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.
7
votes
5answers
470 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 ...
3
votes
3answers
137 views
How does relativity explain gravity, without assuming gravity [duplicate]
I have seen the "objects pull down on space-time" explanations, but they assume a "pull down" force themselves. Could anyone explain the space-time explanation without assuming gravity in the first ...
0
votes
1answer
35 views
How does gravity effects both time and light if they have no mass [duplicate]
I've been reading about how black holes can effect both time and light with gravity. So I was wondering, doesn't something have to have mass to be effected by gravity? And if so, does this mean both ...
5
votes
2answers
290 views
Does rotational energy have effect on gravity/metric?
Intuitively, if energy can be stored in rotational motion, it has to obey $E=mc^2$.
Does rotation of typical stellar-sized objects - BHs, pulsars, binaries - have measurable effect on their overall ...
2
votes
1answer
45 views
Questions about the Jeans length
I have a couple of questions about the Jeans length. Suppose the universe has a homogeneous energy density, except there's a spherical region that is overdense.
I understand that if the region is ...
3
votes
3answers
740 views
Gravitational time dilation at the earth's center
I would like to know what happens with time dilation (relative to surface) at earth's center .
There is a way to calculate it?
Is time going faster at center of earth?
I've made other questions ...
-2
votes
0answers
39 views
Whats the theoretical max speed to go around the world? [closed]
I light of the new Scramjet tests I have a question thats been bogging me.
I live in Denmark, about the furthest i can travel is to New Zeeland which is roughly on the opposite side of the earth.
...
2
votes
1answer
50 views
Acceleration due to gravity?
I was looking into orbitals and found something I haven't been able to understand.
http://www.math.ubc.ca/~cass/courses/m309-01a/hunter/satelliteOrbits.html
There is a part on the page which states ...
1
vote
4answers
560 views
Does matter with negative mass exist?
Or does it exist mathematically?
Is it really inconsistent with a common-sense, mathematics or known physical laws?
As far as I understand, if it exists, it must be far away from the "positive" ...
0
votes
2answers
102 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 ...
0
votes
1answer
88 views
can be exist the negative mass? [duplicate]
I'm not sure about this but I guess there must be negative masses in the universe because of the symmetry. If the gravity is one of the main forces in nature it must has negatives mass to be able to ...
1
vote
1answer
28 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 ...
2
votes
0answers
33 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 ...
17
votes
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 ...
-8
votes
0answers
59 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 ...
2
votes
1answer
62 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 ...
-1
votes
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.
...
6
votes
2answers
131 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 ...
13
votes
1answer
146 views
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 ...
1
vote
1answer
53 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 ...
0
votes
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 ...
10
votes
1answer
1k 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 ...
91
votes
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 ...
5
votes
0answers
87 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 ...
-1
votes
2answers
51 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?
1
vote
1answer
81 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 ...
1
vote
4answers
248 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, ...
-8
votes
0answers
42 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 ...
-2
votes
2answers
94 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 ...
1
vote
4answers
2k 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?
1
vote
3answers
252 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 ...
0
votes
2answers
139 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 ...
2
votes
2answers
88 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?
1
vote
2answers
104 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 ...
4
votes
1answer
52 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 ...
0
votes
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 ...
10
votes
2answers
272 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 ...
-6
votes
4answers
341 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 ...
1
vote
1answer
200 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 ...
4
votes
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 ...
4
votes
2answers
431 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 ...
1
vote
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 ...
0
votes
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 ...
-1
votes
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 ...
-3
votes
0answers
36 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 ...
0
votes
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 ...
0
votes
1answer
103 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 ...
1
vote
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 ...
9
votes
5answers
330 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 ...
1
vote
2answers
91 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?
