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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Rotating sphere and circular trajectory: minimum speed

I have a sphere (mass = 3 kg), constrained to a fixed length rope, rotating (radius = 5 m) on a vertical plane. My textbook ask me about the minimum speed in the highest point in order to keep the ...
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2answers
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How Newton found Gravitational acceleration?

How Newton found Gravitational acceleration (g) of The Earth? Can you describe the methods to find and prove it(g value)? (may be describe both classical methods and modern methods)
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Gravity stronger than electromagnetic force in a black hole?

Well, the question has somewhat been answered before, but there's one part missing, which - I'd think - is in conflict with the physical laws. The earlier reply says that the gravitational pull even ...
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2answers
846 views

Velocity required for Horizontal Rain

Related to my previous question - Change in appearance of liquid drop due to gravity... Ok, I think we all have noticed this practical phenomena (a kind of illusion though)... During rain (while ...
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3answers
512 views

Is the Schwarzschild black hole unphysical?

To obtain the Schwarzschild metric from Einstein equations of general relativity, we suppose that the energy density is a distribution : $$ \rho (\vec{r}) = M \delta(\vec{r})$$ The Schwarzschild ...
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1answer
502 views

Causal and Global structure of Penrose Diagrams

What kind of global and causal structures does a Penrose diagram reveal? How do I see (using a Penrose diagram) that two different spacetimes have a similar global and causal structure? Also, I ...
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688 views

Dropping an object on the moon [closed]

If you are dropping an object on the moon would it fall on to the moon's surface or fall towards earth?
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1answer
292 views

What is the gravity at the center of the Earth? [duplicate]

Possible Duplicate: Would you be weightless at the center of the Earth? Supposing there is a cavity at the center of the Earth, what is the gravity there? What will be its direction and ...
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5answers
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Why does no physical energy-momentum tensor exist for the gravitational field?

Starting with the Einstein-Hilbert Lagrangian $$ L_{EH} = -\frac{1}{2}(R + 2\Lambda)$$ one can formally calculate a gravitational energy-momentum tensor $$ T_{EH}^{\mu\nu} = -2 \frac{\delta ...
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1answer
10k views

How do I get the total acceleration from 3 axes?

For a project I'm working on I'm using an accelerometer which measures acceleration in 3 directions, x, y and z. My question is: How can I calculate the total acceleration in a certain direction from ...
10
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1answer
534 views

Why doesn't my particle simulation end in a flat disc?

I've made a 3d particle simulator where particles are attracted to each other by the inverse of the square radius. The purpose of my experiment is to see if this alone would create a flat disk (like ...
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3answers
249 views

How many pieces of toast would you need to make a black hole?

I am trying to find the following: How many pieces of toast would you need to make a black hole? From what I've learnt so far I need to find an equation for the compression force the massive amount ...
4
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1answer
290 views

Would two astronauts one in a satellite, one on top of a tower have the same experience?

Imagine two astronauts. The first is in a satellite in geostationary orbit around the earth. The second is on top of a 35,786 km tall tower that has been built from the equator to space. This means ...
4
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1answer
210 views

Formation of a black hole and Hawking radiation

From the perspective of an outside observer it takes infinitely long for the black hole to form. But if the black hole is no extremal black hole, it emits Hawking radiation. So the outside observer ...
3
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1answer
475 views

Wave pulse on a freely falling rope

Consider a rope hanging from the ceiling (massive / massless irrelevant, I suppose). A wave pulse is set up on the rope. Just as the wave pulse starts propagating on this rope, the top of the rope is ...
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2answers
2k 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. ...
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2answers
427 views

A change in the gravitational law

What would happen if the force of gravitation suddenly starts varying as $1$$/$$r^3$ instead of $1/r^2$ ? Would the symmetry of universe now seen be disrupted?
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1answer
3k views

What is the relationship between mass, speed and distance of a planet orbiting the sun?

After reading this fascinating story about a new exoplanet, I was wondering about how mass, speed and distance determine a circular orbit of a planet around a star. Given the mass of the sun and ...
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0answers
38 views

Do the changes of gravity travel across distances instantly or limited to the speed of light? [duplicate]

Possible Duplicate: The speed of gravity It takes a long time for a radio signal to travel to the planet Mars. What if we made a special type of radio that could detect small changes in ...
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1answer
89 views

Gravity measurements above volcanic chamber

I am trying to find out what formula to use for the above issue. If I do a gravity measurement above a volcanic chamber, what do I have to do? I thought about adding the Bouger corrections, as it is a ...
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3answers
1k views

How to measure force of impact inside container?

I am in 7th grade and for my science fair project, I need a way to measure the force on a dropped object when it hits the ground. What I am trying to determine is which packing materials provide the ...
7
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2answers
578 views

Is Dyson Sphere a stable construction?

Suppose that a star is encompassed by a Dyson Sphere. Do we need a position control system for the Dyson Sphere to keep its origin always aligned with the center of the star? Will it stay aligned ...
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2answers
767 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 ...
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1answer
551 views

Why are we talking about space curvature as if we know what space is? [closed]

1) Why are we talking about space curvature as if we know what space is? Every question about gravity seems to evoke an answer involving "space curvature" which seems like an undefined placeholder ...
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2answers
173 views

Most suitable metric for the Solar system?

If I wanted to solve the Einstein equations for the solar system, which choice of $g_{\mu\nu}$ and $T_{\mu\nu}$ is more suitable? I thought about using a Schwarzschild metric near each planet, but ...
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0answers
117 views

Good introductory books on AdS/CFT correspondence [duplicate]

Possible Duplicate: Introduction to AdS/CFT Since my question in a similar topic was deleted, I'll ask away and hope ppl won't come here telling me: this was already asked! :\ I have a ...
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2answers
401 views

Relationship between Alcubierre drive space-time evolution and speed of gravity

The top rated answer to this question about the Alcubierre drive asserts, "spacetime can dynamically evolve in a way which apparently violates special relativity," but according to the Wikipedia ...
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1answer
2k views

is gravity always the weakest force [duplicate]

Possible Duplicate: What does it mean to say “Gravity is the weakest of the forces”? Strongest force in nature It is usually taught that the gravitational force is the weakest ...
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4answers
2k views

Faster Than Light Communication Using Gravity?

Suppose two parties $A$ and $B$, some distance apart and at rest relative to each other both had control of one massive local object each. If party $A$ lifted their local object upward relative to ...
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1answer
431 views

Is $w=mg$ the right way to calculate mass?

How can you find the mass of an object by using a weight and a gravitational acceleration from another planet then earth? I was given $5.8\text{ N}$ and an acceleration of $4.0\ \mathrm{m/s^2}$. The ...
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1answer
121 views

Are all static solutions of Einstein's equations spherically symmetric?

Is it true that all static solutions in GR are also spherically symmetric? Is there a proof of this? Similarly, are all stationary solutions axisymmetric?
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3answers
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How did Einstein derive general relativity?

How did Einstein derive general relativity (GR)? Did he use: the equivalence principle? The principle of least action? Anything else? Note, I'm not looking for a full mathematical derivation of GR! ...
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0answers
108 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 ...
3
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1answer
881 views

What is the longest distance that can be jumped after swinging from a rope?

In the movie Mission Impossible 3, the main character Ethan Hunt tries to enter a building in Shanghai by swing through the sky, as shown below: The jump consists of 2 sections, the red part, which ...
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2answers
1k views

What do you feel when crossing the event horizon?

I have heard the claim over and over that you won't feel anything when crossing the event horizon as the curvature is not very large. But the fundamental fact remains that information cannot pass ...
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3answers
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If the earth would stop spinning, what would happen?

What would happen if the earth would stop spinning? How much heavier would we be? I mean absolutely stop spinning. How much does the centrifugal force affect us? If you give technical answers (please ...
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3answers
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Why doesn't a fly fall off the wall?

Pretty simple question, but not an obvious answer at least not to me. I mean you can't just place a dead fly on the wall and expect it to stay there, he will fall off due to gravity. At first I ...
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5answers
3k views

Where does the “g” force that pilots experience come from?

I understand that it has to do with acceleration. Say a pilot does a quick maneuver and experiences a force of 5g. What exactly is happening here? And what is this force relative to? If someone ...
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1answer
461 views

Could this fountain, under the right conditions, technically be able to lift me up?

There is a big fountain in a lake in my city. I was talking with a friend and we were wondering whether it would be able to lift me up. I sent a few emails and obtained information about the ...
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3answers
256 views

What will be the shape states of water in a centrifuge in 0G

Consider the concept of these videos http://www.youtube.com/watch?v=Zip9ft1PgV0&sns=em and http://www.youtube.com/watch?v=cUhgKFV5Ri4&sns=em but set in a zero gravity environment and in a ...
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1answer
672 views

What gives matter Gravitational Mass? [duplicate]

Possible Duplicate: Does the equivalence between inertial and gravitational mass imply anything about the Higgs mechanism? In Higgs mechanism, Higgs field, which likes syrup, slows down ...
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2answers
972 views

Lever Mechanics - How to formulate an ideal lever launch

Let's say I have a simple lever as shown below, and the lever is massless and the pivot is frictionless and there is no air resistance. I'm thinking the cradle for the projectile would have to have a ...
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3answers
177 views

How to incorporate effects of gravity in a many-electron system?

In the weak interaction limit, behaviors of electrons can still be described by a 1-partitcle equation with a modified mass, where the change in mass can be understood as effects of other neighboring ...
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2answers
159 views

Why is the critical density of the Universe non-zero?

I'm missing something and I wondered if someone could enlighten me. As I understand it, Einstein's equations say, among other things, that matter curves spacetime. I also understand, the Universe has ...
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1answer
115 views

picture of newtonian gravity and black hole

We assume that the earth planet is a black hole. When a light beam is fired. The only way that even light can move is A and B which means this is impossible that light move on other ways C, D, E ...
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1answer
294 views

detect movement - how does it relate to G force

I would like to detect if a user is moving by using an android smartphone - walking or jumping doesn't matter - want to know if the user wearing the smartphone is moving. I have found this code ...
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1answer
508 views

Space Rocket Lift Efficiency and the Gyroscope?

I'm no physicist apart from basic 3d web animation, I'm just curious and please feel free to correct my misuse of terms or inadequate speculations. I've been reading a lot on gyroscopes and ...
4
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1answer
232 views

Is the fuel burn for a satellite launch affected by the position of the moon relative to the launch site?

The gross mass of a satellite rocket is tiny compared to that of Earth, and Luna. Between them, however, the two bodies set up tides in bodies of water which itself is again considerable mass. At ...
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4answers
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Could we make a trebuchet that could launch objects to a stable orbit?

Inspired by this xkcd, which calculated the energy requirements for accelerating individual humans to escape velocity (regardless of consideration for what that would do to your organs), I am ...
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6answers
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Why is there an escape velocity?

I've been trying for days, but I just can't understand why escape velocities exist. I've searched the web and even this site, and although I've read many explanations, I haven't been able to truly ...