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.
4
votes
4answers
404 views
what is the difference between a blackhole and a point particle
Theoretically, What is the difference between a black hole and a point particle of certain nonzero mass. Of-course the former exists while its not clear whether the later exists or not,
but both have ...
4
votes
1answer
67 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 ...
4
votes
4answers
669 views
Does the mass of an object change as it moves away from the earth?
The mass of a helium nucleus is less than the mass of two isolated protons and two isolated neutrons. When the component hadrons are assembled, this mass is lost as energy ($E=mc^2$). This makes it ...
4
votes
1answer
4k views
Why is gravitational potential energy negative, and what does that mean?
I usually think of gravitational potential energy as representing just what it sounds like: the energy that we could potentially gain, using gravity. However, the equation for it (derived by ...
4
votes
1answer
493 views
Finding the position of a planet between two other planets of known mass and distance
Here is the question:
A planet with mass $m$ and a second with mass $M$ are separated by a
distance $d$. A third planet with mass $m_3$ happens to be midway
between $M$ and $m$. Where could ...
4
votes
1answer
829 views
Gravity and Planetary Differentiation
During solar system formation, many bodies achieved hydrostatic equilibrium, a spherical shape where their self gravitational force was balanced by internal pressure. Many also achieved ...
4
votes
1answer
326 views
Quantum Gravity and Calculations of Mercury's Perihelion
In an astronomy forum that I frequent, I have been having a discussion where the state of quantum gravity research came up. I claimed that Loop Quantum Gravity theories couldn't prove GR in the ...
4
votes
1answer
53 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 ...
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
1answer
165 views
Does conformal gravity explain the Bullet cluster lensing effects?
Conformal gravity is an "alternative" theory of gravity, where instead of using the Einstein-Hilbert action composed of the Ricci scalar, the square of the conformal Weyl tensor is used. It was ...
4
votes
2answers
136 views
Is Brian Cox right to claim that Gravity is a strong force for large masses, is it wrong, or is it only a matter of interpretation?
I watched a program of his in which it was claimed that since mass bends space in accordance to General Relativity, then in the case of very large stars it becomes a strong force to the point of being ...
4
votes
1answer
124 views
Is this closed time-like curve in a Godel universe a “circle”?
Would the observer moving along the circle in this Godel space-time diagram feel fictitious forces as though he is accelerating along a circular path or would he simply arrive at an earlier point in ...
4
votes
2answers
433 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 ...
4
votes
1answer
187 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 ...
4
votes
1answer
249 views
Electric potential energy in curved space-time
In flat space-time the electric potential energy between two charges is $\frac{k Q_1 Q_2}{r_{12}}$, where $Q$'s are charges and $r_{12}$ is the distance between them. What would happen if the two ...
3
votes
4answers
574 views
Why does an object thrown parallel to the ground eventually fall down?
Suppose an object is thrown parallel to the ground. The gravity acts downward (ie. perpendicular to the direction of motion of the object). The work done by gravity on that object will be given by :
...
3
votes
5answers
567 views
Confused about the role of mass
I'm far from being a physics expert and figured this would be a good place to ask a beginner question that has been confusing me for some time.
According to Galileo, two bodies of different masses, ...
3
votes
5answers
778 views
How does the curvature of spacetime induce gravitational attraction?
I don't know how to ask this more clearly than in the title.
3
votes
6answers
600 views
Do you feel gravity?
I have been reading a few articles about the question why we don't feel/notice gravity in everyday life, but I couldn't understand why exactly we don't feel/notice it, that is, why we don't feel a ...
3
votes
4answers
389 views
Can a black hole be explained by newtonian gravity?
In the simple explanation that a black hole appears when a big star
collapses under missing internal pressure and huge gravity, I can't see
any need to invoke relativity. Is this correct?
3
votes
6answers
3k views
How does gravity affect bullets?
I read recently that if you hold a bullet in one hand and a pistol in the other, both hands at the same height, and subsequently fired the pistol at the same time as dropping the bullet, both bullets ...
3
votes
3answers
835 views
Is it possible/correct to describe electromagnetism using curved space(-time)?
Comparing the simples form of the forces of both phenomena: the law of Newton for gravitation $V\propto \frac{1}{r}$, and the Coulomb law for electrostatics $V\propto \frac{1}{r}$, one might think ...
3
votes
4answers
157 views
Can a huge gravitational force cause visible distortions on an object
In space, would it be possible to have an object generating such a huge gravitational force so it would be possible for an observer (not affected directly by gravitational force and the space time ...
3
votes
2answers
748 views
If it was possible to dig a hole that went from one side of the earth to the other
And you jumped in.
What would happen when you got to the middle of the earth? Would you gradually slow down, until you got to the middle and once you were in middle would every direction feel like it ...
3
votes
2answers
217 views
Deviation from Earth's orbit
How much orbital deviation is required for the Earth to get knocked out from current orbit so it either moves away from Sun or towards the Sun?
3
votes
4answers
554 views
Two masses in deep space - collide or orbit?
If two identical masses are somehow "released" into deep space (that is, they're subject to no other gravitation forces but their own, and are initially at rest to each other). What decides whether ...
3
votes
3answers
392 views
Gravitation as the source of redshift of light beams
According to Hubble's law, light and other kinds of electromagnetic radiation emitted from distant objects are redshifted. The more distant the source, the more intense is the redshift. Now, the ...
3
votes
5answers
1k views
How do we know that earth moves around sun and that too in elliptical orbit
I know the basics of solar system like how earth moves around sun, and that we have so many planets, elliptical orbit of earth, and how far is sun from earth etc etc. I want to take a step back and ...
3
votes
3answers
327 views
Can a planet cover the whole sky as seen from its satellite
Sorry if this is a trivial question, but I do not know enough physics to answer this on my own.
Can there exist a satellite that revolves around a planet so large such that the planet covers the ...
3
votes
1answer
33 views
Why doesn't our solar system run into other solar systems over billions of years?
Why doesn't our solar system run into other solar systems over billions of years.
There is plenty of time for it to be attracted by gravity to other objects. So I thought, it should be a common ...
3
votes
2answers
85 views
Orbit in the vacuum
As the space is a vacuum and there is no friction in space,
Can we assume that, if we place an object in gravity in exactly the right distance from a planet with gravity and in the right acceleration, ...
3
votes
3answers
191 views
Does the equivalence principle mean that the Earth is expanding?
Popular books suggest that for an observer in the Einstein lift the following situations are equivalent:
1, the lift hangs motionless (relative to the Earth) on a cable in the gravitational field of ...
3
votes
2answers
232 views
What would happen if I take a glass of water in space?
What would happen if I take a glass of water in space i.e. outside the gravitational influence of earth?
My teacher said that the water would vaporize but I am not completely satisfied by the answer.
...
3
votes
2answers
49 views
Why do clusters of stars eventually dissipate?
Why don't the stars in a star cluster attract each other gravitationally, forming one big star? What causes a cluster to disperse the stars in it?
3
votes
3answers
346 views
Why all rocks are not orbiting bigger rocks in space?
Why only big rocks (planets) have satellites and not small ones? Why cosmic dust doesn't orbit rocks that are many times heavier than the dust grains? If dust is still too heavy then what about ...
3
votes
3answers
661 views
Why would Antimatter behave differently via Gravity?
Confinement of antihydrogen might help provide a future answer. http://arxiv.org/abs/1104.4982
3
votes
3answers
145 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 ...
3
votes
2answers
200 views
Has anyone else thought about gravity in this way?
Picture yourself standing on a ball that is expanding at such a rate that it makes you stick to the ball.
Everything in the universe is expanding at this same rate.
To escape the earths gravitational ...
3
votes
2answers
255 views
Astronaut on a rotating asteroid
This might sound like the silliest question ever, but can someone shed some light on what is actually happening in the following situation:
There is an asteroid in outer space what is rotating with ...
3
votes
3answers
742 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 ...
3
votes
2answers
2k views
Why do helium filled balloons move away from the Earth?
From my understanding objects do not fall but are pulled to the earth from gravity.
With this in mind, I can't understand why if helium filled balloons are not pulled by gravity then shouldn't they ...
3
votes
5answers
193 views
Is the gravitational constant $G$ a fundamental universal constant?
Is the gravitational constant $G$ a fundamental universal constant like Planck constant $h$ and the speed of light $c$?
3
votes
2answers
229 views
Consequences of destroying a space elevator
Suppose there is a fully functional space elevator built on Earth. The base is attached to coordinates
$
(\lambda, \varphi) = (0,0)
$
e.g., on the equator on the zero-meridian.
What would happen ...
3
votes
2answers
416 views
Zero divergence of energy-momentum tensor and gravitational energy
Trying to teach myself general relativity and have just hit yet another confusion. I'm reading that in curved spacetime the energy-momentum tensor has zero divergence, ie
...
3
votes
2answers
37 views
How could they tell things weigh a sixth of their “Earth weight” on the moon?
I frequently keep hearing stuff about "gravity on the moon is only a sixth of that as Earth's" and "of course you'll weigh more on Jupiter". I do know the relative sizes of those bodies allow for ...
3
votes
3answers
517 views
How does gravity force get transmitted?
How does gravity force get transmitted?
It is not transmitted by particles I guess. Because if it was, then its propagation speed would be limited by the speed of light. If it is not transmitted by ...
3
votes
2answers
160 views
What would be the optimal weight of a ball if I want to throw it as far as possibile?
Assume the force behind my throw to be X. Assume the point of release is 2 meters above ground. Asume the ball is made of shiny steel. I'm not sure if the material matters, I'm just thinking that ...
3
votes
1answer
298 views
Gravitational waves as dark energy?
Is the energy carried by gravitational radiation a viable candidate for $\Lambda$ / dark energy?
3
votes
2answers
1k views
Measuring acceleration of earth due to its fall around the sun
Every orbiting of a satellite around a mass is nothing else but a constant fall - and therefore acceleration - towards this mass. In a way it is a "falling around" that mass.
My question Is it ...
3
votes
1answer
1k views
Bubbles in zero gravity
What happens if you blow bubbles into a glass of lemonade on the international space station?
Since you are weightless in orbit, there's no up, down, left nor right. We define down on the Earth ...
