty understanding gyroscopic precession. I understand that mathematically by convention torque is perpendicular to the force and so is angular momentum but surely that force ...
The homeschooler's user avatar

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