# 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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1answer
14 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 ...
0answers
21 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 ...
6answers
2k views

### How can a black hole emit X-rays?

Considering that a black hole's gravity prevents light from escaping, how can a black hole emit X-rays? Visible light and X-rays are both electromagnetic radiation, shouldn't the black hole's gravity ...
0answers
43 views

### Are Dark Energy, Dark matter invented concepts to cover up our lack of understanding? [closed]

Dark Energy?, Dark matter? mean two entities that we have no proof of its existence, we do not know what they are and we just invented those names to cover up what we do not know or understand, while ...
2answers
74 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 ...
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
60 views

### Earth and Moon computer simulation [closed]

So I want to simulate the solar system but want to start simple with one orbiting body. However, I never did anything like this before and was wondering if anyone here could give me some hints. ...
2answers
127 views

### What are the limitations of performing music in space?

Canadian astronaut Chris Hadfield's song Space Oddity is making news around the world today. It makes me wonder: What are the limitations of performing music in space? Clearly, there is no point to ...
1answer
134 views
+50

### Is period of rotation relative?

My question is inspired by the following answer by voix to another problem: "There is a real object with relativistic speed of surface - millisecond pulsar. The swiftest spinning pulsar currently ...
1answer
52 views

### Defining the star as the ellipse focus rather than the barycenter, what does the other focus do? [duplicate]

There are a lot of images and animations on the internet depicting two bodies orbiting around their common barycenter. The barycenter is defined as the (let's say right) focus of the ellipse. If we ...
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
958 views

### Does a guitar sound different in zero (or micro) gravity?

Seeing a video of astronaut Chris Hadfield playing a guitar on the International Space Station made me wonder if a guitar or other stringed instrument played in zero-G would sound any different than ...
13answers
5k views

### How does gravity escape a black hole?

My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to ...
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 ...
2answers
49 views

### If there were two equal masses with one mass in the middle which way would gravity pull it?

If there were two masses with a third exactly in the middle of those two, which way would the middle mass be pulled?
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 ...
4answers
239 views

### Why do objects accelerate as they fall?

Most importantly, what must change in order for the falling object to change its speed? Is it the distance to the centre of the planet? If you pull the earth away from the object as the object falls, ...
0answers
39 views

### not all items fall at the same rate some items have outside facters gain speed in which they “fall” [closed]

I was reading one of the post this morning and found it fasanating with all the formulas that were being poured out about items will falling at the same rate given the same hight that they were droped ...
2answers
92 views

### Repulsion does not exist; Gravity assist slingshot as a repulsive force [closed]

unification theory might say that there is only one force in universe that is called gravity(attraction). So at atomic level for same charges that repel eachother(electrons etc.), if thought of as a ...
4answers
1k views

### Upward force on a object at rest

Is there an upward force on a object at rest? If yes, where does it come from?
3answers
242 views

### Similarity between the Coulomb force and Newton's gravitational force

Coulomb force and gravitational force has the same governing equation. So they should be same in nature. A moving electric charge creates magnetic field, so a moving mass should create some force ...
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 ...
2answers
87 views

### Why don't black holes within a galaxy pull in the stars of the galaxy

visit http://www.nasa.gov/audience/forstudents/k-4/stories/what-is-a-black-hole-k4.html If black holes can pull even light, why cant they pull the stars in the galaxy?
2answers
100 views

### Negative potential energy of gravity

Does the negative potential energy in the gravitational field have to be considered in calculating the total mass of the system in question (because of $E=mc^2$)? If so it seems to me that the ...
1answer
50 views

### Gravitational resonances for satellites

I read on the website of European Space Agency that the altitude of Galileo satellites, which is 29600 Km from the center of the Earth, is chosen to avoid gravitational resonances so that station ...
0answers
69 views

### Is there any proof that the speed of gravity is limited? [duplicate]

I must warn that though I'm argumenting with black holes I'm not asking how does gravity escape the black hole!. I want to know if the absolute speed of gravity waves were proven bu an experiment. We ...
2answers
250 views

### Does relativistic mass have weight?

If an object was sliding on an infinitely long friction-less floor on Earth with relativistic speeds (ignoring air resistance), would it exert more vertical weight force on the floor than when it's at ...
4answers
337 views

### Violation of Newton's Second Law (?)

Here the big circle denotes the circular path of a stone(small circle on path) tied to a string from the centre of the circular path . This is COMPLETELY HORIZONTAL At an instant the velocity in ...
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 ...
4answers
413 views

### Why do we still need to think of gravity as a force?

Firstly I think shades of this question have appeared elsewhere (like here, or here). Hopefully mine is a slightly different take on it. If I'm just being thick please correct me. We always hear ...
1answer
58 views

### How would gravity change on a planet rotating around itself very fast?

Let's take a planet identical to Earth, but with rotation speed multiplied by ten thousand. What would happen with the gravity if it was spinning madly around itself? Would the centrifugal force make ...
2answers
425 views

### How can I calculate the flow rate necessary for a given cross sectional area, angle, and length

I'm making a toy for my kids and this problem came up. I have a channel on a slight angle (angle is between ground and length of channel) and I'm pouring water into it. I want to know how quickly I ...
5answers
137 views

### How universal gravitation falls short

As a non physicist I can understand how Newtonian mechanics falls short in cases of high velocity etc. and is properly generalized by the special theory of relativity. What is not clear to me is how ...
2answers
74 views

### Will a helicoper which is hovering inside a closed box move with the box when we move it? [duplicate]

I got this micro helicopter to play with, you know that cheap double bladed ones you can find for \$25 or so. Anyway last night I was on bed trying to sleep and I got this idea, what if we put the ...
1answer
81 views

### Is the speed of light related to the mass of the universe?

If the mass of the universe were cut in half, would it affect the speed of light? Would it be twice as fast? Would it stay the same? Do we have instruments that are sensitive enough to measure the ...
0answers
35 views

### effect of gravity on chemical reaction rates [closed]

a chemical reaction is done on earth in very vacuum and that chemical reaction is also done in space so that gravity higgs field not affect that reaction. Which reaction will be fast wrt gravity and ...
1answer
102 views

### Relation between Higgs boson and force of gravitation

I don't know and I can't handle it. I am so much curious about the recent discoveries in science. European Space Agency and NASA are doing great work. Hats up to these institutions. My nights are ...
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 ...
2answers
79 views

### The potential and the intensity of the gravitational field in the axis of a circular plate

Calculate the potential and the intensity of the gravitational field at a distance $x> 0$ in the axis of thin homogeneous circular plate of radius $a$ and mass $M$. Could anybody describe how to ...
5answers
325 views

### What happens to light and mass in the center of a black hole?

I know that black holes are "black" because nothing can escape it due to the massive gravity, but I am wondering if there are any theories as to what happens to the light or mass that enters a black ...
2answers
90 views

### What happens to things when things get crushed in a blackhole [duplicate]

When a black hole destroys things until they are smaller than molecules, where does it go and what happens when it clogs up?
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?
5answers
937 views

### The square in the Newton's law of universal gravitation is really a square?

When I was in the university (in the late 90s, circa 1995) I was told there had been research investigating the $2$ (the square of distance) in the Newton's law of universal gravitation. ...
3answers
69 views

### What is the cause the light is affected by gravity? [duplicate]

I know that photons have no mass and that a photons exist only moving at the speed of light. So what is the cause that a massive astronomical object can bend a ray of light? I have two thoughts, but I ...
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 ...
2answers
49 views

### Can the effects of a person's mass upon the local gravitational field be detected and measured remotely?

As the title suggests, Can the effects of a person's mass upon the local gravitational field be detected and measured remotely? I am aware any mass produces and effects gravity but couldn't find ...
4answers
1k views

### Would you be weightless at the center of the Earth?

If you could travel to the center of the Earth (or any planet), would you be weightless there?
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
65 views

### Orbital mechanics and rocketry: Is it ever a good idea to intentionally lower periapsis?

tl;dr: Hohmann Transfer appears to be the optimal way to achieve a circular-to-circular orbit, but is it possible to lower the periapsis in order to achieve a more elliptical orbit with apoapsis at ...
2answers
98 views

### What is the fastest a spacecraft can get using gravity-assist?

Assuming normal spacecraft and space objects (no neutron stars, black holes, etc). To what speed can a spacecraft accelerate using gravity-assist? For example, if a spacecraft is moving at ...
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