The Newtonian model of gravity in which the force between two objects is given by GMm/r^2.

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2answers
60 views

Are parts of objects part of that object's mass?

Disclosure: I'm not a physicist or a scientist, so there is a chance that this will seem like a silly question for some of you. But I'm really curious about this and I want to know the answer. ...
38
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8answers
6k views

“Falling upward” - how far you have to be from Earth to start falling to the Moon?

Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus ...
3
votes
1answer
74 views

Limitations of 2D point-mass Dynamics of the solar system

To model the solar system, I took the planets to be point masses, used newtonian mechanics and modelled the orbits as circular (only Mercury's orbit has appreciable eccentricity). The entire system is ...
3
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2answers
63 views

Would there be a “freefall zone” in the centre of a centrifugal artificial gravity?

In the novel Schismatrix, Bruce Sterling writes about people living on the inside of rotating cylinders (similar to Larry Niven's Ringworld and Iain M. Banks' Orbitals, but the atmosphere extends ...
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2answers
110 views

Orbital mechanics: will a satellite crash?

For a satellite given an initial speed and position above a planet, I'd like to know whether it will experience an elliptical, circular, parabolic or hyperbolic trajectory, or crash into the planet. ...
0
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2answers
67 views

Can we decrease the value of $g$ (gravitational pull) without gaining height? [closed]

The value of $g$ on earth surface is $9.8\ \text{m/s}^2$ and when we gain height, then the value of $g$ decreases. Pilots experience same phenomena in air, but can we decrease the value of $g$ by ...
-1
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1answer
63 views

Gravitational Potential Energy misunderstandings [closed]

I'm really confused by the derivation of Gravitational Potiential Energy. I took a screenshot of the wikipedia entry below. My major confusion lies in: 1. The use of the position vector r in the ...
0
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3answers
70 views

How is Earth orbit possible? [duplicate]

What force other than gravity acts on objects in Earth orbit? Without such force, continued orbit would be impossible, as the object would be drawn to the Earth.
0
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1answer
52 views

Gravitational potential of planets

How do you work out the gravitational potential of planets in the solar system? Do orbiting planets have gpe? And what would it be relative to? The equation I know for it is mgh. But what would the ...
1
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0answers
29 views

Derivation Poisson's gravity equation by divergence theorem

I'm trying deduce the poisson's equation $\nabla^2\Phi (x)=-4\pi G\sigma(x)$ by divergence theorem Let $D:x^2+y^2+z^2\leq 1$ and $\sigma:D\to \mathbb{R}$ be the mass density function of $D$ (suppose ...
2
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1answer
60 views

The source of work by gravity force [duplicate]

I learn that any work needs a source of energy, the work of gravity force (like keeping the Moon in the orbit by the Earth gravity force) comes from where? Can we find any mass reduction (transform ...
0
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1answer
87 views

Why do the satellites revolves on a circular path around a planet at orbital velocity? [duplicate]

We know that there is a particular velocity with which a satellite will move in a circular orbit ( orbital velocity ) but if its velocity is more than the orbital velocity but less than the escape ...
1
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0answers
40 views

Free Falling Bodies [closed]

An object is thrown vertically upward. It has a speed of 10ft/s when it reached one-half its maximum height. since this is a thrown object it's initial velocity is a mystery, is it right to make ...
1
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0answers
22 views

The amount of force needed to bend and break a magnesium cylinder? [closed]

I want to determine how much force in Newtons will be need to bend a solid cylinder of magnesium. The cylinder is (x) cm in diameter, and it is (l) cm long. The density of normal magnesium alloy is ...
0
votes
0answers
21 views

Can you do an n-body simulation in terms of energy and momentum?

An N-body simulation typically works directly in terms of the gravitational forces and accelerations. If you can solve the equations exactly, this is fine, but there are many instances where this is ...
0
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1answer
32 views

Falling objects with different masses

I know that free falling objects with different masses fall at the same rate but that does not explain why objects with big masses are heavier to lift? what is gravity anyway I know it isn't a force ...
4
votes
1answer
657 views

If a bullet is fired vertically upwards, when it comes back does it fall to the same spot? [duplicate]

What I'm basically asking is that if a body is projected with sufficiently high velocity so that it doesn't escape from the earth's gravitational field but reaches an appreciable height with respect ...
0
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2answers
617 views

How to determine velocity vector direction with respect to acceleration.

I'm currently writing a program that attempts to simulate particle movement in a gravitational field with more than one object exerting a force on it. I decided that I'd have the particle move by ...
0
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1answer
56 views

Why does Goldstein's derivation of orbits from Newton's law not yield the orbit consisting of symmetric oscillation along a line thru center of Sun?

Why does Goldstein's derivation of Keplerian orbits from Newton's law of gravity not yield the orbit consisting of symmetric oscillation along a line passing thru the center of the Sun? The fact that ...
1
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2answers
55 views

How is the equation for force by charge or by mass discovered?

It's a newbie question. How is the formula for force between two charges or gravitational force between two particles derived. Is it just intuition or is there any method used to derive these ...
1
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1answer
65 views

Potential energy in gravitation

The gravitational potential energy of a two particle system is: $$ U = -G \, \frac{m_1 \, m_2}{r} $$ where $r$ is the distance between the two particles with mass $m_1$ and $m_2$, and the potential ...
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3answers
100 views

Law of gravitation

How can I prove the Newton's Law of Gravitation wrong? I saw 2-3 videos but could not understand. What are the flaws that Einstein came up with in Newton's Law of Gravitation?
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2answers
70 views

What would happen if the gravitational force was repulsive? [closed]

Since my young age, I have always been somewhat afraid of looking up at the sky for one weird reason. It always made me think about this irrational deep dark fear that I have: What if we stopped ...
0
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1answer
49 views

Lollipop 3d choreography animation

I initially wanted to recreate the little dancing dots android lollipop's boot animation in javascript: For that, I've implemented a 3d $n$-body simulation (with css and js): ...
3
votes
3answers
730 views

Do the planets really orbit the Sun?

This is a duplicate of this question on Space Exploration.SE. So why would I ask it again here? Read below. The question reads We often say that the planets orbit the Sun, which is usually a ...
2
votes
1answer
57 views

How much microgravity is there on ISS due to tidal forces? [closed]

I used the equation for the tidal force: $$ F = \frac{2GMd}{r^3} $$ where $M$ is the mass of the Earth ($5.972 \times 10^{24}$ kg), $d$ is half the length of ISS ($50$ m), $r$ distance from center ...
69
votes
4answers
8k 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 ...
0
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1answer
152 views

What would happen if the Earth were hollow? [duplicate]

This is a question of my exam. What would a person experienced if he were put in any point inside the Earth that is hollow (all its mass is concentrated in the surface). Finding the gravitational ...
0
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2answers
49 views

Question about Cavendish experiment

I got a question. In Cavendish experiment was used lead in both of spheres. Can we repeat that experiment with balls made from stone or anything else. Because lead could be a byproduct of radioactive ...
3
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1answer
58 views

Effective mass of two objects

I read that if we want to perform conservation of energy taking one object as frame of reference, we need to assume that the other object has the effective mass and velocity. And effective mass is ...
3
votes
0answers
58 views

How can escape velocity be independent of the direction of projection of a body?

I read that the escape velocity of a body is independent of the direction of projection. For example, I could throw a ball at $11.2$ km/s velocity horizontally, and it would still leave earth. I am ...
2
votes
3answers
39 views

What is the gravitational pull if one has as much mass above them as below them? [duplicate]

I understand the previous responses to other answering if you were at the center of a planet, would you be weightless? Yes you would due to the forces of gravity pulling at you from all sides. ...
2
votes
2answers
44 views

The way planets rotate and revolve [duplicate]

Why is that the all planets move in the same plane?(correct me if I'm wrong) Why not some of them in anticlockwise direction and others in clockwise direction?
5
votes
1answer
223 views

Do all celestial objects have stationary orbit?

Do all celestial bodies of sufficient mass such big asteroids, moon, planets etc. have orbit such that the period of revolution is same as the sidereal period of that object (such as geostationary ...
1
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1answer
58 views

Angular Momentum and Kepler's Second Law

Let me preface by saying that I get the gist of the conservation of angular momentum, at least qualitatively. To better illustrate my question, I will consider the case of a planet orbiting a star. ...
2
votes
2answers
203 views

Vehicle Driving off Cliff (Thelma and Louise movie)

What is the slowest speed at which a vehicle driven off a cliff can clear the cliff without catching its underside between the wheels? In the usual frictionless vacuum, this would appear to depend on ...
9
votes
2answers
3k views

Will a bullet dropped and a bullet fired from a gun horizontally REALLY hit the ground at the same time when air drag is taken into account?

In a world without air, I understand they definitely would. However, with drag taken into account, I think they wouldn't. Since the drag force varies proportional to the square of speed (ignoring the ...
1
vote
1answer
46 views

Gravitational field within a solid sphere with a hole on it [closed]

I want to calculate the gravitational field at $p'$, we know that the sphere has a uniform density $\rho$. Basically what I tried to do to solve this problem was to calculate the potential exerted by ...
35
votes
6answers
8k views

Which ball touches the ground first?

This is a very well known problem, but I can't find an answer in the specific case I'm looking for. Let's consider two balls : Ball 1 weighs 10 kg Ball 2 weighs 1 kg Balls have identical volumes ...
0
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1answer
52 views

How long would a non-gravitationally assisted flight take to Pluto?

Say you want to fly to Pluto from Earth and orbit it. So you're going to launch it from Earth with such a speed that as it approaches Pluto, it's traveling (roughly) the same linear speed of Pluto. ...
0
votes
2answers
44 views

Remotely finding the mass of a satellite

I have a question similar to one I've asked earlier (Calculating mass of an orbiting body with force and acceleration), but without a specific formula... How can you find the mass of a satellite body ...
2
votes
1answer
80 views

Calculating mass of an orbiting body with force and acceleration

I'm new to physics, and it's a lot to take in- but there is a problem that I really can't seem to wrap my head around- finding the mass of an orbiting body, like an asteroid. I've looked around a lot ...
-1
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2answers
106 views
2
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0answers
53 views

In Newton's gravity, is the inverse square law related to the fact that our Universe has (apparently) 3 dimensions? [duplicate]

I read, long ago, a book, whose title I can't remember but I think the author was Carl Sagan. In the book was saying that gravity follows the inverse square law because of our Universe was 3 ...
1
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0answers
22 views

Rotation of earth and effect on $g$ [duplicate]

How do we calculate the change in the acceleration due to gravity (consider earth as a uniform sphere) with a change in the rotation speed of earth along its axis? How do we build up a mathematical ...
5
votes
1answer
367 views

What is the critical mass of a planet to have an atmosphere like Earth's? [closed]

Small planets/orbits like Moon cannot have atmosphere because of their masses. They don't have enough gravity to hold an atmosphere. Then what is the critical mass that makes enough gravity to keep an ...
-5
votes
6answers
4k views

If there is no gravity, does that mean there's no mass as well? [duplicate]

I have demonstrated that weight only is measured based on the gravitational pull of where you live. For example, the gravity on the surface of Mars is three times smaller than the gravity on the ...
3
votes
3answers
88 views

Why does a teeter-totter (see-saw) tend to incline towards the heavier end?

Why does a teeter-totter (see-saw) tend to incline towards the heavier end? Since objects of different mass tend to fall at the same speed (assuming a vacuum), why do then heavier objects push harder ...
0
votes
0answers
26 views

Is it possible to determine perigee distance from distance recordings from a spacecraft to Earth?

If you are on a spacecraft orbiting earth in an elliptical orbit and were able to take measurements of the distance between the spacecraft and earth as well as the time intervals between those ...
0
votes
1answer
173 views

Why is only angular momentum conserved for a planet and not linear momentum?

Suppose a planet is moving in an elliptical orbit with the Sun at one of its focii. I know that the forces in existence will be gravity which provides the necessary centripetal force. Now my book ...