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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Why do 3d spheres and gravity tend to rotating discs on one plane?

Whether is it our solar system or a whole galaxy, there is usually a massive object (star or black hole) at the centre with gas and objects rotating around it. The gravitational effect of the ...
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Gravitational sphere

I was reading this article that mentions a blackhole as having a gravitational sphere of 4,000 light-years. I'd not heard of the term (gravitational sphere) before ...
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Basic Free Fall Problem [closed]

So I am trying to remember my childhood doing some Physics problems, but seems that I forgot almost everything. But it's not a big thing... that's why I'm training! My problem is pretty basic ...
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1answer
260 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 ...
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273 views

interaction at a distance: how it works

In classical physics, I know the forces due to gravity and electrostatics. I know a lot of work has been done, but don't know the status. Is there a generally accepted theory of how the force is ...
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314 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 ...
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Action principle for the Electromagnetism and Gravity

Here is the formula for the stress energy tensor: $$ T_{\mu\nu} = - {2\over\sqrt{ |\det g| }}{\delta S_{EM}\over \delta g^{\mu\nu}} $$ (This follows from varying the total action $S ...
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Formation of black holes

If stars start with a finite density and light can escape from them, how can they be compacted to form a mass with infinite density which light cannot escape? The black hole will have the same mass as ...
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The Sun as a gravitational lens

Since the Sun is a gravitational lens with as focal length of 550 AU for visible light, with an immense amplification factor, shouldn't it light up objects hanging out there? We should get solar ...
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Calculating the gravitational acceleration inside of a planet

Plant X has a radius of 5000 km and is composed of two layers. The first inner layer ranges from the centre to 2000 km from centre, it's density is 8 kg / dm^3. The second layer ranges from 2000 ...
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Escape velocity of a rocket standing on Ganymede (Moon of Jupiter)

I want to calculate the escape velocity of a rocket, standing on the surface of Ganymede (moon of Jupiter) and trying to leave Ganymede. My thinking was, the kinetic energy $E_{\text{KIN}}$ must be ...
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2answers
215 views

Inertial Mass of a scalar field

Does it make sense to talk of the inertial mass of a scalar field? By the equivalence principle, it must be equal to its gravitational mass. We know that the scalar field contributes towards the ...
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Does gravitational differences affect the distance light travels? (a thought experiment)

My question is: How is light passing through different gravitational densities affected? The value of "c" is constant in a vacuum. I'm curious about if various time frames have any effect. This is ...
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Why do some satellites fall to Earth?

In another question How does Newtonian mechanics explain why orbiting objects do not fall to the object they are orbiting?, one can read an affirmative answer. They how do you explain satellites ...
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why do some orbiting objects have a changing position?

I know that the reason why they stay in orbit but why do some move away from the earth(the moon) or come closer and eventually fall? And why does the moon move away from earth?
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why do all objects of varying mass acelerate the same? [duplicate]

Possible Duplicate: Confused about the role of mass why is it that two object of varying mass will fall at the same speed in a frictionless enviorment like the moon? Is it because the ...
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1answer
418 views

centrifugal force in static frame of reference

The other day we derived Kepler's third law. $$ \left( \frac{T_1}{T_2} \right)^2 = \left( \frac{r_1}{r_2} \right) ^3 $$ In order to derive this, you can look at a given planet that revolves around ...
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Alternate layman's metaphors for illustrating curved space-time

The metaphor of a surface (typically a pool table or a trampoline) distorted by a massive object is commonly used as a metaphor for illustrating gravitationally induced space-time curvature. But as ...
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Quantum mechanical gravitational bound states

The quantum mechanics of Coloumb-force bound states of atomic nuclei and electrons lead to the extremely rich theory of molecules. In particular, I think the richness of the theory is related to the ...
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1answer
139 views

Question regarding escape velocity

Haliday-Resnick-Walker state that the escape velocity will cause a projectile to move upward forever, theoretically coming to rest only at infinity. I was wondering what the forces forcing the body ...
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122 views

Does Spacetime have a “This Side Up” arrow? [duplicate]

Possible Duplicate: Does the curvature of spacetime theory assume gravity? Forgive my naivete as I am not schooled in Physics or Mathematics. I was watching NOVA's "The Fabric of the ...
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Can a photon passing by an open space barycenter of a system of masses be modeled as if all the system's mass is at the barycenter?

To be clear, this example can't apply to the Solar System, since the barycenter is within the Sun, similarly the Earth/Moon system's barycenter is within the Earth. But, given a system of ...
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weighing big box by carrying the box and stepping on the weighing machine yourself …?

I have seen people counter the problem of big box and small weighing machine by stepping on the machine while carrying the box in arms and subtract the body weight. Do you think it'll give accurate ...
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1answer
727 views

Gravitational waves as dark energy?

Is the energy carried by gravitational radiation a viable candidate for $\Lambda$ / dark energy?
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6answers
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Is there any anti-gravity material?

I want to know if there is any anti-gravity material. I am thinking of making flying vehicles which are made up of anti-gravity material so that they will not experience any gravity on them and can ...
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204 views

A wonky gravitational potential and its critical points

I have tough problem I am not sure how to solve: For this question, we are confined to a plane. Consider a gravitational field that is proportional to $\frac{1}{r^3}$ instead of $\frac{1}{r^2}$, and ...
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If Earth Were Filled with a Compressible Fluid and Spun Really Fast, What Hollow Would Form? [duplicate]

Possible Duplicate: What nonlinear deformations will a fast rotating planet exhibit? This is really a basic physics question that I wanted to check myself on because I'm not entirely sure ...
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1answer
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Why does Jupiter emit more energy than it receives?

I hear that Jupiter and Saturn emit more energy than they receive from the Sun. This excess energy is supposedly due to contraction. Is this accepted as fact (or is it controversial)? Does this ...
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141 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?
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230 views

At what size will self-gravitation contribute more to stability than surface tension?

The governments of Earth have embarked on an experiment to place a massive ball of water in orbit. (umm... special water that doesn't freeze) Imagine this to be a fluid with a given density, $\rho$ ...
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How does Newtonian gravitation conflict with special relativity?

In the Wikipedia article Classical Field Theory (Gravitation), it says After Newtonian gravitation was found to be inconsistent with special relativity, . . . I don't see how Newtonian ...
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1answer
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Is the number of wavelengths of light spanning a distance invariant with respect to spacetime distortion?

I was recently asked by a friend how the expansion of spacetime affects photons. I gave him what I feel is a satisfactory general response, but it got me wondering how, exactly to calculate this ...
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0answers
87 views

When will the moon fall down? [duplicate]

Possible Duplicate: Are tidal power plants slowing down Earth's rotation? I've heard that there are turbines converting energy from tidal waves. Tidal waves are created by the moon's ...
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2answers
459 views

Black hole complementarity

Black hole complementarity states that two observers, one falling into a black hole, and one observing outside, experience two different histories but since they can not communicate there is no ...
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Initial vs Constant Orbital Velocity

I am working on some basic physics simulation for a game and need to simulate gravity. I have a system working that is behaving more or less correctly so far, but I want to see if I can send a ...
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1answer
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How does one calculate the force applied on an object by a magnetic field?

I've tried very hard to find an answer to this question, and every path leads me to an abstract discussion of fundamental forces. Therefore, I will propose two very specific scenarios and see if they ...
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Could the acceleration of universe expansion be caused by gravity itself?

Dark energy is suggested to be a repulsive force in the universe causing an accelerated expansion. If the amount of mass outside our observable universe is greater than inside (higher mass density), ...
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experimental bounds on spacetime torsion

Did Gravity Probe B provide any bounds on Einstein-Cartan torsion? is a non-zero torsion value at odds with the results regarding frame-dragging and geodetic effects?
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Charging a black hole?

What would happen if we have a black hole and we start shooting at it a single electron at a time, and go on doing it forever? Would the electrons start to bounce off eventually?
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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 ...
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4answers
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What is the difference between gravitation and magnetism?

If you compress a large mass, on the order of a star or the Earth, into a very small space, you get a black hole. Even for very large masses, it is possible in principle for it to occupy a very small ...
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Apple falls for which of these 2 reasons?

Needles to say I am a visitor here. I do not belong to the science world;) But I have read both of these things before: Apple falls to the ground because curved spacetime pushes it there (same ...
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Would you feel weightless if falling or flying up in a vacuum?

Ignoring that you can't survive in a vacuum. If you were to jump of a ledge in a vacuum (in a vertical tube eg). While falling would you feel weightless? Surely the earths gravity is still pulling ...
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Relationship between Weak Cosmic Censorship and Topological Censorship

The weak cosmic censorship states that any singularity cannot be in the causual past of null infinity (reference). The topological censorship hypothesis states that in a globally hyperbolic, ...
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1answer
281 views

A loop quantum gravity toy inspired by an Aharonov-Bohm ring

Comparing my question to Give a description of Loop Quantum Gravity your grandmother could understand what I'm looking for here is a toy for a toddler ($\approx$ a pre-QFT graduate student). I seek ...
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Neutrino unaffected by gravity

Are neutrinos affected by gravity? If not, could that be a plausible reason for a neutrino taking a shorter path than light, since light is affected by gravity?
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438 views

Fluid Mechanics Problem - Ball in Reservoir

I'm having a problem starting and solving the following problem: My attempt at the solution was to realize you need to balance the upward forces with those of the downward forces in which case you ...
5
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1answer
432 views

Is this static mechanical balance possible without trickery?

On damnlol.com, I came across this picture: http://www.damnlol.com/hello-god-i-have-a-fault-to-report-7549.html My question is: Is this possible without glue? If not are there similar situations ...
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1answer
839 views

Schwarzschild black hole geometry in Novikov coordinates

As I am a layman in gravitation I would really appreciate any suggestions on how to approach and tackle the following problem: Choose a time-symmetric 3-geometry for the initial geometry of ...
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2answers
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Silly bet about the best way to walk up a hill

I have a silly bet with a friend of mine. One of us argues that walking up a hill whilst leaning forward requires less exertion, due to your centre of mass being further forward, and therefore ...