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 come we talk about gravitational potential energy and not gravitational potential?

With regards to gravity the equation learned is $$U=-\frac{GMm}{r}$$ And the relationship to force is $$F=-\frac{dU}{dr}$$ In electrostatics we instead talk about electric field and electric ...
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Radiometric dating of lithosphere and the Schwarzchild's critical radius [on hold]

It is reported that the earth's lithosphere is constantly recycling over time. If that is so, then would this alter the validity of radiometric dating? It is theorized that a singularity the size ...
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1answer
59 views

How does gravity's space time distortion access energy to induce freefall? [on hold]

An observer, standing on top of a building, has a gravity switch (use your imagination) which allows him to turn gravity on and off. He turns the switch off, then releases a pool ball over the edge ...
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2answers
82 views

Is the Moon freely falling towards the Earth?

The Moon is revolving around the Earth and its centripetal acceleration is towards the Earth. Does it mean that the Moon is freely falling towards Earth? What is the cause of the Moon orbiting around ...
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surface tension versus gravity in a zero-gravity environment, size limit of water and air bubble [on hold]

surface tension versus gravity in a zero-gravity environment, size limit of water and air bubble. When do the forces balance?
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Is there, and if there is, what is the name of the concept that matter is made out of void/space?

I can't recall where I heard about this, but it was rather intriguing. I recall the notion that matter is compressed void/space, where I remember an example of a bedsheet representing space, and the ...
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21 views

Finding equation for acceleration using mass1, mass2, gravity, theta, and Mu [on hold]

So, the only question I could not solve in this chapter for AP Physics, and I really want to understand it. We have a situation like this: There is a box sitting on an incline. We are trying to ...
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Does the distance two weights are from you alter the difficulty of lifting them?

If I am weightlifting and I choose to do the deadlift, does it matter how far away from me the weights are on the bar, given that they are equally distance from me on both sides. E.g. I am deadlifting ...
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3answers
56 views

Why does unequal mass distribution cause orbital decay?

It's well known that in low orbits around astronomical bodies with uneven mass distribution, orbits will shift around, and the orbiting object may crash into the surface. Why is this the case? ...
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26 views

How big could a bubble of air and water be in zero gravity? Assume air surrounds and fills a bubble of water

I am designing an all water planetoid for a new science fiction novel I am writing. The Planetoid is a spherical bubble of water filled with air, and covered by a sufficiently thick atmosphere to ...
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1answer
38 views

Expansion of the universe [closed]

If matter cannot be created nor destroyed, I would assume that the universe expands only to the point that energy is expressed from the objects inside it until it collapses onto itself, given as the ...
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1answer
36 views

Spaceship Doppler frequency

A spaceship starts falling under gravity with an acceleration $g$ as measured by an observer Barry at rest on Earth. At the instant that the ship starts to fall, an astronaut Harry at the base of the ...
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1answer
68 views

Does Newton's first law apply also to non-newtonian physics?

I think I understand than an example of Newton's first law (intertial frames) would be a single asteroid in vaccum with no other bodies around, a comet in such free space or likewise. Then I think the ...
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1answer
43 views

An alternate approach to find variation of gravitational acceleration with respect to depth of an object in Earth

As we go down the Earth, assuming it to be a uniform sphere abiding only to Newtonian physics, the gravitational acceleration imparted to an object by the Earth goes on decreasing, and decreases to 0 ...
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40 views

Standard gravitational parameter - different formulas

Why we have two formulas for Standard Gravitational Parameter: $$\mu=GM \ \,{\rm and}\, \mu = rv^2 \ .$$ I don't see any direct connection between the two formulas. How can we derive the second from ...
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1answer
73 views

Gravity of the accelerated particle in General relativity?

Suppose heavy particle or body ("billiard ball") is accelerated with very thin and very rigid (not absolutely rigid) rod of infinitesimal (not zero) mass ("cue stick") Acceleration is constant in ...
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1answer
43 views

Spiral Galaxies and Gravity Lenses

Spiral Galaxies must have a great deal more mass than elliptical galaxies of the same size in order to account for the flat velocity curve. I've seen references of eight to ten times the visible ...
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65 views

Can one model a spiral galaxy with a similar manifold to one for a whirlpool?

My question is more of a phenomenological question than literal, since the forces involved are very different. When one looks at the distortions (or wrinkles or whatever you wish to call them) in the ...
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Does gravity affect magnetism, vice-versa, or do they “ignore” each other?

I am suddenly struck by the question of whether gravitation affects magnetism in some way. On the other hand, gravity is a weak force, but magnetism seems to be a strong force, so would magnetism ...
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1answer
32 views

Calculating change in gravitational potential energy [closed]

A satellite mass $500\ kg$ is elevated from altitude $10000\ km$ to altitude $20000\ km$. Radius of earth is $6378\ km$. Mass of earth is $5.97 \cdot 10^{24}\ kg$. Gravity is given to be $9.81\ m/s $ ...
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49 views

Forces on objects orbiting a black hole?

Firstly, please excuse my elementary knowledge and lack of eloquence when writing about astrophysics. I am a dentist, who occasionally thinks about the how the universe works. I'm both fascinated and ...
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3answers
95 views

Why airports are generally near sea? [closed]

The generic way to measure the height is from the sea-level. If the airport is near the sea i.e. not at a very height from the sea, then the fuel required to make the air-plane above the sea level ...
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131 views

Can a bullet really fly through space forever?

Some people says that if it would be possible to shoot a bullet so high that it would get over the Earth gravitational pull, the bullet could fly through space forever, because of no deceleration of ...
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1answer
28 views

Finding the velocity of a ball thrown upwards [closed]

A ball is thrown upwards with a speed of 16.1 m/s. What is the ball's velocity 1.6 seconds after it is released? I tried to solve this using the equation: $v = u+at$, substituting u for 16.1, a for ...
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52 views

Gravitational potential energy and Center of Mass

I've been reading about the Nordtvedt effect, and how Gravitational Binding Energy (GBE) affects total mass. According to the WP article, experimental evidence rules out the existence of this effect. ...
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61 views

How to prove the energy of gravity in general relativity is non-local?

Every textbook in general relativity containing the energy of gravity all says that the energy of gravity is non-local and every energy-momemtum density received is pseudo-tensor, but "having not ...
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73 views

At what speed would you die upon impact with water?

If we hit the water at great speeds, we die. This is because the water has no time to "move out of the way" and acts as a "solid" surface. At what falling speed can we consider the water as a ...
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46 views

Recommendation on ADM mass and Bondi mass

I want to learn some advanced topics in GR, such as ADM 4-momentum and Bondi 4-momentum. However nearly no textbooks on GR contain this area, such as Wald, MTW, Hawking, Carroll and Zee's. Can anyone ...
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22 views

Numerical tests of Titius–Bode formula for artificial planetary orbits [duplicate]

Titius–Bode law is an empirical formula stating that positions of planets in Solar system is described by: $$a=0.4+0.3 \times 2^m [AU],$$ where $m$ are natural numbers. All planets, with exception ...
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1answer
73 views

Velocity of satellites greater than required velocity

We often find velocity required to keep a satellite in orbit by the formula $v=\sqrt{\frac{GM}{r}}$ where $v$ is perpendicular to the gravitational force. It is very intuitive that the object will go ...
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1answer
135 views

How do objects even move due to gravity?

I am an newbie general relativistic learner and I learnt that gravity is bending of space-time and since objects move in straight-lines but since its curved they follow curved movement through space ...
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1answer
42 views

DO the condensed photon particles-waves-longitudinal-waves exist? [closed]

This is kind of hard to explain, because weird as it sounds, i have experienced a phenomenon that i would like to see if it exists and if i can explain it mathematically. The longitudinal waves of ...
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3answers
76 views

What happens to objects pulled in by black hole? [closed]

What happens to the particles/elements/objects that sink into the gravitational pull and ultimately go to the interior part of a black hole? If, according to popular notion, it is crushed by the ...
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2answers
290 views

If gravitation causes constant acceleration why moon does not fall into earth? [duplicate]

If moon travels with constant speed in one direction and earth gravitation causes constant acceleration in perpendicular direction why moon does not eventually fall into earth? I mean if gravitation ...
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63 views

Is it possible to determine whether distant galaxies are gravitationally bound

In a previous question, one issue was related to the potential energy of cosmic structures. This raised in particular the question of whether these structures are gravitationally bound. If you ...
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Can a number of gravitational slingshot stop a planet?

The answers to Where does the extra kinetic energy come from in a gravitational slingshot? state that in a gravitational slingshot the object being accelerated "steal" speed from the planet (or moon). ...
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4answers
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Why is velocity zero at a neutral point in a gravitational field?

It is said that at neutral points in a gravitational field the net force on a moving mass becomes zero, which means the mass should be moving with constant velocity-and not zero! However, certain ...
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1answer
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Conservation of angular momentum in Earth-Moon system

We all know about the fact that tidal friction is slowly slowing down the Earth's rotation about its axis, and that subsequently the Moon is slowly drifting away, in order for the angular momentum of ...
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1answer
124 views

From Paris to … London [closed]

(Excuse the pun in the title, couldn't resist) Paris and London are connected by a straight underground tunnel, as shown in the diagram below. A train travels between the two cities powered only by ...
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1answer
63 views

Does the gravitino contribute to the gravitational interaction?

I have a very basic question with respect to supersymmetry. Actually, I no clear idea at all what the effect of the superpartners (called gauginos??) of the exchange particles of interactions ...
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Do we weigh less in the morning? [duplicate]

A question on Biology SE got me thinking, ignoring weight naturally lost during the night, "Do we weigh less in the morning?" During the night, the sun is above us, and the earth below us. ...
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How can you calculate how fast a spinning ring/cylinder will accelerate a mass via gravitomagnetism? [duplicate]

Lets say you have a cylinder of length L, radius R, and mass M. How fast will it accelerate a mass of mass M2 that is entering the "throat" of the cylinder, considering the effects of ...
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1answer
41 views

Regarding the 'Arrow of Time'

I read according to 'Newtonian' Mechanics any set of physical activity of particles can be reversed ( I think) so a set of complicated dynamic systems of particles and matter can reverse their ...
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1answer
47 views

Graviton polarization in higher dimensions

It's not difficult to see that the graviton in $D$ spacetime dimensions has $(D-3)D/2$ polarizations. In $D=4$ there are two $\epsilon^{\pm}_{\mu\nu}$. What I find curious is that in $D=4$ I can ...
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1answer
47 views

Is the spacetime generated by isolated system always asymptotically flat?

I read a saying in wiki of asymptotically flat spacetime http://en.wikipedia.org/wiki/Asymptotically_flat_spacetime "In general relativity, an asymptotically flat vacuum solution models the exterior ...
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1answer
38 views

How is dark matter meant to explain the faster than expected rotation of galaxies? [duplicate]

The stars on the outer edges of galaxies go around faster than they should be. How is dark matter meant to account for this? if you just add more mass wont that just give it a greater gravitational ...
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Why does time slow down the closer you are to a mass?

When ever i look this up all I get is sites saying how its because general relativity says "-" why does it do it though? it is because there is more motion near gravity than further away? Or is it ...
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Why graviton is spin 2 instead of spin 1? [duplicate]

Why graviton is spin 2 instead of spin 1? Is it because the traceless symmetric tensor has rank 2? In many articles, they say the graviton is spin 2 particles, but they don't mention why. I find ...
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1answer
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How come the atmosphere moves with Earth? [duplicate]

now, I have read a lot of explanations on that but still can't really understand why it would happen so if you can give some examples for a such a thing happening. I mean lets say gravity attracts the ...
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1answer
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Why is gravitational potential energy negative? [duplicate]

Why is gravitational potential energy negative? How is it different from other forms of energy? I recently saw a video by Dr. Michio Kaku, he said that the total energy content of the universe is ...