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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Why don't electrons fall on the ground, off the charged wire?

Please, don't mind if my question is silly, but I was always wondering... Imagine we have a negatively charged metailc wire. Let's assume it's just hanging on a thin rope. What force keeps electrons ...
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1answer
73 views

How can I use Einstein's field equations? [duplicate]

Every time I try to find the answer to this question I get redirected to different pages that ultimately do not end up answering my question. I have some understanding of Riemannian geometry but have ...
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2answers
777 views

Why is it hard to detect a black hole

I've read in some texts that we can't directly observe a black hole in space because not even light can escape from its gravity. Some of the indirect observational methods mentioned are, gravitational ...
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0answers
26 views

Has galaxy merger and universe expansion been proven?

I understand the basic argument for hierarchical merging of galaxies during expansion of the universe. I also understand peculiar motion and the Hubble flow. So I understand what's going on with ...
2
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2answers
46 views

Is there a difference between Weight and Force of Gravity?

Is there a difference between Weight and Force of Gravity or they just mean the same thing? My textbook does not clarifies this point. Thank you in advance for your help!
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1answer
69 views

How can I use Einstein's field equations to find the metric tensor? [duplicate]

I have watched and read a lot on the topic of General Relativity and the geometry behind it. I am confident that I can derive an approximation of the the stress-energy-momentum tensor with just the ...
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0answers
41 views

How does Einstein's gravity work? [duplicate]

I'm a chemistry student interested in physics. Hope the question doesn't sound funny. As opposed to Newton's gravity, which doesn't explain how gravity works, Einstein explained gravity as a result ...
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1answer
34 views

Is an elastic, gravitating object stable?

As I understand, an object can contract under its own gravitantional pull. Then as it does so, different parts of the object must move with some velocity. This in turn means that their relativistic ...
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1answer
50 views

If there were only 2 particles in the universe, at rest, separated by some space, what would happen to them and why? [on hold]

If there were only 2 particles in the universe, at rest, separated by some space, what would happen to them and why? What if there were 3 particles arranged in an equilateral triangle? 5 particles ...
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0answers
45 views

How Electromagnetic Force is stronger than Gravitational Force [duplicate]

I had read this in text book..Can some explain this. When we hold a book in our hand, we are balancing the gravitational force on the book due to the huge mass of the earth by the 'normal force' ...
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1answer
132 views

Is my diagram of spacetime curvature valid (relatively)?

I've been wracking my brain trying to understand what "curved spacetime" really is, and I think replacing one dimension with the time dimension then drawing the world-lines through time was the "aha!" ...
2
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1answer
65 views

Can observations of cosmic super voids teach us anything about time, dark energy, or dark matter?

Can observations of cosmic super voids teach us anything about... Time - If gravity affects time is there a chance we could gain any further information from studying a region with lower than ...
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0answers
64 views

Do physicists believe there is anti-gravity? [closed]

There are forces and their counter parts in the universe such as the four basic forces: the strong, weak, positive electromagnetic and negative electromagnetic forces. What about gravity? Is ...
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1answer
68 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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1answer
19 views

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 ...
3
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0answers
45 views

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

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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1answer
47 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
34 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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1answer
75 views

In what direction will this magnet move? [closed]

Two plain ringshaped magnets joint together with the same poles.The pulley hardly attached to small ringshaped magnet that has a rod on its center.
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0answers
31 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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0answers
17 views

Stabilizing the flow of instability [closed]

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
65 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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1answer
32 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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42 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
42 views

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

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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76 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
92 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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0answers
40 views

Can it happen that in less gravity clock goes slower but the actual time remain same? [closed]

It is said that clock in satellite goes slower than that on Earth because of time dilation .(by 'clock' I mean the mechanical instrument that measures time)
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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
46 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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1answer
37 views

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

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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5answers
2k views

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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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
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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0answers
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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2answers
58 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
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
63 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
79 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
77 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
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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0answers
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
87 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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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 ...
1
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1answer
49 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
63 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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0answers
26 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 ...
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0answers
25 views

Could the gravitional pull of all the travelling photons “relativistic mass” in our galaxy explain “dark matter”? [duplicate]

Does that make sense mathematically? If we could sum up all the photons in-transit in our galaxy and calculate the total gravitational pull, could it provide an alternate explanation instead of ...
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1answer
27 views

Turning point of ball, when throwing it into the atmosphere

I am currently reading an introduction chapter about gravitation. One exercise was about calculating the escape velocity on earth. At one in the book it reached the following formular: $ r_{max} = ...