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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386 views
Force inversely proportional to the squared distance
Newton's law of universal gravitation:
"Newton's law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to ...
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2answers
622 views
Newtonian gravity vs. general relativity: exactly how wrong is Newton?
Is there a simple function I can use to describe the difference between simple Newtonian dynamics and the actual observed motion? Or maybe some ratios for common examples of, say, the motion of stars ...
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2answers
164 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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118 views
Apparent non-aberration of gravity waves
Since GR assumes that gravity waves travel at speed c, we expect we would be able to some day detect an aberration effect similar the that of light. Of course, gravity waves are so tiny in magnitude, ...
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3answers
571 views
Still trying to understand gravitational potential and Poisson's equation?
A week or so back I asked a question about the gravitational potential field
$$\phi=\frac{-Gm}{r}, \qquad r\neq 0, $$
and how to show the Laplacian of $\phi$ equals zero for $r\neq 0$? Eventually, ...
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2answers
192 views
Is this alternate theory of gravity as cause instead of effect plausible?
I came across this video today on YouTube that presents an interesting alternate theory of Gravity and the "missing" matter in the Universe that Dark Matter/Energy theories try to account for.
If I ...
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4answers
759 views
Would you feel weightless if falling or flying up in a vacuum?
Ignoring that you can't survive in a vacuum.
If you were to jump of a ledge in a vacuum (in a vertical tube eg). While falling would you feel weightless? Surely the earths gravity is still pulling ...
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2answers
111 views
Gravity pull on objects accelerating towards the earth's center?
Accelerating away from a mass mitigates gravity's pull on the accelerating object. Would the same be true for an object accelerating towards the center of the mass?
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2answers
327 views
Bowling ball on a rubber sheet analogy - what pulls the ball down [duplicate]
Possible Duplicate:
Does the curvature of spacetime theory assume gravity?
Since I read Cosmos long ago, I see the same analogy about the balls rolling on a rubber sheet used to explain how ...
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2answers
669 views
If the earth would stop spinning, what would happen?
What would happen if the earth would stop spinning? How much heavier would we be? I mean absolutely stop spinning. How much does the centrifugal force affect us?
If you give technical answers (please ...
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2answers
136 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 ...
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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?
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2answers
2k views
Why does a parachute decrease terminal velocity?
Terminal Velocity depends on two things- surface area and speed. These are inversely proportionate.
If both these variables affect terminal velocity, why do parachutes slow you down? Initially you ...
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2answers
731 views
Negative g-force: difference between gravity and centrifugal force?
I was having a discussion with a friend about human tolerances of g-force. He believed that the maximum human tolerance of negative g-force is low, in the order of .5 g. I countered, saying that ...
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2answers
418 views
Does gravity spread instantly? [duplicate]
Possible Duplicate:
The speed of gravity
I am real noob in physics, so sorry if this question is really stupid.
Today in a casual conversation I claimed that if the sun were to instantly ...
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1answer
154 views
Sun-Earth Virtual Gravitons?
How many virtual gravitons do the sun and earth exchange in one year?
What are their wavelengths?
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2answers
1k views
Forces in a Pendulum
Help me visualise the forces found in a pendulum. I know weight can be decomposed into $F_x$ and $F_y$.
Centripetal force, acceleration and velocity are three words that I'm confused. Centripetal ...
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2answers
78 views
Will a helicoper which is hovering inside a closed box move with the box when we move it? [duplicate]
I got this micro helicopter to play with, you know that cheap double bladed ones you can find for $25 or so. Anyway last night I was on bed trying to sleep and I got this idea, what if we put the ...
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2answers
112 views
What mechanisms exist for generating lift on a static object?
What mechanisms exist for generating lift on a static object?
Condition is: Other than propellers
I know that generating lift on a static object in a sense of anti-gravity for e.g. drone is not ...
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1answer
135 views
Do all black holes spin in the same direction?
My question is as stated above, do all black holes spin the same direction?
To my knowledge, the spin in the direction of the spin of the matter that created them. Another similar question was asked ...
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1answer
42 views
Gravitational potential energy with regards to annihilation
Given particles A, B, C and D, where:
A and B have an equivalent mass
C and D have an equivalent mass, both larger than A (or B)
D is the antiparticle of C.
A and B start close to C, but with ...
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1answer
190 views
Understanding the math behind a falling object attached to a parachute
I'm trying to understand the mathematics behind calculating the total speed and force of a $40 kg_f$ object hooked up to a parachute, falling to Earth.
From what I understand, the formula for this is
...
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2answers
126 views
Why is the critical density of the Universe non-zero?
I'm missing something and I wondered if someone could enlighten me. As I understand it, Einstein's equations say, among other things, that matter curves spacetime. I also understand, the Universe has ...
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2answers
258 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 ...
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2answers
287 views
When we throw an object in the presence of gravitational attraction is the mass of the object proportional to the time of fall or not?
I was confused when i saw two object of different masses (A and B)falling from the same height(h) but both of them strikes the ground at same time. Is this possible, that masses of the objects are ...
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4answers
1k views
Gravitational force between two masses
I get it that there will be a gravitational force between objects attracted towards gravity but can there be a gravitational force between two objects resting on horizontal plane? In other words, does ...
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4answers
236 views
A Question on Singularity
I am not aware of GR, but due to curiosity i have a question in my mind. Please let me know if it is inappropriate to ask here.
My question is about singularity. I am under the assumption that ...
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1answer
413 views
Why do we define such a thing as singularity?
According to general relativity, an observer not in the close proximity of a black hole, observing a mass fall into that black hole, will never see that mass cross event horizon(it will gradually fade ...
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1answer
67 views
Do all forces act in the same way where gravity is close to zero?
Suppose that I put in the outer space (where gravity from other bodies is negligible) a large, perfectly round sphere totally filled with water. At the bottom (even though "bottom" doesn't make much ...
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1answer
108 views
Would an object hit the ground faster when it gets launched vertically upwards or downwards?
A while ago I had a discussion with someone about the speed of an object when it hits the ground. When does an object hit the ground faster:
When you launch the object to the sky so it can fall ...
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1answer
82 views
Nothing escapes BHs, gravitons mediate gravity, so why do BHs gravitate? [duplicate]
Possible Duplicate:
How does gravity escape a black hole?
Nothing escapes black holes, gravitons mediate gravity, so why do black holes gravitate?
My question is, "where is the hole (no ...
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1answer
69 views
A Decrease In Momentum?
The specifics of a question I am working on are, "After a 0.280-kg rubber ball is dropped from a height of 1.80 m, it bounces off a concrete floor and rebounds to a height of 1.45 m."
Why doesn't the ...
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1answer
384 views
Does the Higgs Mechanism contradict Entropic Gravity?
Does the Higgs Mechanism contradict Entropic Gravity?
It seems like it probably does. But then again, one is a microscopic theory and the other is macroscopic. Can they live together in harmony? or ...
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1answer
94 views
Rotating sphere and circular trajectory: minimum speed
I have a sphere (mass = 3 kg), constrained to a fixed length rope, rotating (radius = 5 m) on a vertical plane.
My textbook ask me about the minimum speed in the highest point in order to keep the ...
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1answer
315 views
Atmospheric Escape of Gas Molecules
Most of the Bodies and Objects in space are likely to have an atmosphere. Since the space is empty, the gases in atmosphere should have either dissolved or emptied into space. But, Why doesn't this ...
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3answers
167 views
If a star were to suddenly dissapear, would it still have gravity?
I am wondering whether or not literally anything can travel faster than the speed of light. For example, if gravity from a star immediately ceases to have an effect if it suddenly and magically ...
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1answer
64 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 ...
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3answers
257 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 ...
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1answer
62 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). ...
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2answers
1k views
Jumping in an elevator?
I've always been under the impression that jumping in an elevator wouldn't help at all, especially after reading a snippet of physics where Einstein said that free fall was identical to zero gravity.
...
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1answer
3k views
To calculate pressure at point in a vertical water pipe of given diameter
Is it possible to calculate pressure at a point in a pipe, where a solenoid valve shall be placed, given that
the pipe carries water (utility supplied, regular water)
from an overhead tank (at a ...
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2answers
212 views
How far does physics influence russian roulette?
Prompted by this discussion on the math exchange
My thought was that the added mass of a bullet in an otherwise empty revolver would bias the chamber spin such that the bullet would remain in one of ...
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2answers
215 views
Does gravitational differences affect the distance light travels? (a thought experiment)
My question is: How is light passing through different gravitational densities affected?
The value of "c" is constant in a vacuum. I'm curious about if various time frames have any effect.
This is ...
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1answer
79 views
why do some orbiting objects have a changing position?
I know that the reason why they stay in orbit but why do some move away from the earth(the moon) or come closer and eventually fall?
And why does the moon move away from earth?
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1answer
107 views
Question regarding escape velocity
Haliday-Resnick-Walker state that the escape velocity will cause a projectile to move upward forever, theoretically coming to rest only at infinity.
I was wondering what the forces forcing the body ...
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1answer
181 views
Is relativistic motion equivalent to fluctuating gravitational fields?
The theory of relativity makes very precise predictions about how an object's motion through space-time affects the passage of time for both the object and observers in other frames of reference.
I ...
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1answer
42 views
Calculate magnitude of force acting on some area by falling object
I have simple question: is it possible to calculate magnitude of force acting on some area by falling object?
Let's say I have an object with mass $5\text{ kg}$. I drop that object in height 1 ...
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1answer
59 views
Can the multiverse theory be tested by gravity waves?
I was wondering if the presence of multiverse could be detected by the fact that there is probably matter disturbing space-time that would cross boundaries?
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1answer
50 views
Question about weightlessness
Let us consider +Y as upward and -Y axis as downward. So the acceleration that works these objects is always $-g$.
For the first picture, if the lift is going upward with acceleration $a$, then ...
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2answers
110 views
What is the mass of a photon moving at the speed of light? [duplicate]
What is the mass of a photon moving at the speed of light?
And if it does not have mass, how is it affected by gravity?
Also why does Einstein's general relativity support that a gravitational wave ...
