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Questions tagged [gravity]

Questions regarding the attractive force which exists between any two bodies of matter.

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Why is the moon's motion more complicated than that of those planets?

I am of course not an expert on astronomy. But from the literature, I have the impression that the lunar motion is very complicated. It seems that Kepler can make sense of the motion of the planets ...
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How does gravity have an effect from the inside the event horizon of a black hole with the rest of the universe?

According to general relativity, as I understand it, the space around a black hole's event horizon is distorted by gravity, such that the distance to a point approaching the event horizon from an ...
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Why black holes have positive energy?

How could Hawking get the conclusion "[...] and black holes have positive energy. That’s why empty space is stable. Bodies such as stars or black holes cannot just appear out of nothing. But a whole ...
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363 views

Requirements for a satellite/planet to be tidally locked to a planet/star

The Moon is tidally locked to the Earth, the Four Galilean are tidally locked as well, and the recently found planetary sistem TRAPPIST-1 has seven tidally locked planets, but Venus or Mercury are not....
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Is the Roche limit used this way?

Has the Roche limit affect been observed having an effect on an constructed object gravitational wise to cause damage? Could the Roche limit be used to slow an object?
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The gravitational effect of a neutron star in our solar system

Imagine a neutron star with a mass of $3\:M_{\odot}$ is present at $10\:AU$ from the Sun, perpendicular to the plane of the solar system. The neutron star traverses over the sun (and the solar system),...
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Can a bullet be fired on the moon and sent it into orbit?

I know you need oxygen for the ignition, but presumably, if the bullet is impervious (or water-tight) and if there is a little air enclosed in the bullet case, wouldn't that be sufficient to fire it ? ...
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What does Gauss' divergence theorem say about compression of a body under self-gravitation?

Below this question there is a comment that caught my interest: First, there's a misconception. Compressing a body does not increase the force of gravity on outer-lying masses. That's Gauss's ...
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Where do rocks come from?

Consider a loose pile of rocks in an early solar system, say a tiny asteroid. As it accretes more and more material, the pressure at its center eventually becomes great enough to collapse the gaps ...
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Gravitational waves

We know that gravitational waves stretch space-time so light travelling through that space should be stretched as well. So my question is how do we know anything is stretching if the effect is ...
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In zero gravity - I'm floating inside a torus as they spin it up, do I float do the “ground”?

Imagine this scenario - I'm in a space station with one of those huge torus shaped constructions, that you spin to simulate gravity through centripetal force. But it's turned off right now, so I'm ...
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Is energy required for gravity to have effect, or is it an energy itself?

If magnetism requires some energy to exert change of motion to attract ferrous metals, then, does gravity require energy to exert attraction, or is gravity energy itself, creating attraction through ...
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If dark matter bends light, how do we know the stuff in the sky is where we think it is?

We measure movement, position, and many other things of an object in space because of its light and what we can measure with it. But as far as I know there's supposed to be a HUGE amount of dark ...
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Can Earth have a ring?

Why doesn't the earth have a ring? If solid material such as dust and moonlets, and common components of satellite systems are brought nearer to the Earth, then what will happen? Will it take the ...
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567 views

If earth changed its orbit, what would happen to the moon?

Let there be an accelerating force acting on earth over an arbitrarily long time to change its orbit around the sun (e.g. by asteroids repeatedly passing by close to earth (intentional or not) or ...
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How can gravitational waves escape from the gravity of black hole?

I know that even light can't escape from a black hole's gravity and the velocity of light and gravitational waves are the same. How can only gravitational waves escape from its gravity?
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How can X-rays be emitted from a black hole? [duplicate]

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 prevent X-rays ...
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Why the comets can cross the orbits of planets?

Why are comets able to cross planetary orbits without colliding with the planets - Why is the comets' orbits elliptical, intersecting all the other planets' orbit. Why don't they get attracted by the ...
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Why is the eccentricity of Triton so low?

Triton is very special, since its orbit is retrograde respect Neptune, its obliquity is near 0° and its orbit is almost circular (according to Harris (1984) is $5 \times 10^{-4}$). I'm familiar with ...
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Horseshoe orbits and integration in C

I'm studying a particular case of the restricted three-body problem. It's been found that some objects follow a horseshoe orbit pattern, and I'm trying to sort out something through an integration ...
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1answer
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How can a gas planet become tidally locked with a star?

Supposing a planet is made entirely of gas, is it possible for the planet to become tidally locked with the star it orbits? I know when a terrestrial planet orbits a star in a different amount of ...
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How can gravity lead to the Big Crunch scenario?

According to modern cosmology, space is expanding, causing proper distances (but not comoving distances) to increase between galaxies. In the Big Crunch hypothesis, gravity halts and reverses the ...
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What causes the Earth to have gravity?

I've always wondered how our planet Earth or any other large spheroid-shaped thing in our universe has gravity? Does it have a small black hole on its core causing it to have gravitational force? ...
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Revolution of Planets [duplicate]

Why do planets revolve around the sun? I understand that gravity plays a huge role in allowing it to move around. But what caused it to revolve in the first place? One possible solution is explained ...
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Which of the following statements about gravitational waves are true? [closed]

a) The emission of gravitational waves from merging black holes is predicted by Einstein's general theory of relativity. b) The first direct detection of gravitational waves, announced in 2016, came ...
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Gravity of other objects in black holes?

If something falls into a black hole, but has not yet hit the singularity in the center, can we (outside of the black hole) feel its gravity? On the one hand, I've learned that there's no distance ...
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Are there upper or lower mass limits for black holes?

Natarajan & Treister (2008) describes a practical upper limit for black hole masses at $\sim 10^{10} M_\odot$. This is all due to the black hole's interactions with nearby matter. However, is ...
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The deduction of the Hill Sphere formula

This question may be a little lazy, but can anybody give me a proof of the Hill sphere formula? Acording to wikipedia, the formula for the radius, $r$, is $$r\approx a(1-e)\left(\frac{m}{3M}\right)^{...
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Modifying Heisenberg's unified field theory with gravity [closed]

How would Hisenberg's unified field equations be represented if gravity is incorporated. Would it result in a case similar to the Heisenberg-Ivanenkov equation, or am I just confused here? I would ...
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1answer
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Examples of astrophysical objects NOT in virial equilibrium?

Astronomers always talk about astrophysical objects being in virial equilibrium (e.g., a stellar system, or a disk of gas within a galaxy, etc.). But I never hear about thermodynamical equilibrium. ...
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Why are galaxies distributed as walls and voids, or spider webs?

I have wondered why the "walls and voids" of the cosmological galaxy distribution seem to resemble a "foam", like rising bread that's ready to fall, with "voids" corresponding to empty bubbles in the ...
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How can we map masses orbiting the outer Solar System by determining the orbits of planets and smaller objects?

Neptune was discovered 170 years ago with the help of observations of Uranus' orbit. A great demonstration of the scientific principle for its time. Recently, Earth and cis-Lunar based radio ...
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Energy needed to create a black hole vs energy needed to run the Large Hadron Collider?

I was disappointed to hear Brian Cox's flippant reply to whether or not the LHC could create a black hole big enough to swallow the earth in this video: https://www.youtube.com/watch?v=QFy87tFTZwY I'...
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How did the Milky Way and Andromeda Galaxy separate before they began their collision course?

How did the Milky Way and Andromeda Galaxy separate before they began their collision course? I understand that gravity is pulling together the two galaxies, but what separated them in the first ...
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Is there mass i.e a physical bottom to a black hole? [duplicate]

All matter has a gravitational pull, a ball, a rock, a planet and a star - the bigger the matter the more it affects the space fabric around it thus having a greater gravitational pull. I see black ...
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How to correctly create planetary orbits from kepler elements

I would like to build a gravity model (with 3D graphics) that would build the orbit of the planets based on orbital elements input and the gravity calculation for each element. My initial code is ...
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Are gas giants supported by thermal pressure?

I've heard gas giants are supported because there is an equilibrium between thermal pressure and gravity. That is, if Jupiter were to suddenly begin compressing, temperatures would increase to the ...
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How can a supermassive black hole cause so much energy to enlighten its matter when its massive gravity prevents light to escape?

To cite the German newpaper article Astronomen beobachten erwachendes Schwarzes Loch: Das Materie-Monster sitzt den Angaben zufolge im Herzen der 42 Millionen Lichtjahre entfernten Polarring-...
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Do differentials in the Earth's density cause the flyby anomaly?

The flyby anomaly is an occasional, unpredictable, and as yet unexplained discrepancy in acceleration of satellites when they pass close by the Earth. It causes an increase of up to 13 mm/s during ...
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How fast would Earth need to rotate to fling someone off due to centrifugal force?

If Earth started spinning fast enough, would the centrifugal force eventually overcome Earth's gravity enough enough to fling someone (let's assume 75 kg) into orbit or into space? If so, how fast ...
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Are white dwarf stars supported by proton degeneracy as well?

In general, fermions form a degenerate gas under high density or extremely low temperature. It's clear that white dwarf stars are supported by electron degeneracy pressure. However, there are still a ...
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Why don't globular clusters flatten with a galactic disc?

Globular clusters lie in the galactic halo, outside of the disc. However, galaxies are more or less a collection of material and objects — why is it, then, that most stars form a plane due to the ...
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1answer
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How close would merging black holes have to be to feel gravitational waves? [duplicate]

Recently LIGO discovered gravitational waves caused by two black holes that were orbiting each other, and then collapsed into one black hole. A few months later, we find out that this actually happens ...
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Could this three moons system be stable?

In my fictional world I have selected all parameters to maximize the Hills Sphere of my planet. The planet has three times Earth's mass and it is located in 2 AU from its star that's 1.4 times more ...
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Techniques for locating origin of gravitational waves

The gravity wave detections (GW150914 and GW151226) have both been ambiguous in their exact position, the LIGO papers (GW150914 and GW151226) give no precise information about the location of the ...
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Is it possible for a moon to continuously have a side facing its star whilst orbiting a planet?

For example; is there any possibility for the moon to always have a side facing the Sun whilst orbiting Earth? And if so then what would the day cycle be like?
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Can a gas giant be hollow?

Let's think of Saturn, because of its density. Can there be a place near the center of the planet where the gravity upwards is more than towards the center, and with an atmosphere that is more ...
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Higgs Bosons in the centre of Black holes or extremely large stars [closed]

With the discovery of the Higgs Boson as the mass carrier, what is the possibility at the centre of a black hole being so dense and gravity so infinite that the structure of a proton or neutron are ...
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Would the (orbitting) speed of the Earth change if suddenly the sun disappear and the Earth is thrown into space?

Usually an orbit is considered as an acceleration. So if the Earth has a speed of about 100.000km/h around the sun. Now I think there is some relativity involved, so from which point of view shall we ...
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Could asteroid mining affect gravitational balance?

I'm just a guy who is interested in all topics around space, so forgive me if it is a dumb question. I have read some things about asteroid mining in the past and I think it's a really interesting ...