Questions tagged [newtonian-gravity]
This tag is for questions regarding the Newtonian model of gravity in which the force between two objects is given by $~GMm/r^2~.$ It is a natural phenomenon by which all things with mass or energy – including planets, stars, galaxies, and even light – attract one another. On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the ocean tides.
839
questions
268
votes
11
answers
66k
views
Don't heavier objects actually fall faster because they exert their own gravity?
The common understanding is that, setting air resistance aside, all objects dropped to Earth fall at the same rate. This is often demonstrated through the thought experiment of cutting a large object ...
59
votes
4
answers
141k
views
How does gravity work underground?
Would the effect of gravity on me change if I were to dig a very deep hole and stand in it? If so, how would it change? Am I more likely to be pulled downwards, or pulled towards the edges of the hole?...
88
votes
9
answers
219k
views
Why doesn't the Moon fall onto the Earth?
Why doesn't the Moon fall onto the Earth? For that matter, why doesn't anything rotating a larger body ever fall onto the larger body?
82
votes
7
answers
11k
views
Why is the Earth so fat?
I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy?
The ...
39
votes
5
answers
34k
views
Would you be weightless at the center of the Earth?
If you could travel to the center of the Earth (or any planet), would you be weightless there?
118
votes
10
answers
16k
views
How can I stand on the ground? EM or/and Pauli?
There is this famous example about the order difference between gravitational force and EM force. All the gravitational force of Earth is just countered by the electromagnetic force between the ...
29
votes
7
answers
239k
views
Why do two bodies of different masses fall at the same rate (in the absence of air resistance)?
I'm far from being a physics expert and figured this would be a good place to ask a beginner question that has been confusing me for some time.
According to Galileo, two bodies of different masses, ...
28
votes
5
answers
40k
views
The Time That 2 Masses Will Collide Due To Newtonian Gravity [closed]
My friend and I have been wracking our heads with this one for the past 3 hours...
We have 2 point masses, $m$ and $M$ in a perfect world separated by radius r. Starting from rest, they both begin to ...
64
votes
15
answers
21k
views
Why do we say that the Earth moves around the Sun?
In history we are taught that the Catholic Church was wrong, because the Sun does not move around the Earth, instead the Earth moves around the Sun.
But then in physics we learn that movement is ...
43
votes
6
answers
8k
views
Why are so many forces explainable using inverse squares when space is three dimensional?
It seems paradoxical that the strength of so many phenomena (Newtonian gravity, Coulomb force) are calculable by the inverse square of distance.
However, since volume is determined by three ...
19
votes
3
answers
4k
views
If it was possible to dig a hole that went from one side of the Earth to the other...
...And you jumped in.
What would happen when you got to the middle of the Earth? Would you gradually slow down, until you got to the middle and once you were in middle would every direction feel like ...
18
votes
2
answers
10k
views
Gravitational field intensity inside a hollow sphere
It is quite easy to derive the gravitational field intensity at a point within a hollow sphere. However, the result is quite surprising. The field intensity at any point within a hollow sphere is zero....
33
votes
4
answers
7k
views
How can we recover the Newtonian gravitational potential from the metric of general relativity?
The Newtonian description of gravity can be formulated in terms of a potential function $\phi$ whose partial derivatives give the acceleration:
$$\frac{d^2\vec{x}}{dt^2}=\vec{g}=-\vec{\nabla}\phi(x)=\...
247
votes
8
answers
43k
views
Does Earth really have two high-tide bulges on opposite sides?
The bit that makes sense – tidal forces
My physics teacher explained that most tidal effect is caused by the Moon rotating around the Earth, and some also by the Sun.
They said that in the Earth - ...
18
votes
5
answers
6k
views
Why do all the planets of the solar system orbit in roughly the same 2D plane?
Most images you see of the solar system are 2D and all planets orbit in the same plane. In a 3D view, are really all planets orbiting in similar planes? Is there a reason for this? I'd expect that the ...
20
votes
5
answers
135k
views
Why is Earth's gravity stronger at the poles?
Many sources state that the Earth's gravity is stronger at the poles than the equator for two reasons:
The centrifugal "force" cancels out the gravitational force minimally, more so at the equator ...
9
votes
3
answers
1k
views
Negative Mass and gravitation
Since Newtonian gravity is analogous to electrostatics shouldn't there be something called negative mass? Also, a moving charge generates electric field, but why doesn't a moving mass generate some ...
92
votes
7
answers
13k
views
If dark matter only interacts with gravity, why doesn't it all clump together in a single point?
I'm a complete layperson. As I understand, dark matter theoretically only interacts with the gravitational force, and doesn't interact with the other three fundamental forces: weak nuclear force, ...
39
votes
6
answers
91k
views
Why don't planets have Circular orbits?
This might be a completely wrong question, but this is bothering me since many days ago. Given the mass (Sun) curves the space around it, gravitation is the result of such curved space (Correct me if ...
34
votes
4
answers
138k
views
What is the difference between center of mass and center of gravity?
What is the difference between center of mass and center of gravity?
These terms seem to be used interchangeably.
Is there a difference between them for non-moving object on Earth, or moving objects ...
18
votes
2
answers
1k
views
Is it possible to prove that planets should be approximately spherical using the calculus of variations?
Is it possible to use the Lagrangian formalism involving physical terms to answer the question of why all planets are approximately spherical?
Let's assume that a planet is 'born' when lots of ...
30
votes
4
answers
10k
views
What's the difference between centre of mass & centre of gravity for massive bodies?
My book says:
For most of the small objects, both are same. But for mammoth ones, they are really different ones. And in a gravity-less environment, COG is absent; COM still exists.
Ok, what's the ...
10
votes
1
answer
3k
views
Kepler problem in time: how do two gravitationally attracted particles move? [duplicate]
Two particles with initial positions and velocities $r_1,v_1$ and $r_2,v_2$ are interacting by the inverse square law (with G=1), so that
$$ {d^2r_1\over dt^2} = - { m_2(r_1-r_2)\over |r_1-r_2|^3} $$
...
84
votes
4
answers
15k
views
Where does the extra kinetic energy come from in a gravitational slingshot?
I read in this answer in this site that the KE a free-falling ball acquires is not originated by the attracting body but that energy was actually stored in the ball when it had been lifted to the ...
22
votes
7
answers
5k
views
The distance square in the Newton's law of universal gravitation is really a square?
When I was in the university (in the late 90s, circa 1995) I was told there had been research investigating the $2$ (the square of distance) in the Newton's law of universal gravitation.
$$F=G\frac{...
7
votes
1
answer
3k
views
Radial fall in a Newtonian gravitational field [duplicate]
Suppose an object of mass $m$ starts at rest at a radial distance $ r_0$ from a perfectly spherical mass $M$ (where $m << M$), $r_0 > R =$ radius of $M$.
Can we analytically determine when ...
5
votes
1
answer
7k
views
Is acceleration due to gravity constant?
I was taught in school that acceleration due to gravity is constant. But recently, when I checked Physics textbook, I noted that
$$F = \dfrac{G m_1 m_2}{r^2}. $$
So, as the body falls down, $r$ ...
25
votes
8
answers
29k
views
Would it help if you jump inside a free falling elevator?
Imagine you're trapped inside a free falling elevator. Would you decrease your impact impulse by jumping during the fall? When?
15
votes
5
answers
3k
views
Near Earth vs Newtonian gravitational potential
Newton's Law of Universal Gravitation tells us that the potential energy of object in a gravitational field is $$U ~=~ -\frac{GMm}{r}.\tag{1}$$
The experimentally verified near-Earth gravitational ...
2
votes
1
answer
3k
views
Weightlessness for astronauts
Well, this question has been puzzling me for kinda long time, many people believe that orbiting astronauts feel weightless because they are "beyond the pull of Earth's gravity"...How far from the ...
57
votes
9
answers
21k
views
Are we slightly lighter during the day and slightly heavier at night, owing to the force of the Sun's gravity?
Using $g = \frac{Gm}{r^2}$, the force on a point mass located at 1 AU from the Sun ($m = 2 \cdot 10^{30} \text{ kg}$) is about ~0.006 N/kg.
Does that mean that, e.g., a 70 kg person is ~42g lighter ...
46
votes
4
answers
11k
views
Has gravity ever been experimentally measured between two atoms?
Has there been any experiments, or are there any references, demonstrating gravity between atoms? If so, what are the key experiments/papers? Or if not, what is the smallest thing that has actually ...
43
votes
10
answers
9k
views
How does a spinning object "know" that it is spinning?
I am constructing a thought experiment about a spinning object that is floating in intergalactic space. I assume that this object is about the size of a planet so that it will have enough gravity so ...
40
votes
7
answers
87k
views
Why is gravitational potential energy negative, and what does that mean?
I usually think of gravitational potential energy as representing just what it sounds like: the energy that we could potentially gain, using gravity. However, the equation for it (derived by ...
18
votes
3
answers
6k
views
Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?
Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?
17
votes
5
answers
15k
views
Derivation of Kepler's laws
How can analytically be derived the Kepler's laws?
I found some extremely synthetic equations which from the Newton's laws (in particular $\mathbf{F} = m \mathbf{a}$) tried to obtain the Kepler's ...
12
votes
5
answers
3k
views
Intuitive explanation of the inverse square power $\frac{1}{r^2}$ in Newton's law of gravity
Is there an intuitive explanation why it is plausible that the gravitational force which acts between two point masses is proportional to the inverse square of the distance $r$ between the masses (and ...
53
votes
9
answers
22k
views
Why are $L_4$ and $L_5$ lagrangian points stable?
This diagram from wikipedia shows the gravitational potential energy of the sun-earth two body system, and demonstrates clearly the semi-stability of the $L_1$, $L_2$, and $L_3$ lagrangian points. The ...
12
votes
3
answers
6k
views
How does Newtonian mechanics explain why orbiting objects do not fall to the object they are orbiting?
The force of gravity is constantly being applied to an orbiting object. And therefore the object is constantly accelerating. Why doesn't gravity eventually "win" over the object's momentum, like a ...
10
votes
2
answers
1k
views
Bending of light - photon's inertia instead of mass
Using classical mechanics, the formula for gravitational attraction is
$$F = G\frac{m_1m_2}{r^2}.$$
This formula does not work for photons, and we need to use Einstein's theory of gravity to ...
8
votes
3
answers
6k
views
Integrating radial free fall in Newtonian gravity [duplicate]
I thought this would be a simple question, but I'm having trouble figuring it out. Not a homework assignment btw. I am a physics student and am just genuinely interested in physics problems involving ...
32
votes
3
answers
15k
views
What are the precise statements by Shouryya Ray of particle dynamics problems posed by Newton which this news article claims have been solved?
This recent news article (here is the original, in German) says that
Shouryya Ray, who moved to Germany from India with his family at the age of 12, has baffled scientists and mathematicians by ...
31
votes
8
answers
3k
views
What is potential energy truly?
I have a problematic question for which I have been unable to attain a satisfactory answer. What is potential energy truly?
-
I have read about how potential energy can be seen as the "highering" ...
24
votes
3
answers
6k
views
Significance of the second focus in elliptical orbits
1.In classical mechanics, using Newton's laws, the ellipticity of orbits is derived. It is also said that the center of mass is at one of the foci.
2.Each body will orbit the center of the mass of ...
17
votes
3
answers
11k
views
Why does the moon drift away from earth?
I once saw on TV that the moon is slowly drifting away from the earth, something like an inch a year. In relation to that the day on earth what also increase in time.
I wonder why is that?
16
votes
1
answer
2k
views
What are common methods for calculating the time dependency of elliptical orbit?
After playing a game called "Kerbal Space Program" I got interested in orbital mechanics and started messing with simplified calculations to determine $\Delta v$ requirements. In which I compared two ...
12
votes
4
answers
6k
views
Can gravity be shielded, like electromagnetism? [duplicate]
If I remember well, they said that it can't, but I do not know why.
Yes, I meant if gravity can be shielded using something like a Faraday cage
(or something else?).
Thank you.
9
votes
2
answers
2k
views
Gravity in 2+1D spacetime and inverse linear law
In our 3+1D universe, gravity obeys the inverse square law. In a 4+1D universe, gravity would be expected to obey the inverse cube law et cetera.
In a 2+1D universe, one would similarly expect gravity ...
7
votes
2
answers
2k
views
Why don't we consider centrifugal force on a mass placed on earth?
Let us say a block of mass is placed on the surface of earth. Then while drawing the forces on that body, we say:
Force $F = mg$ acting towards the center of Earth.
Normal reaction $N$ offered by the ...
6
votes
3
answers
5k
views
How was Newton able to guess that gravitational force is inversely proportional to distance squared?
This question is puzzling me since I learnt about the gravitation law in school. Why did Newton guess/assume that gravitational force is inversely proportional to the square of distance?
Did he ...