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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1answer
63 views
What is the shortest run needed for a “dropped skateboard” rider to reach 160 mph?
I just saw a great clip on the fastest skateboarder to date. He wants to reach 160 mph from a dropped board, but says his speed is limited by the roads currently available. What is the shortest run ...
1
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3answers
164 views
The theory of moon creation when a Mars size planet hit Earth
As we know the predominant theory where does the moon come from is that a Mars size planet hit the earth and took a chunk out of it which eventually materialized into moon.
My question is that if a ...
1
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3answers
274 views
Can gravity be absent?
Can gravity be absent? Not weightlessness as an astronaut experiences it because the astronaut's body still has gravity which will probably manifest in the presence of another smaller/larger body. For ...
4
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3answers
260 views
Do rotating bodies emit gravitational waves?
Suppose we have a cylinder of mass $m$, radius $R$ and height $h$ in rotation with speed $\omega$ around its symmetry axis with no friction (ideal situation).
I'd expect this cylinder to emit ...
1
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1answer
104 views
Are there still Big Bang Gravitational effects
If all matter was concentrated at one point before the big bang, I assume that the gravity was at a huge scale. For a long time many believed that the universe would eventually collapse back upon ...
0
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0answers
67 views
How to control heavy ball in x and y direction? [closed]
I'm trying to build a device that holds a rather heavy ball and I want to be able to control it's "movement" in the X and Y directions (they are horizontal as well as orthogonal).
My current ...
1
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1answer
124 views
vector-scalar gravity
Has anyone developed a vector-scalar theory of gravity using an acceleration field equation that is based on a ratio in which the numerator is the familiar Newtonian vector sum of all masses times the ...
3
votes
1answer
63 views
Can extremely charged objects simulate some black hole effects?
If we had a positive point charge of incredible quantity, does there exist an imaginary sphere about it, such that regardless of the initial speed and direction of any electron, that electron could ...
2
votes
1answer
490 views
The trajectory of a projectile launched from a hilltop
Here is the problem:
A boy stands at the peak of a hill which slopes downward uniformly at angle $\phi$. At what angle $\theta$ from the horizontal should he throw a rock so that it has the greatest ...
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1answer
174 views
Gravitational force?
We know that two mass particle attract each other with a force
$$F~=~\frac{G M_1 M_2}{r^2}.$$
But what is the reason behind that? Why does this happen?
5
votes
1answer
141 views
Confused about indices of the Ricci tensor
In an intro to GR book the Ricci tensor is given as:
$$R_{\mu\nu}=\partial_{\lambda}\Gamma_{\mu \nu}^{\lambda}-\Gamma_{\lambda \sigma}^{\lambda}\Gamma_{\mu \nu}^{\sigma}-[\partial_{\nu}\Gamma_{\mu ...
3
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2answers
200 views
Has anyone else thought about gravity in this way?
Picture yourself standing on a ball that is expanding at such a rate that it makes you stick to the ball.
Everything in the universe is expanding at this same rate.
To escape the earths gravitational ...
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3answers
248 views
Laws of gravity for a universe that only consists of two objects?
So, we know that when two objects of normal matter get away from each other, the gravitational pull they feel from each other, decreases.
I wanted to see how that would work. And in my ...
1
vote
3answers
321 views
What entities create a gravitational field?
It is well known that masses create a gravitational field. Photons are affected by gravitation, but do they generate a gravitational field as well? What about the other gauge bosons?
Do gravitons ...
1
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1answer
118 views
Determining Orbital Velocity
Is there any way to determine the orbital velocity of a point around another stationary point, if I don't know the mass of either of the points but know the force that gravity exerts and the distance ...
0
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0answers
50 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 ...
3
votes
6answers
588 views
Does gravity slow the expansion of the universe?
Does gravity slow the expansion of the universe?
I read through the thread http://www.physicsforums.com/showthread.php?t=322633 and I have the same question. I know that the universe is not being ...
2
votes
1answer
50 views
What parameters determine the acceleration a spacecraft can acquire using the gravity of a celestial body?
Contemporary spacecraft frequently rely upon gravity to accelerate/decelerate.
Given a spacecraft already in space - What factors determine the acceleration it can acquire from the gravity of another ...
8
votes
1answer
720 views
Why does water flow out of an upside-down bottle? (Rayleigh Taylor Instability)
I am currently reading the excellent book An Indispensable Truth: How Fusion Power Can Save the Planet by Francis F. Chen and I came across this explanation.
The Rayleigh–Taylor Instability
...
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2answers
135 views
How do we end up with a gravity-dominant macroscopic universe from a quantum world having weakest gravity?
At quantum scale, gravity is the weakest force. Its even negligible in front of weak force, electromagnetic force, strong force.
At macroscopic scale, we see gravity everywhere. Its actually ruling ...
4
votes
2answers
434 views
How can I calculate the flow rate necessary for a given cross sectional area, angle, and length
I'm making a toy for my kids and this problem came up. I have a channel on a slight angle (angle is between ground and length of channel) and I'm pouring water into it. I want to know how quickly I ...
3
votes
4answers
157 views
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 ...
-4
votes
1answer
153 views
Why did Standard Model never sense a requirement to include gravitational quantum?
Standard Model is advanced version of Quantum physics. It tried to include everything which came in the way while understanding quantum world. It even didn't bother to include even Higgs Boson which ...
6
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3answers
2k views
Why is Higgs Boson given the name “The God Particle”?
Higgs Boson (messenger particle of Higgs field) accounts for inertial mass, not gravitational mass.
So, how could it account for formation of universe as we know it today? I think, gravity accounts ...
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2answers
97 views
What is gravitational speed for a black hole? [duplicate]
Possible Duplicate:
What is the escape velocity of a Black Hole?
This is gravitational speed for earth: $$v=\sqrt {g_{e}r}.$$
What is gravitational speed for a black hole? I want an ...
2
votes
3answers
188 views
Can a photon be made to orbit a known (or undiscovered theoretical) body?
Can a photon through some process be made to orbit a celestial or any other object?
Two follow-up questions.
Can this orbit be described as the photon crossing its own path.
Will this ...
2
votes
3answers
341 views
What happens to matter in extremely high gravity?
Though I am a software engineer, I have bit interest in sciences as well. I was reading about black holes and I thought if there is any existing research results on How matter gets affected because of ...
0
votes
2answers
158 views
What happens if an object has more kinetic energy than the Gravitational Binding Energy?
So the binding energy of an object is the amount of energy needed to move it an infinite distance away from another mass to essentially “escape” its gravitational field. But what happens if you give ...
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2answers
309 views
Electric Force is to Magnetic Force as Gravitational Force is to …?
One can no nothing about the magnetic force and yet arrive at it by taking the relativistic effects of a current and a moving charge system into account. I ask whether there exists such an inherent ...
2
votes
4answers
1k views
If gravity is a bend in Space-time then what is magnetism?
Einstein postulated that gravity bends the geometry of space-time then what does magnetism do in to the geometry of space-time, or is there even a correlation between space-time geometry and ...
0
votes
1answer
61 views
Continuum model of the n-body problem?
Consider a collection of $n$ point masses evolving under the influence of gravity. For very large $n$, one might be interested in making a continuum approximation of this system (for cosmology, say). ...
4
votes
4answers
2k views
Why can't a rope be pulled completely straight?
I have found several discussions on how to calculate the sag of rope that is tied off at two points (like a tightrope), and I understand it to a certain extent. What I can't wrap my head around is how ...
5
votes
0answers
149 views
Why Brown York stress tensor == dual field theory's energy momentum tensor?
From the AdS/CFT dictionary, how to argue that the Brown York stress tensor for a gravity
system near the boundary is exactly the same as the energy momentum tensor of the dual field theory?
In the ...
1
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0answers
44 views
strength calculations - scooter [closed]
I need some help to compute strength calculations of my homemade scooter.
the plate (where you stay on) are 20mm x 500mm x 10mm
In the back there are 1 wheel, and in the front there are 2 wheels ...
5
votes
2answers
183 views
Do objects with mass “suck in” spacetime?
I don't really understand the general theory of relativity (GTR) really deeply, but according to my understanding, the GTR say that gravitation is caused by the curvature of spacetime by objects with ...
0
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1answer
69 views
Space station gains enough mass to lose orbit?
I.S.S is constantly being improved (add-ons).
Will the space station need to be moved to a higher orbit at some point?
2
votes
4answers
544 views
How long must escape velocity be maintained?
Escape velocity from Earth's surface is 11.2 Kilometres/second
How long would one need to maintain this escape velocity to actually escape Earth's gravitational pull? Must this 11.2 km/s velocity ...
0
votes
0answers
182 views
The Gravity Spring [closed]
This problem examines the scenario of a tunnel drilled through the center of the Earth, and the length of time required for an object to fall through the hole and back again.
use trigonometry to ...
5
votes
1answer
119 views
What would gravity be like on a hemispherical planet?
On a hemispherical world half the size of Earth (ignoring that the planet should break apart and become a smaller sphere) what would the gravity be like? How would it change as you traveled from pole ...
7
votes
1answer
334 views
Is this a quaternion representation of the equations of motion of General Relativity?
In The Quaternion Group and Modern Physics by P.R. Girard, the quaternion form of the general relativistic equation of motion is derived from
$du'/ds = (d a / d s ) u {a_c}^* + a u ( d {a_c}^* / ...
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votes
2answers
251 views
The measurement of $g$ and air resistance
Suppose I am trying to determine the value of the acceleration due to gravity by simply dropping objects from a ledge. If I did not account for air resistance, would the acceleration due to gravity ...
4
votes
4answers
671 views
Does the mass of an object change as it moves away from the earth?
The mass of a helium nucleus is less than the mass of two isolated protons and two isolated neutrons. When the component hadrons are assembled, this mass is lost as energy ($E=mc^2$). This makes it ...
1
vote
1answer
106 views
How do gravitational waves sustain and propagate large scale spacetime curvature?
I understand that gravity in GR is a manifestation spacetime curvature dictated by the field equations by the principle that objects follow the geodesic path in spacetime.
And, I get how ...
3
votes
1answer
141 views
Understanding the E=MC2 for multiple objects
Im actually not a very good at physics, but I was playing with this functions (just for fun and education, so this question might end at the Scifi forum), and would like to know if this logic is ...
1
vote
3answers
181 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, ...
1
vote
1answer
66 views
Computing gravitational deflection of light knowing $\phi$ and $-\nabla \phi$?
I have a 3D cartesian grid an in each grid I know the gravitational potential $\phi$ and the 3D gravitational field $-\nabla \phi$ (with a Newtonian approach). How to compute the path of a photon in ...
2
votes
1answer
117 views
Calculating a Deflection of Light Integral from Einstein's 1907 paper on Gravity and Light
$\frac{1}{c^2}\int_{\theta=-\pi/2}^{\theta=\pi/2} \frac{kM}{r^2}cos\theta ds=2\frac{kM}{c^2\Delta}$
An observation: $\cos\theta=\frac{\Delta}{r}$
1
vote
1answer
260 views
Does the Relative density of water change based on the state it is in
Will Relative density of water change based on the state it is in? Ie solid, liquid, gas.
What causes this change(if any) in Rd?
2
votes
1answer
142 views
Can a charged black hole interact via electromagnetism? [duplicate]
Possible Duplicate:
Detection of the Electric Charge of a Black Hole
Light cannot escape from a black hole. However light is also interpreted as the carrier of the electromagnetic force. So ...
0
votes
1answer
83 views
Does General Relativity require that Spacetime must be a orientable? [duplicate]
Possible Duplicate:
Can spacetime be non-orientable?
Apart from the constraints put on the topology of spacetime by QFT (Parity For example), if the global topology of a universe is that of ...
