Questions tagged [newtonian-gravity]

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

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1answer
141 views

Motion described by $a=\frac{k}{x^2}$

Say a particle in one dimension experiences acceleration inversely proportional to the square of displacement. What is its displacement as a function of time? $$ a=\frac{d^2x}{dt^2}=\frac{k}{x^2} \\ \...
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Newtonian geodesic equation in barycentric frame

In a paper I was reading, I came across the Newtonian geodesic deviation equation $${\ddot{\eta}}^a + K{^a}{_b}{\eta^b}=0$$ Where $\eta{^a}$ are the components of the 3 -vector separating the two ...
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1answer
102 views

Forces in a body falling with air resistance

I would like to know why when a body falls with air resistance, we have that the drag caused by the air is $-kv$ and the acceleration is $-mg$. What I don't understand is how they both have the same ...
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5answers
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Ambiguity in applying Newton's shell theorem in an infinite homogeneous universe

Newton's shell theorem has two corollaries: The gravitational attraction of a spherically symmetric body acts as if all its mass were concentrated at the center, and The gravitational acceleration ...
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2answers
126 views

Gravity as a force [on hold]

I want to know if my reasoning is correct. If you drop an object at a height, the object accelerates downward at a constant rate due to gravity. Because we defined force as the ability to accelerate ...
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3answers
4k views

What is the shape of the upper boundary of water hitting a screen?

When I saw that thing I did not understand how that shape is formed? To be ideal, take a vertical smooth plane. Now aim at the wall with a thin water tube. Then the outer layer forms a "parabolic ...
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Newtonian gravity for cylinder [duplicate]

Given a cylinder of uniform mass density $ \rho $ . It has a unit length and radius. For any point situated at a distance $ r $ radially away from the central axis is the gravitational field strength ...
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1answer
2k views

Velocity of satellites greater than required velocity

We often find velocity required to keep a satellite in orbit by the formula $v=\sqrt{\frac{GM}{r}}$ where $v$ is perpendicular to the gravitational force. It is very intuitive that the object will go ...
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2answers
133 views

Assumptions in calculating potential energy of a system

In calculating the gravitational potential energy of a system of two masses $m, M$, it is often assumed that $M\gg m$ in order to neglect motion of the larger mass $M$. As a result, the potential ...
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2answers
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Shouldn't ball has to move in the circle even after cutting the rope by which it was attached?

Since the tension force is a central acting force, the torque on an orbiting ball about that center is zero. But if the rope is cut down during motion the torque would still remain zero. This would ...
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1answer
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How much energy would it take to blow up the Earth?

There is a common statement running around that we as a species has enough nuclear weapons to blow up the earth several times over. What I want to know is: by how many orders of magnitude is that a ...
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2answers
317 views

Why doesn't the vacuum of space rip the Earth, or other celestial bodies, like the Sun, apart?

Since vacuums are a lower pressure than the atmosphere of the earth(and probably many or all other planets) then why doesn't the vacuum of space rip the earth apart or suck all the gasses on it away ...
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32 views

Gravitational two-body problem: Same period

Specifically in circular orbits of two stars around the center of mass: How can I show that the periods are equal?
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1answer
40 views

What is the period of the binary orbit? [closed]

Problem There are 2 masses, one with 90% the mass of earth and one with 50% the mass of earth. Let's call the bigger one m1 and the smaller one m2. m1 has a radius of 95% that of earth and m2 has a ...
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0answers
42 views

Do objects actually fall at the same rate? [duplicate]

The force of gravity takes the mass of both objects into account. For the sake of argument we are doing the experiment on the moon. The moon + the hammer have a greater combined mass than the moon ...
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1answer
58 views

Gravitational force experienced by a moving object (classical physics)

Is the force of gravity experienced by an object, higher, if its moving (Eg: in a perpendicular/tangential direction to the center of gravity), as compared to when it is at rest? My understanding is ...
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1answer
49 views

What is the gravity on a “partial” ringworld?

This was inspired by https://worldbuilding.stackexchange.com/questions/149706/life-on-the-broken-ring-an-issue-of-size. Let's say I have a part of a Ringworld (see link for specifications). ...
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1answer
45 views

Experiment for Constant Acceleration

I am conducting an experiment to find how long the ball free falls at a certain height. I would drop a ball at a certain height, and I would predict (using math) the time the ball would take to hit ...
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3answers
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Gravity formula explanation needed

Why, in finding gravity, are the masses of two objects multiplied? I don't understand the value of mass over gravity and why the masses are multiplied and then divided by radius^2. In one sentence I ...
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2answers
170 views

How do you derive a quantum field theory from a spacetime metric?

What are the first steps in converting a metric into a quantum field theory? I know roughly what to do once I have a pair of non-commuting operators, but how do I get to that point? Specifically, I'd ...
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62 views

Which orbit planet will take with 2 stars?

Is there a law or an equation that shows which star gravity will win and suck a a planet that has 2 stars Assume planet P has 2 stars A & B A is smaller than B Planet P is closer to A more than ...
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Might a planet perform figure-8 orbits around two stars?

Might a planet perform figure-8 orbits around two stars? I'm thinking that if the two stars were equal mass (and not orbiting each other) then a planet that were to go right between them would ...
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5answers
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Does mass affect speed of orbit at a certain distance?

Does the mass of both the parent object, and the child object affect the speed at which the child object orbits the parent object? I thought it didn't (something like $T^2 \approx R^3$) until I saw ...
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How much of a gravitational change would be noticeable? [closed]

Assuming we could turn the gravitational constant in a gym up or down on a dial, what percent would the change need to be before it became noticeable?
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2answers
55 views

What length can a metal wire (cable) hang vertically?

What length can a cable hang vertically with no additional weight attached to it. I tried figuring this out with 5/8 aircraft cable specs by dividing the rated weight capacity of the cable by the ...
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44 views

Escape Velocity and its effect on the atmosphere

I’ve been doing some world building and wound up with a bit of a physics puzzle that’s stumped me. So rather than a long winded explanation I’ll simply state the primary parameters. Under otherwise “...
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29 views

Angular velocity of a ring [closed]

There is a ring of mass $M$ formed by a very large number $n$ of particles of mass $M/n$. I would like to find the orbital velocity of the ring in function of $M,R$. I considered a mass $dm$ and ...
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1answer
32 views

How to calculate the water pressure generated from a waterfall? [closed]

The Latourell waterfall in Portland, Or. falls 224' with an average flow rate of 50.0 cu ft/sec. How do you calculate the water pressure generated where the water meets the ground below? How does ...
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2answers
137 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. ...
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15 views

Effective gravitational force curve within an equilibrated self-gravitating cloud?

Suppose we have a uniform-density spherical cloud of gravitating dust (i.e., zero-pressure matter with negligible non-gravitational interactions) at non-zero temperature. Within such a sphere, the ...
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1answer
27 views

Finding suitable element to perform integration upon [closed]

Is there any precise (proper) method or technique to specify the element on which integration will be performed. Is it the same for all properties like moment of inertia, gravitational potential, ...
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2answers
78 views

How to calculate gravity inside of a circle of mass?

I've been reading about the Shell Theorem and how the gravitational force inside of a sphere is equal to zero. I was wondering if the same was true for a circle, but couldn't find any definitive ...
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0answers
51 views

Most precise way to measure $g$ at home?

I've tried measuring acceleration due to gravity, using a photogate with free falling picket fence. I've also tried photogate with picket fence on a low-friction cart and inclined plane. So far, ...
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1answer
43 views

Tension in string b/w blocks on wedge [closed]

In this Question: Why is A option correct? The option A being correct why is it assumed that the blocks are in rest. They can be accelerated as well. Also in this case component of the 4kg block ...
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3answers
78 views

Why bodies with different masses reach gound at the same time in vacuum? [duplicate]

According to Newton's Gravitation Equation $$F=G \dfrac{M\cdot m}{r^2}$$ force of gravity depends upon mass of both objects, and $$F=ma$$ means force is proportional to acceleration. Then why do ...
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1answer
32 views

How would classical physics explain the sorting of objects according to density in a gravitational field?

I'm wondering how classical physics (i.e., using Newtonian Gravity) would explain why it is that objects are sorted by density in the presence of a gravitational field?
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2answers
71 views

Could the moon move balls on perfectly flat and level surface?

I've been looking into the concepts of highly flat surfaces and highly round objects, and I'm wondering if an experiment like this would work / has been done: Take a perfectly flat surface Level it ...
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0answers
12 views

$Gm/r$ in momentum equation of compressible flow for cylindrical geometry

In the case of compressible flow, due to the gravitational field, the source term added in the momentum equation (which is gravitational potential) is $Gm/r$ in the case of cylindrical geometry and $...
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26 views

Negative masses [duplicate]

If there existed negative masses how would one use Newton's law of gravitation (since it doesn't spit out the direction of the gravitational force but only the magnitude like Coulomb's law)? Would ...
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3answers
78 views

Is a set of objects starting with zero velocity gravitationally bounded?

Given a random set of objects in 3D space each with zero velocity and each with mass $M$. Now let them move under gravity. They will collapse in on each other. But will they then fly appart? Or is ...
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1answer
40 views

Newtonian gravity for cylindrical geometry

What is Newtonian gravity for and thus gravitational potential for cylindrical geometry? For spherical geometry the gravitational potential is Gm/r^2. But what is it for cylindrical geometry?
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3answers
68 views

Why are the surfaces of liquids always perpendicular to the gravitational force?

I am not a physicist nor do I have a good knowledge of the topic. Pardon me if I use terms erroneously. I observed that when I put some water in a bottle, without regards to how the bottle is placed,...
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1answer
33 views

Jumping vs hitting the ground

Whats the difference between jumping and hitting (kicking) the ground, in both situations the normal force has the same direction, so why when we hit the ground the ground wont push us back, ...
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1answer
55 views

Jumping and falling force same? [closed]

When someone tries to jump he/she can only jump for a height of few inch and doesnt break his leg which requires a small force but when someone falls from a small building he/she will have much higher ...
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2answers
224 views

Orbital period and nodal precession & apsidal precession

In classical orbital mechanics as per Newton/Kepler, the time taken to complete one orbit around a body of mass $M_e$ is: $$ T = 2 \pi \cdot \sqrt { \frac {a^3} {G \cdot M_e} } $$ But also, for ...
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1answer
151 views

Could you give boundary conditions to the gravitational potential given the density distribution?

We´re doing a project that's all about solving differential equations with separation of variables. We´re trying to find the gravitational potential given the density distribution (that has azimuthal ...
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2answers
219 views

Would a space elevator theoretically be possible on a tidally-locked planet?

It's my understanding that many of the theorized methods of creating a space elevator rely on the upward centrifugal force of earth's rotation on a counterweight high above to keep the structure taut ...
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5answers
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Does friction always oppose motion?

Recently I had the following misconceptions: Static friction always opposes the motion of body. The force of friction cannot initiate motion in a body. Now I came to know that my understanding was ...
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1answer
101 views

Ellipsoid form of a sphere deformed by gravity of its own weight

So I have a sphere of radius $R$, and density $\rho$, elastic modulus $E$ and Poisson ratio $\nu$. With no gravity it is a sphere, but when gravity comes with a direction $(Oz)$ it becomes an ...
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3answers
75 views

How can the first Maxwell equation be valid in non-static cases?

I am thinking in the framework of Classical Gravity, where the speed of the interaction is infinite. Now it is also known that there is a correspondence between Classical gravity and electrostatics, ...