10
votes
Accepted
Why does a spinning top get back upright when knocked?
The torque that rotates a top upright, as happens in that youtube video, is due to sliding friction between the top and its supporting surface.
Crucial to this effect is the fact that the top in that ...
- 7,581
9
votes
Accepted
How to tell if an orbit is closed given the potential?
It is not enough to have $r(t)=r(t+\Delta t)$: you must simultaneously have $\phi(t)=\phi(t+\Delta t)$ where $\phi$ is the angular degree of freedom. To put it differently, your need the radial and ...
- 42.9k
9
votes
Accepted
Newtonian contribution to perihelion precession of Mercury
You are essentially correct to assume that the precession is sometimes backwards as well. The frequently quoted value of 531"/cy is only the long term net average. In reality there is a ...
- 1,966
9
votes
Accepted
Does motorcycle counter-steering work by precessional motion?
A precessing effect might be there, but it isn't necessary.
In order to negotiate a turn you need to initiate the lean first. To obtain a lean to the right you need to steer the contact patch of the ...
- 18.4k
9
votes
Antigravity force in powered precession gyroscope
This scheme gets invented every 20 years or so and then gets promptly disproved both by experiment and by dynamical analysis. For example, I thought that I had invented this very scheme with my ...
- 83.8k
8
votes
Details of Newtonian Prediction for Mercury's Precession
The Newtonian Mechanics prediction for the Mercury's Precession is actually $532''$ per century.
The general result
If the central force is attractive, there is a circular orbit of radius $r_0$. ...
8
votes
Accepted
What gives energy for precessional motion of gyroscope?
The centre of mass of the rod and the spinning disc drops a little.
Thus there is a loss of gravitational potential energy and a gain in the precessional kinetic energy.
You can think of it as the ...
- 86.8k
6
votes
Accepted
Without using rotational mechanics, why does a gyroscope precess the direction it does?
The reason why a gyroscope does behave in this strange way is that if you try to rotate it's axis in some direction, the "endpoints" of this axis have to be pushed perpendicular to what our first ...
- 1,201
6
votes
Accepted
How was the neutron's spin measured?
The spin of the neutron was measured by the Stern-Gerlach experiment by Sherwood, Stephenson and Bernstein (1954) (sadly paywalled, free links welcome),
Abstract: A neutron beam was polarized by ...
- 26.6k
6
votes
Accepted
Confusion on conservation of angular momentum
Angular momentum is a much more complicated beast than linear momentum.
$\renewcommand{\tensor}[1]{\overleftrightarrow{#1}}
%% ugh, the \over arrows have different spacing than \vec
\renewcommand{\vec}...
- 6,389
5
votes
Accepted
Precession of perihelion of Mercury?
The GR precession angle correction for the Sun-Mercury pair is proportional to $G M_{\odot} / L$, where $M_{\odot}$ is the mass of the Sun while $L$ is the angular momentum of Mercury relative to the ...
- 4,878
5
votes
Is it possible to estimate the 23.5° tilt of Earth's axis?
So out of curiosity, I decided to go through some more fundamental models and their reasonable simplifications in deriving the principle behind Earth's rotational axis' precession. If you want, read ...
- 2,307
4
votes
Accepted
Gyroscope precession
This gyroscope precession isn't very intuitive but you can understand in terms of rotational motion that you probably do have an intuition for.
As the torque from the supporting cable tips the wheel, ...
- 8,087
4
votes
Accepted
Earth precession
A top's precession is caused by torque from the point not being aligned with the center of gravity, this tends to try to get the top to rotate away from the stable axis.
In the case of Earth, the ...
- 4,458
4
votes
General relativity and precession of orbits
The difference between the observed precession rate and the calculated precession rate excluding GR is called the anomalous precession. The magnitude of the anomalous precession rate depends mostly on ...
- 345k
4
votes
Accepted
Do stellar black hole precess like Earth do?
Yes, black holes can precess.
In binary consisting of two black holes, both spins and the orbital angular momentum will precess about the total angular momentum.
In April, LIGO and Virgo announced the ...
- 9,741
4
votes
Accepted
Is there still an anomalous precession in Mercury's orbit that GR doesn't predict?
The wikipedia section on the precession of Mercury's orbit is in serious need of an update. The presented observational data is centuries old. Its main reference is the Clemence paper from 1947. Since ...
- 22.5k
4
votes
Antigravity force in powered precession gyroscope
There is no magic anti gravity force. The force F comes entirely from the table and is the same whether the gyroscope is spinning or not. If the table is removed the gyroscope simply falls with ...
- 85.8k
3
votes
Why does a spinning top get back upright when knocked?
The top is a symmetric rigid body. The equations of motion of a rigid body around its center of mass are given by: (Please, see for example: Marsden and Ratiu , (page 6).
$$I_1\dot\Omega_1=(I_2-I_3)\...
- 29.9k
3
votes
Accepted
Why does gravitational torque on a precessing gyroscope turn the spin momentum instead of making the gyroscope fall?
Actually - if you start from rest, the gyroscope does fall. A tiny little bit. And it is that initial downward motion that leads to the forces needed to start the precession. The downward motion leads ...
- 118k
3
votes
Accepted
Mercury's precession
If two bodies orbit each other and the mass of one of them is much larger than the mass of the other body, the equations of motion can be solved exactly. In polar coordinates the solution is given by:
...
- 4,779
3
votes
Can precession happen with no external forces?
Any spinning body with a non scalar inertia tensor (i.e. the inertia tensor is not proportional to the identity matrix) will have its angular momentum and angular velocity vectors in different ...
- 87k
3
votes
Accepted
Lagrangian of a precessing wheel: gyroscope in Lagrangian mechanics?
I) choose coordinate system $(x,y,z)$
II) choose the generalized coordinates $\varphi\,,\psi$
Gyroscope
III) create the rotation matrix $R$ via the generalized coordinates and in your case also ...
- 10.9k
3
votes
Accepted
Can you explain gyroscopic precession using only Newton's three linear laws without applying their angular cousins?
"Intuitive" is a tricky word. Most people find gyroscopic effects unintuitive no matter what we do. And by far the most intuitive way to understand gyroscopic effects is through angular momentum ...
- 44.7k
3
votes
Accepted
Is the torque on a gyroscope a conventional virtual force perpendicular to rotation?
This is a good question. Gyroscopic precession is also what has baffled me the most of all classical mechanics I've encountered.
The force comes from the inertia of the spinning mass. Gravity tries to ...
- 49.5k
3
votes
Accepted
"Precession" of a free electron
The talk about precession is misleading. The electron is in an eigenstate of the Hamiltonian, and is thus in a stationary state. The only time evolution is a global phase:
$$ \exp{(-i\frac E {\hbar} t)...
- 29.1k
3
votes
Is the Laplace-Runge-Lenz vector applicable for test particle motion around black holes?
There is no direct generalization of the LRL vector for geodesic motion in black hole spacetimes, however there are some concepts in general relativity that play analogous roles:
Carter constant, a ...
- 13.9k
2
votes
How to show circular orbit as a precessing ellipse
I am unsure how this is the precession of the ellipse and how the
precessing ellipse can be proved to give a nearly circular orbit. Will
showing that the angular velocity $w_0$ being nearly ...
- 17k
Only top scored, non community-wiki answers of a minimum length are eligible