# 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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### 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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### Is it possible/correct to describe electromagnetism using curved space(-time)? [duplicate]

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 ...
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### Transfer of energy from gravity back to other “more familiar” forms of energy?

In this question I've mentioned an account of the recently reported 2nd observation of gravitational waves, LIGO and Virgo Collaborations, Phys. Rev. Lett. 116, 241103, 15 June 2016, where 1 of the ...
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### When a ball is tossed straight up, does it experience momentary equilibrium at top of its path?

This question has been asked many times all over the Internet and answers can be found on places such as yahoo and ask.com, but I'm not satisfied with those answers and I don't trust the validity of ...
4answers
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### 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 ...
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### 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 ...
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### Gravitational Time Dilation with big masses

From Wikipedia I read that the formula for calculating the time dilation caused by a mass is $t_0 = t_f{\sqrt{1-\frac{2GM}{rc^2}}}$ where: $t_0$ is the proper time between events A and B for a slow-...
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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 ...
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### 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 ...
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### 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 ...
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### Why Can a Skydiver Hit the Ground and Be Killed? [duplicate]

This is a follow on question from Physics SE Question "Can a Skydiver Land On a Large Slide and Survive?". User Steeven gives this answer here. User Dargscisyhp asks: What is it exactly that ...
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### What's the biggest cube you could have before gravity rounded it?

I took this question from Quora because it seems interesting and this community would have some fun with it. I would assume that one would use the modulus formulas for force to counteract the gravity, ...
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### Orbit reversal in a gravitational well

A friend recently asked me this, and without wishing to spend a huge time thinking about it, I wondered if anyone knew the answer (or at least had an informed guess), or was familiar enough with these ...
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### With gravitational lensing could we see our selves from a right angle?

When looking at a black hole the ability to see ourselves is compromised by the small window for the light trajectory to come back to us. The light from the Earth that is bent by (almost) 180° and ...
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### Does the escape velocity of a black hole exceed $c$ *before* a singularity is created?

As an offshoot of the question Can we have a black hole without a singularity? I'm curious if the point of no return at which the massive object is condemned to become a singularity happens before its ...
1answer
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### How quickly does gravity extend from created mass? [duplicate]

I apologize in advance if this is a stupid or off-topic question. Since Energy can be converted into matter according to $E=mc^2$, how fast does it take for the gravity of that new matter to extend ...
5answers
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### Where does the idea gravity=curvature of spacetime really come from?

I have been searching for quite a while but mostly found the answer: Einstein's genius. Quite unsatisfactory. I know and understand that the idea gravity=curvature of spacetime works. Furthermore I ...
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### What truly is mass, and is there a direct way to measure it?

We know a mass of an object of one kilogram as an object that weighs W = mg = 9.8 N and we reference it to that, (when it should as a fundamental parameter describe weight not the opposite). But if we ...
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### 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 ...
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### 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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### Where does air pressure come from?

Where does air pressure come from? I thought it was from gravity or the speed of the gas resulting from its heat. However, analyzing my own hypotheses, I think that my 'heat conjecture' is probably ...
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### Why isn't all of the dust in a nebula used in the formation of a star?

I was watching a show on discovery and according to it, in a nebula the dust and gases slowly come together and as the gravity increases and the pressure rises in the core the gases fuse together and ...
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636 views

### What kind of energy gravitates, and why?

When listing energies for the purposes of keeping track of conservation, or when writing down a Laplacian for a given system, we blithely intermix mass-energy, kinetic energy and potential energy; ...
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### Can Vesta dominate the orbits of other asteroids?

I think I remember a talk where a professor said that Vesta is a particularly important asteroid because its gravity is strong enough to perturb other asteroids. In spite of Vesta's size, this effect ...
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### Wavelength of photon changes as it rises from a planet's surface(acc. to this equation)?

The setup assumes a large mass(Earth?) an a photon launched from its surface initially. The wavelength of the photon on launch is known. Then the new energy of the photon is compared with energy it ...