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.

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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 ...
ErikE's user avatar
  • 2,926
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?...
Mia Clarke's user avatar
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?
Adir Peretz's user avatar
  • 1,023
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 ...
Mark Eichenlaub's user avatar
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?
freeside's user avatar
  • 543
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 ...
Kivanc Uyanik's user avatar
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, ...
merwaaan's user avatar
  • 409
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 ...
MGZero's user avatar
  • 503
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 ...
HappyDeveloper's user avatar
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 ...
Ryan David Ward's user avatar
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 ...
RoboShop's user avatar
  • 387
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....
Gummy bears's user avatar
  • 1,572
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)=\...
Beyond-formulas's user avatar
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 - ...
Benjohn's user avatar
  • 3,090
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 ...
a06e's user avatar
  • 3,702
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 ...
Tejas Ramdas's user avatar
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 ...
SN77's user avatar
  • 559
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, ...
user151841's user avatar
  • 1,619
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 ...
aravind ramesh's user avatar
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 ...
DarkLightA's user avatar
  • 1,422
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 ...
Jose Javier Garcia's user avatar
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 ...
user avatar
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 $$ {d^2r_1\over dt^2} = - { m_2(r_1-r_2)\over |r_1-r_2|^3} $$ ...
MaiaVictor's user avatar
  • 1,952
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 ...
Cory Baumer's user avatar
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{...
Alessandro Jacopson's user avatar
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 ...
Simon S's user avatar
  • 885
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, $r$ ...
user13107's user avatar
  • 657
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?
user avatar
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 ...
Bobbie D's user avatar
  • 343
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 ...
Four Seasons's user avatar
  • 2,547
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 ...
John Feminella's user avatar
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 ...
Lance's user avatar
  • 2,118
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 ...
user1928764's user avatar
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 ...
BlueRaja - Danny Pflughoeft's user avatar
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)?
Heet Kansagra's user avatar
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 ...
BowPark's user avatar
  • 757
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 ...
Julia's user avatar
  • 1,682
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 ...
Ehryk's user avatar
  • 3,241
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 ...
YWE's user avatar
  • 223
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 ...
Gummy bears's user avatar
  • 1,572
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 ...
Kam's user avatar
  • 93
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 ...
Zev Chonoles's user avatar
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" ...
Just_a_fool's user avatar
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 ...
jimjim's user avatar
  • 1,447
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?
Sebastian Godelet's user avatar
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 ...
fibonatic's user avatar
  • 5,876
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.
user6090's user avatar
  • 1,436
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 ...
rwallace's user avatar
  • 281
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 ...
claws's user avatar
  • 7,235
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 ...
Amr's user avatar
  • 562

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