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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203 views

How does time relate to mass and velocity

I understand that the larger the mass the greater gravity is and the slower time is, as well the faster an object is traveling the slower time passes. My question is that since the faster an object ...
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1answer
116 views

What limits range of gravitational attraction concerning space expansion?

Wikipedia says: Metric expansion is a key feature of Big Bang cosmology and is modeled mathematically with the FLRW metric. This model is valid in the present era only on large scales (roughly ...
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1answer
71 views

Why are trajectories in General Relativity speed-dependant?

I'm a secondary student who never studied any general relativity level maths (yet!), but I heard that in general relativity objects with mass and energy deform space-time, and therefore bodies move in ...
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1answer
125 views

Do electrons have structure in quantum gravity?

I have read Luboš Motl's blog and he says in QG electrons must be composite and have structure because QG says there can be no point like particles and everything above the Planck size can be resolved ...
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2answers
161 views

Does influence on gravity increase with mass when velocities approach the speed of light? [duplicate]

In other words, when a particle's mass increases significantly as it approaches the speed of light (c), does its influence on gravity increase, and if so, does it increase proportionally?
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93 views

Trajectories of particles with spin in Einstein-Cartan theory

The Einstein-Cartan theory is a generalisation of General Relativity insofar as the condition that the metric affine connection is torsion-free is dropped. In other words, the space time is a ...
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84 views

Perihelion Advance in curve space

I've been doing some work that involves particle's motion subjected to central forces. I was trying to compute the perihelion advance of the particle's path. For that I've been following this ...
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5answers
678 views

Why won't protons revolve around the nucleus containing electrons and neutrons?

In case of solar system,we can explain "Why Sun would not revolve around any other planet?",by giving the reason that Sun is heavier than any other planets. Heavier the body,greater will be the ...
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63 views

Is projectile trajectory on Earth affected by the fact that it isn't a singular point?

So in high school physics, we were told that projectiles traveled in a parabola, but Google searches have said that it's more like an ellipse, since the Earth isn't flat. However, I was thinking that ...
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27 views

Modeling Spacetime In A Corpuscular Or Statistical Way

When I first read about Newton's gravity, it really bothered me. I didn't understand how shaking a lighter at the other end of the universe would instantly influence the lighter that was in front of ...
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4answers
171 views

Why is there no gravity in the center of a celestial body, yet there is gravity on the surface?

I've heard few explanations, but none in good detail that make it clear to those with less mathematical knowledge on the equations of complex physics. I would like to think that since you are on the ...
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0answers
125 views

How could they be hit twice by debris?

In the movie Gravity the two astronauts (Bullock and Clooney) are hit twice by the fast moving debris that bring havoc to their locations. I think this is a bad scientific mistake in the movie plot. ...
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343 views

How Come gravity doesn't affect itself?

If gravity is this "unexplainable force" that pulls everything to the center of a planet or stellar remnant you stand upon, why doesn't gravity pull itself? If gravity effects anything with energy, ...
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221 views

Why aren't atoms affected by gravity, but molecules are?

I read this book here: http://tiny.cc/Gravity-Atom-Myth It claims that while gravity does affect individual atoms, it's not quantified like molecular mass due to lack of binding proteins which render ...
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3answers
174 views

What is gravity? [closed]

I am not asking why it occurs, but what it is. Everything understood is something. Something would entail the existence of a property, whether or not tangible. We know photons exist, but we know ...
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1answer
1k views

Do we have theories about why gravity exists/HOW it works?

For years I have been very fascinated by the "mystery" aspect of gravity. Functionally, we understand it perfectly for our all applications, but in my (limited to undergrad General Physics 3 quarters) ...
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160 views

Given finite speed of gravity, why didn't Earth fell into the Sun already?

When Sun and Earth are moving, at each moment $t$ they are attracted not to the current position of each other, but to the position of each other at $t-\Delta t$, where $\Delta t$ is the time required ...
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1answer
207 views

Why does mini quadcopter doesn't maintain altitude?

I have a mini quadcopter controlled by remote controller. I have set the motors (e.g 50% thrust) it will remain at altitude at 10 cm~ but once I increase to 80% and then follow by decrease to 50% I ...
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1answer
398 views

Sandstone getting soaked with water

I have seen someone putting a sandstone in water. With only about 10% of the stone sitting in the water. One could see the stone getting soaked with water. So there must be a force, which lets the ...
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8answers
418 views

Why in Newton's law of gravity, we do $M_1 \times M_2$ and not $M_1 + M_2$?

In Newton's law of gravity $$F=\frac{G M_1 \cdot M_2}{r^2}$$ we do the product $M_1 \times M_2$ and not the sum $M_1 + M_2$. Why's that so?
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1answer
89 views

How faster is the effect of force exerted by gravity? [duplicate]

We all know that Earth revolves around the Sun due to Gravitational force of the Sun. Also we know that it takes just over 8 minutes for the Sun's light to reach Earth. Now let us say, hypothetically ...
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1answer
203 views

The physics equation for a perpetual seesaw

I am trying to create a game that envovles two or more people jumping on a seesaw and propelling each other in the air. I need help with the physics equations Assume no friction no air resistance ...
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1answer
76 views

Trying to understand infinite gravitational energy

Google has not been helpful because so many derivations of gravitational potential energy discuss $r$ at infinity. My understanding of this https://www.youtube.com/watch?v=IcxptIJS7kQ is: Dark ...
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1answer
380 views

What is the buoyant and viscous force acting on a ball in free fall?

I was thinking about a situation were I have a long cylindrical jar with some viscous liquid in it. I also have a spherical ball with me. I drop the ball into the liquid inside the jar with some ...
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1answer
103 views

How does Hawking radiation grow as a black hole evaporates?

The temperature of Hawking radiation is inversely proportional to the mass of a black hole, $T_{\rm H}\propto M_{\rm BH}^{-1}$, and so as the black hole shrinks the temperature of the radiation should ...
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1answer
128 views

How can you calculate the tidal gradient for an orbit?

In the movie Gravity, two characters are dangling from the international space station by a long tether. I've previously wondered exactly how you could calculate the tidal forces that act on an ...
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Present experimental status of Moffat's Modified theory of Gravity

Modified theories of Gravity have been discussed before in this 2-year old question, Are modified theories of gravity credible? I was going through Moffat's modified gravity, given in ...
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3answers
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Why do sand dunes, hills and mountains assume the shape they normally do?

What about something taken in a tablespoon? This shape, as I understand, could be explained by gravity, friction and similar concepts, but why the shape isn't cubical or cylindrical couldn't be ...
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2answers
173 views

Horizontal Gravitational Force

So I was writing a lab report for Physics I, where I was describing projectile motion. I had written Since gravity on Earth does not have a horizontal force, the object will not lose horizontal ...
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3answers
738 views

Calculating Atmospheric Pressure on an Imaginary Planet

I am planning a series of science fiction novels that take place on an imaginary binary planet system. Both planets have a lower surface gravity than the Earth and one has slightly more mass than the ...
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2answers
97 views

How does one determine black hole mass based on the curvature of a photon?

How much mass would a black hole need to create a Schwarzschild radius that would trap a photon, whereby the photon would (to an outside observer) be continually curved 0.004km/s at the horizon? ...
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1answer
90 views

What is the physics behind this invention? [duplicate]

Could someone explain the physics behind this US patent ? The apparatus is supposed to generate electric energy from gravity. The idea of the invention is to use a trapdoor kind of mechanism to push ...
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1answer
73 views

What is most infeasible about a sky hook (as in space travel not Batman)?

We need to get things into space but it costs too much. Lots of people talk about space elevators but no one talks about the skyhook idea. I'm referring to this: Skyhook (Wikipedia), not the thing in ...
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2answers
172 views

Can a scalar field model gravity? How accurate would be the results? Are there any difficulties with such a model?

Newtonian gravity can be described by the equation: $$ \nabla^2 \phi = 4 \pi \rho G $$ where $\rho$ is the mass density, $\phi$ is the gravitational potential, and G is the universal gravitational ...
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1answer
335 views

Are the perpetual machines in this video really perpetual?

This video lists a number of perpetual motion machines that have been proposed throughout history: Robert Boyle's Self-Flowing Flask A "perpetual train" using rolling cones F. G. Woodward's hoop ...
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0answers
176 views

Angular Momentum Conservation in Gravitational Interaction

thanks for any help. I'm trying to show that in a 2body problem, angular momentum is conserved given that $\dfrac{dp}{dt}=\dfrac{-GMm(rv)}{r³}$, where p is momentum, t time, G gravitational constant, ...
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153 views

Angular Momentum Conservation in Gravitational Interaction [duplicate]

thanks for any help. I'm trying to show that in a 2body problem, angular momentum is conserved given that $\dfrac{dp}{dt}=\dfrac{-GMm(rv)}{r³}$, where p is momentum, t time, G gravitational constant, ...
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1answer
51 views

Testing the speed of gravitational waves in superfluids

Ok. Light can be slowed down in superfluids. I would like to know whether if a Cavendish like experiment could be possible within inside superfluids i.e, to rotate two balls such that to observe their ...
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2answers
83 views

Is centre of gravity a point?

Is the centre of earth a point? If it is so, then it should attract objects both horizontally and vertically. But, it always attracts vertically. Why is it so?
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2answers
65 views

Neutrinos travels through our bodies: do they contribute for gravitational atractive force?

Are neutrinos traveling through our bodies what really make us attracted to the Earth?
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352 views

Why gravity is an attractive force? [duplicate]

Why gravity is an attractive force? One may say that it is because of space time curvature but General Relativity is built on this law: $\displaystyle G \frac{m_1 \times m_2}{r^2}$ (To be more ...
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2answers
155 views

Newtonian gravity equation in a 2 dimensional world [duplicate]

I am wondering if my line of thought is correct - and thus the resulting answer to the problem above would be correct. As we know the gravitational force (of two point masses) is given by $$F = ...
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1answer
84 views

General Relativity and Time Dilation [duplicate]

Is time affected by the gravitational force? If so, what might be the effect on time at the centre or near centre of earth ?
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2answers
173 views

Gravitation not instantaneous = non elliptic orbits?

When I studied physics some time ago my teacher explained that if we consider the gravitational atraction not instantaneous, such as the General Relativity says, the planets would be attracted towards ...
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79 views

Surface Area Relative to Element Distribution

We tend to find heavier elements like iron and others toward the center of celestial bodies. Lighter elements like carbon, hydrogen, and oxygen are almost always towards the surface. That makes ...
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1answer
97 views

Vacuum to hold an object

How can i check and calculate if a certain vacuum pump can hold an object that weights X kg? By "hold" I mean not let that object fall to the floor due to gravity
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1answer
97 views

How are fictitious forces related to my feeling?

This question arises when I am studying fictitious forces in an undergrad introduction to physics course. Suppose I am standing in an elevator with an acceleration $a$ directed upward. From the ...
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1answer
225 views

Does heat always rise?

Does heat always rise in a gravitational field? I recently read that heat could be traveling down to the deep part of the ocean. Is there some new or old physics that makes heat sink?
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1k views

If starting speed is faster than terminal velocity then what?

If an object is say thrown down (vertically) at an initial speed that is faster than its terminal velocity, what would happen to that objects speed? Would it slow down?
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1answer
378 views

Can masses move in 2+1 gravity?

I would like to understand basic concepts of the general relativity in 2+1 spacetime. As far as I know, GR predicts that such a spacetime is flat everywhere except for the point masses which create ...