# Tagged Questions

Gravity is an attractive force that affects and is effected by all mass and - in general relativity - energy, pressure and stress. Prefer newtonian-gravity or general-relativity if sensible.

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2answers
459 views

### Ski Jumper's vertical velocity after 246.5m record?

What would be the vertical velocity of this ski jumper (ski flyer), after he first touches down, after he breaks the record with a 246.5m jump? What g force would he experience as he slows down? ...
2answers
73 views

### What is a “gravitational cell”?

I am not a physicist, and I don't understand the details of electromagnetism. Anyhow, I was looking for how the batteries work in Google. So, I came across this article: "How batteries work: A ...
2answers
137 views

### General Relativity & Kepler's law

According to Kepler's law of planetary motion, the earth revolves around the sun in an elliptical path with sun at one of its focus. However, according to general theory of relativity, the earth ...
1answer
601 views

### String theory and trace anomaly in semiclassical gravity?

what does string theory have to say about the trace anomaly in the expectation value of the stress energy tensor of massless quantum fields on a curved background and its interpretation as the ...
1answer
384 views

### Gravitational attraction of triangles

Suppose I have two triangles relatively close together (so they probably shouldn't really be treated as point masses). I want to calculate the gravitational force (and potentially torque?) generated ...
1answer
59 views

### Physics of a cold and hot top

Imagine two tops made up of exactly one thousand atoms. One is kept at 4 degrees Kelvin, the other at room temperature. 1. Would they weigh the same given an arbitrarily precise scale in the Earth's ...
1answer
65 views

### Understanding bending light beam perpendicular to motion

I'm just reading a book about gravity. An example it gives is a spaceship accelerating. A beam of light travelling at right angles to the direction of movement of the spaceship enters it via a small ...
1answer
13 views

### Can we build a synthetic event horizon?

If we imagine ourselves to be a civilization capable of manipulating very heavy masses in arbitrary spatial and momentum configurations (because we have access to large amounts of motive force, for ...
1answer
79 views

### Initial position and velocity of rocket to escape earth's gravity

I'm trying to numerically simulate a spacecraft trajectory between earth and mars. I already wrote the solar system model where the sun is at the origin of the x,y,z plane Earth and Mars are orbiting ...
1answer
199 views

### Special relativity paradox and gravitation/acceleration equivalence

One of the features of the black hole complementarity is the following : According to an external observer, the infinite time dilation at the horizon itself makes it appear as if it takes an ...
1answer
79 views

### Energy needed to lift and bring down an object

A mass of 0.5 Kg needs to be moved from point A to another point (B) which is 1 meters above point A. The time for this movement should be 0.2 seconds, then the mass is kept at position B for another ...
1answer
58 views

### Geodetic model for numerical weather prediction

What is the implicit geodetic model used by common numerical weather prediction products? WGS84? NAVD88? For example, NOAA's Rapid Refresh (RAP), North America Mesoscale (NAM), and Global Forecast ...
1answer
102 views

### Do all black holes spin in the same direction?

My question is as stated above, do all black holes spin the same direction? To my knowledge, the spin in the direction of the spin of the matter that created them. Another similar question was asked ...
1answer
72 views

### The effects of heat on gravitational fields

In boiling soapy water, globs of soap coalesce as the temperature increases to boiling. Does this mean that temperature increases the gravitational pull of bodies?
0answers
86 views

### Equation of state of cosmic strings and branes

I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ...
0answers
91 views

### Positivity of Total Gravitational Energy in GR

I read the following statement in the introduction to an article: Over the last 30 years, one of the greatest achievements in classical general relativity has certainly been the proof of the ...
0answers
156 views

### Penrose Conformal diagram for flat 2-dim Lorentz space-time

I have the following metric $$ds^2 ~=~ Tdv^2 + 2dTdv,$$ defined for $$(v,T)~\in~ S^1\times \mathbb{R},$$ e.g. $v$ is periodic. This is the according Penrose diagram: Question 1) Is the ...
0answers
148 views

### Why Brown York stress tensor == dual field theory's energy momentum tensor?

From the AdS/CFT dictionary, how to argue that the Brown York stress tensor for a gravity system near the boundary is exactly the same as the energy momentum tensor of the dual field theory? In the ...
0answers
55 views

### Gravitational effects and metric spaces

Could somebody please explain something regarding the Nordstrom metric? In particular, I am referring to the last part of question 3 on this sheet -- about the freely falling massive bodies. My ...
0answers
78 views

### Materials with different gravitomagnetic permeability?

If you start with general relativity, and assume small perturbations around a nearly flat metric, it is possible to obtain linearized equations of gravity that look a lot like Maxwell's equations, ...
0answers
177 views

### Alcubierre warp bubble effect on gravity and space

Hello I read this question: Faster-than-light communication using Alcubierre warp drive metric around a single qubit? and I had this question: What kind of impact would an alcubierre warp bubble have ...
0answers
237 views

### How does lifting an object effect its entropy

I have figured out that: When photons leak out from a container, the entropy of the photon collection increases, because each photon has a different escape time. Photons that have leaked out from a ...
0answers
86 views

### Finite or ∞ set of masses & ∃ gravity center?

Any finite & non empty set of masses has a computable center of gravity: $\vec{OG} = \frac{\sum_i m_i \vec{OM}_i}{\sum_i m_i}$ . Does the contrapositive permits to conclude that a mass system ...
0answers
135 views

### Early stages of a computational model for object movement charting

We would like to build a computational model capable of accurately predicting the position of any object inside a chamber at any given time. Inside the model we would have a number of smaller ...
0answers
49 views

### Dirichlet's work on gravity in non-Euclidean space?

In the book The Norton History of Astronomy and Cosmology by the late John North I have found the following statement (page 514): "The German mathematician Lejeune Dirichlet studied the law of ...
0answers
159 views

### include the stretch of the spring own weight in potential energy for spring pendulum?

we are given a problem with spring with its own mass $m$. I am confused how to set up the PE term in the Lagrangian. Assume the spring has length of $L_{0}$ when it is laying on a table horizontally. ...
0answers
101 views

### What results from particle collision would ensure the existence of the graviton?

I understand that particles are smashed together to try to enable us to detect some sort of graviton presence but we can't actually detect a graviton due to the fact that it 'exists' in some extra ...
0answers
232 views

### Why does a coin falls faster when it's flipping as well?

From my experiments with measuring how fast a coin falls, I have consistently measured a faster falling rate for a coin that flips as it falls. As an example, a coin dropping on its edge from height ...
0answers
137 views

### Calculation of a Gravity Resonance Keyhole

Can anyone describe the mathematics behind the calculation of a resonance keyhole (for a two-body model)? It seems like the size and position of the keyhole should be a function only of mass and ...
0answers
19 views

### The gravitational random walk

When we shoot a single photon out into space, the chance that it will eventually return to our vicinity from a different direction is vanishingly small, even though spatial curvature exists due to the ...
0answers
37 views

### When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?

It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
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60 views

### What is the critical mass of a planet to have an atmosphere like Earth's?

Small planets/orbits like Moon cannot have atmosphere because of their masses. They don't have enough gravity to hold an atmosphere. Then what is the critical mass that makes enough gravity to keep an ...
0answers
57 views

### Thermal gravitational radiation and its detection

To my poor knowledge on the topic, the gravitational waves that are most likely to be detected by LIGO or other experiments do not have thermal spectrum. But I'm not certain. I know that Hawking's ...
0answers
44 views

### What other factors effect the ability of a propigating field of pressure in a gaseous medium to predict gravitational collapse?

What other factors effect the ability of a propigating field of pressure in a gaseous medium to predict gravitational collapse? Will the only factor influencing the Speed of Sound in this medium be ...
0answers
77 views

### How complete is our understanding of general-relativistic solutions for extremal black holes?

Putting aside quantum mechanics (or at least putting aside the question of fermions), is our knowledge of extremal General-Relativity solutions good enough that we would be able to rule out a ...
0answers
183 views

### How to model an accelerometer measurements on a car wheel?

I am working on kinematically modelling an accelerometer on a car wheel. When working on the initial conditions, I am confused whether or not I should use the gravitational acceleration since there ...
0answers
256 views

### Quantization of Gravitational Field: Quantization conditions

I'm begining to study Quantization of field with the second quantization formalism. I've studied phononic field, electromagnetic field in the vacuum and a generic relativistical scalar field. I ...
0answers
46 views

### Flung out of the galaxy

I watched a video by Dr. Michio Kaku, in which he states a theory about Dark Matter. This theory says that Dark Matter could be just ordinary matter from another parallel universe, which would be ...
0answers
45 views

### Water Stream from a Horizontal Surface

If water was projected from a flat surface where gravity was equal all over the surface. What would happen when the water fell in on itself? The water is in a continuous stream and is perfectly ...
0answers
105 views

### Did force of gravity cause macroevolution?

Did big bang create gravity? What role gravity is assumed to have played in the formation (starting from the big bang) of large structures of our universe and what other important physical mechanisms ...
0answers
66 views

### How to calculate the mass of the Cygnus X-1 black hole?

I have received a question about how to calculate the mass of Cygnus x-1 (black star). Since we are able to find the the mass through this Wikipedia page I know that we can find the mass through ...
0answers
135 views

### What is the pressure in a vertical pipe that has moving fluid through it?

I have a system like this When the valve is closed, pressure along any point along the thin tube may be found thru pgh. 1. What about when the valve is opened? 2. Does the top of the jar need to be ...
0answers
71 views

### Where to go to minimize tidal forces?

Suppose you design an experiment where you need to minimize the effects of tidal forces. Where would you go? There are a few possibilities, and the choice depends on how much effort you are willing to ...
0answers
66 views

### If one were to move withing a tesseract, would the direction of gravity be different within each cell

If you unfold a tesseract into 3D space you get a cross shape (basically). Animated, it looks like the bottom most cube becomes inverted from it's 4D orientation. Would that mean that the pull of ...
0answers
50 views

### Concave Sphere Habitat characteristics

I'm writing a short story set in an artificial planet-sized sphere with an ecosystem in its inner surface, whose "gravity" is created through spinning. Energy sources aside, what other interesting ...
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