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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A central force which enables a torque on a sphere - is it still conservative?

Consider the following example: Two spheres (one big, other small) standing vertically on ground. At first, the small sphere is on top of the big sphere. Then, it starts to roll w/o slipping to ...
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1answer
24 views

Is it possible to have a black hole in empty space?

If the escape velocity of two very massive objects is near the speed of light, and those objects are orbiting each other (let's ignore the Roche limit for this exercise), is it possible that the ...
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0answers
36 views

How much work can a single grain of rice do? [on hold]

I found a website saying a grain of rice contains 1/10 kcal. I'm not a physicist and haven't done maths for a long time. But here's what I came up with: E = m * g * h One small calorie equals ...
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0answers
22 views

The ADM Energy of Gravitational Waves?

I have been looking for books about this question for several days. However, almost all books use Landau–Lifshitz pseudotensor to calculate the energy of Gravitational Waves.And they said the result ...
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2answers
55 views

Bending of space vs. bending of a body - which is larger, and by what factor?

This is not homework, but is a problem I am asking myself. A long rod with known elastic material properties floats in flat space - suspended and kept in place by a number of massless ropes. Now we ...
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2answers
27 views

Does the value of the Ricci scalar determine the strength of the gravitational field?

If I was solving an equation that contains the Ricci Scalar, and I want to solve the equation in the strong and weak gravity regimes, is right to assume that $R>>1$ for first case and ...
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25 views

In what direction will this magnet move?

What will be the direction movement of small ringshaped magnet when place inside the big ringshaped magnet with the same poles?
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28 views

How does Sachs derive QM mass term from general relativity?

I am interested in finding Mendel Sachs' derivation of the mass term in the Dirac equation, using the covariant derivative terms. The discussions as to the merits of Dr. Sachs' work are often prefaced ...
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15 views

Stabilizing the flow of instability [on hold]

Where can I find the technology to print in a sphere an unstable pattern of magnetism surrounded with stable magnetism to provoque flow, And the opposite convex ?
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3answers
58 views

What is the reasoning behind the idea that light cannot escape from a black hole? [duplicate]

According to the definition, light cannot escape from a black hole. How did scientists deduce that light cannot escape from a black hole?
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3answers
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How do we determine the mass of a black hole?

Since by definition we cannot observe black holes directly, how do astronomers determine the mass of a black hole? What observational techniques are there that would allow us to determine a black ...
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1answer
30 views

Energy variations of cosmic background radiation

Has anyone been able to measure a difference in the energy density of Cosmic Background Radiation in a gravity well compared to zero gravity?
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5answers
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Why doesn't matter clump together such that it can't be taken apart again?

Given the inverse square law force of gravity shouldn't two particles that are infinitely close to each other be infinitely attracted to one another? For example, suppose the hands of some super deity ...
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37 views

Are there experimentally verified differences between general relativity and Lorentz invariant “newtonian” gravity?

I used to have (I lost it) a historical article about how Eintein's general relativity theory "won" over a Lorentz invariant generalization of Newton's gravity (I cannot remember the author). This is ...
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2answers
35 views

How does the earth do a negative work on a static body? [on hold]

If a body is in rest and the earth acts with a force on it 10 N Is there a negative work done by the earth though the body doesn't move? how? does it have a common thing with potential energy?
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62 views

What is the Casimir effect caused by gravity?

What is meant by a Casimir effect caused by gravity? Please explain me the concept in detail and how is it caused.
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2answers
79 views

What is the speed of light after it has entered a black hole?

I was just thinking about this the other day. Given Einstein's Relativity etc. assigning the cosmic "speed limit" the value of $c=299792458\:\frac{m}{s}$, we know that the event horizon is the point ...
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2answers
92 views

Concave Sphere Habitat characteristics

I'm writing a short story set in an artificial planet-sized sphere with an ecosystem in its inner surface, whose "gravity" is created through spinning. Energy sources aside, what other interesting ...
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33 views
0
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1answer
34 views

Potential Energy of Interaction Between a Sphere and a Particle Formula Derivation [on hold]

A sphere of radius R has density described by ρ=ρ(r). Derive equation for pontetial energy of interaction between the sphere and some point particle of mass m which is at distance r from the center of ...
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2answers
47 views

Calculation of a normal force with an angle

I'm currently studying forces in physics and in this example, it states that Fn +Fay = mg. Why is this so? Is it because they are both in the same direction?
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0answers
30 views

Time dilation as an inverse correlate of “spatial friction” [on hold]

am i wrong to think of space as either having a density and or a velocity? It seems that higher "spacial friction" is inversely correlated with rate of time. example: traveling through more space in ...
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2answers
76 views

Gravity as a particle-based force vs space-time warp

I'm interested to understand the interpretation of gravity as a result of exchange of force particles (ie gravitons) vs General Relativity-based warping of space-time. Related to this is while a ...
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0answers
45 views

How can spacetime affect massless entities? [duplicate]

Why is light affected and "bent" by spacetime if it has no mass or density?
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3answers
277 views

Understanding bending light beam perpendicular to motion

I'm just reading a book about gravity. An example it gives is a spaceship accelerating. A beam of light travelling at right angles to the direction of movement of the spaceship enters it via a small ...
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2answers
302 views

Why is “space-time” not fully explained as the observational effect of the casimir effect seen from a human perspective?

Simply stated the casimir effect can be observed bringing two plates together microscopically and in vaccum Is this analogous to the observation of gravity? Question: What is casimir effect? ...
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0answers
38 views

What is the mechanism behind gravity? [closed]

For me action at a distance is very spooky. With magnets you can attract or repel objects without touching them, and with gravity you have an attraction between objects no matter how far apart they ...
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0answers
7 views

Water tank, circulation, gravity

Is it possible to use gravity alone to make a water circulation system? -a large container filled with water and a hole in the bottom which attaches to a long hose/tube The other end of this ...
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1answer
657 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
49 views

Gravitational lensing not caused by gravity?

This may be a possible duplicate, though I recently stumbled upon this paper http://arxiv.org/abs/1004.1467 claiming to have an alternative explanation of the interaction of light around massive ...
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43 views

Photons and gravity [duplicate]

A very powerful laser pointing to the empty space next to small object in space. The photons in the laser beam does not hit the object. Will the small object be gravitationally affected by that laser ...
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1answer
60 views

What it would look like to observe people with a different time flows?

As I learned, that the bigger gravity source you are influenced by the more slow time ticks for you, the farther away you are from a gravity source the faster times ticks. So Imagine two different ...
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1answer
101 views

Potential Energy of Springs and Gravity, and Work of a force

I am currently really stuck on this problem and I am not sure how to actually solve it, I think I can reason it out logically but I do not know how to show it. Here it is and under I will show what I ...
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2answers
23 views

Keeping an object from gaining or losing altitude

How can I find the thrust, $t$, needed to keep an object with $m$ mass, in the influence of an object with $g$ gravity, from gaining or losing any altitude?
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1answer
61 views

Different forms of the Einstein field equation

I am working my way through the wonderfully written introduction "General relativity for mathematicians" by Sachs & Wu. I am indeed a mathematics student and find this book to be well suited to ...
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1answer
77 views

Can shadows affect gravity?

I was watching a video on Youtube (Vsauce, great channel, check them out), and they were talking about how much a shadow weighs, and he was talking about sails that were pushed by light. He said an ...
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2answers
375 views

Spacetime geometry around two black holes

For space-time geometry we all know images like he one below. But if I were to take a neutron star and put it right next to but not touching a black hole what effect would that have on the geometry of ...
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0answers
29 views

How can we isolate the gravity from a room or an area? [duplicate]

How can we isolate the gravitional force from something?
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4answers
357 views

Effect of gravity upon light and bagels

According to this question and this web site, photons undergo twice the deflection from gravitational fields as do physical objects. However, the weak equivalence principle states that locally, ...
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1answer
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4answers
1k views

Is there a fundamental reason why gravitational mass is the same as inertial mass?

The principle of equivalence - that, locally, you can't distinguish between a uniform gravitational field and a non-inertial frame accelerating in the sense opposite to the gravitational field - is ...
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17 views

Who was the first person to intuitively explain why gravity weakens by the square of the distance? [migrated]

the surface area of a sphere is 4pir^2 and when you increase it the energy disperses by the square. Who was the first person to realize this?
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2answers
85 views

How much dark matter is needed to gravitationally distort light?

I understand that dark matter only has a gravitational property. It does not interact directly with regular matter or EM radiation. The evidence for dark matter comes from observations that contradict ...
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1answer
140 views

How the center of gravity works in the picture? [duplicate]

How the center of gravity works in the picture? How the other parts of the body able to hold the bal;ance of the center of the gravity of the man?
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3answers
434 views

Theory that gets rid of dark matter/energy

Is there any physics theory that either groups together gravity and dark energy/dark matter or eliminates dark energy/dark matter by modifying standard understanding of gravity or any force? If so, ...
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1answer
40 views

Isolation of a Physical System and Energy

I'm trying to grasp basic concepts of energy and i have a little doubt regarding conservation of energy. According to Feynman book and Wikipedia, the Law of Conservation of Energy states that ...
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3answers
150 views

Einsteins gravity Space time

I'm having a difficulty in understanding Einstein's version of gravity as "space time curvature" (I could be a total physics idiot, please point out !). How does an object on the surface of earth ...
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1answer
45 views

Can I tell where a leak is in a vertical water pipe based on the water pressure at the bottom?

I have a buried water line with a leak in it and I want to locate approximately where it is. The line runs uphill to a storage tank. I've isolated the leak to one section of the pipe. The diameter of ...
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1answer
62 views

What is the speed of a Photon? [duplicate]

How fast does a Photon travel through space? And is gravity able to influence a Photon's path?
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24 views

Available material on Giant Gravitons

I am looking for a pedagogical introduction to giant gravitons (if one exists!). I have basic string theory/SUSY knowledge but no introduction to AdS/CFT. (Do I need to do some reading on AdS/CFT ...