# 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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### Is gravitational potential energy of body by $mgh$ negative?

Consider a 15kg object at 1m from earth ground level, is P.E = 15kg * 9.8m/s^2 * 1m = 147J or P.E = -Gm1m2/r^2 * h = -9.8 * 15kg * 1m= -147J after browsing for a while on debate of potential energy's ...
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### How accurate does the ISS's velocity and altitude need to be to maintain orbit?

We know the the ISS is not drifting weightless in space, but rather is constantly falling as it circles the Earth. To do this it must be at a specific altitude and moving with a specific velocity. ...
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### What would happen to the moon's orbit if we reduce (instantaneously) its mass? [closed]

In my opinion the moon's orbit shouldn't change, because an orbiting body is a free falling object and the trajectory of a free falling object is not affected by its mass (because the inertial mass ...
1 vote
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### Doubt on conservation of angular momentum for Kepler's laws

Just before proving Kepler's laws, my Professor claimed that if $\vec{F}$ is a central force with center $O$ and it is the only force acting on a point $P$, then the trajectory of $P$ is a curve plane....
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### When an object is thrown towards the sky it starts to gain potential energy, why?

I didn't think so because when an object is thrown towards the sky it already has kinetic energy and it looses it's energy due to the gravitational force of the earth. When the object looses all of it'...
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### Has our knowledge of astrophysics and gravity reached the point where we can accurately calculate Lagrange points?

is it possible for us today given the knowledge we possess of gravity and our success with inserting satellites in to steady/ geosynchronous orbit and any knowledge we have on the relative size (and ...
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### Local vs distant gravity effects where is the boundary line of effective control? [closed]

I'm not one of the 'gravity is only a theory' crowd or a flat earther that thinks gravity is fake and that down is always down. but thinking about gravity and what has been put forward by many great ...
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### Error concerning projectile motion in respected textbook?

In the textbook Fundamentals of Physics by R. Shankar of Yale Open Courses, appears the following assertion pertaining to a car driving off a cliff, which seems correct: This is exactly how long it ...
1 vote
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### How much time does it take for an object to fall from space? [closed]

Let's say there's an object of mass $m$ in space, $h$ meters away from the surface of the Earth. $h$ is large enough that $g$ cannot be assumed to be constant. The acceleration varies according to ...
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### How Can there be a Gravitational Potential when there is NO Gravitational Field? [closed]

How does it make any logic that there exist a potential when there is no net field for example when we have a Hollow Sphere with mass we can find out the the gravitational *potential inside the sphere ...
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### Do objects really "fall" at the same rate? [duplicate]

I understand that a hammer and a feather were dropped on the moon and they both landed at the same time. I understand that for all practical intents and purposes all objects do fall at the same rate. ...
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### Gravitational collapse - proof that energy dissipation is required?

As an undergraduate, I took a short course on astrophysics, where I encountered the Jeans mass. This is the critical mass for a spherical cloud of interstellar gas above which the cloud is predicted ...
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### Why is pressure in the outermost layer of a star lower than at its center?

I have done the math and I have obtained the hydrostatic pressure in a star is lower at the outermost layer of a star than in its center, where the pressure is actually maximum. Although the equations ...
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1 vote
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### Why is work done by force $+mgh$ in the situation of throwing something up?

If there is a particle at point A(at rest) and a force moves it to point B(Above point A vertically)(final velocity = 0 at this point), the work done by gravity is $-mgh$. This I understand as the ...
1 vote
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### Why does an accelerometer read geavity when at rest? [duplicate]

This may sound like a stupid question but why does an accelerometer read 9.8 (gravity) on the vertical axis when I set it down on a table? I have just finished AP Physics Mechanics and was under the ...
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### What is the relationship between gravitation, centripetal and centrifugal force on the Earth?

I'm trying to analyze a situation wherein a ship is moving across the surface of the earth. I am trying to analyze this situation in a reference frame that is rotating with the earth (NED frame). I am ...
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### Rocket equation non-constant gravity

I am currently trying to teach myself differential equations and wondered about the following problem. You have an object (a rocket of constant mass $m_r$) and initial velocity $v_0$ that tries to ...
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### Trajectories of projectile based on different speeds of projection [duplicate]

So my teacher was teaching gravitation and an interesting fact that he mentioned was the trajectory of a projectile projected from Earth at a speed equal to escape velocity, is parabolic. Also, he ...
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### Where am I wrong in proving that force perpendicular to motion increases speed and kinetic energy? [closed]

Lets think of a helicopter flying at some height $h$. It throws a food packet to a person on the ground from air. Let's neglect the air drag. The food packet is thrown only with some velocity $v_x$ in ...
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### Integrating acceleration + escape velocity over distance [closed]

I am not sure how to title this question so apologies if it's inaccurate. If I throw an object at thrice the escape velocity of earth, what would be its velocity very far away from earth, (at a ...
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### Pseudo Orbital motion only due to Coriolis force

Planet, say of mass M and radius R is rotating with some angular velocity ω and a object of mass m (initially on the surface and rotating with the planet) was launched with velocity v vertically ...
1 vote
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### The topology of planets [duplicate]

Just a curiosity: Let $g \in \mathbb{Z}_{>0}$. Is it possible for a planet of topological genus $g$ to exist? For example, is there any contradiction (from the point of view of physics) in assuming ...
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### Better equations for modeling and simulating a halo orbit?

I'm trying to model a halo orbit at low altitude (10m from surface). The satellite is using propulsion to trace the circular halo path. It looks like this "from the top" (the blue ball is ...
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### Restricted 3-body: one large mass and two smaller ones

A restricted 3-body problem is usually understood as two large bodies and one much smaller one that doesn't affect the motion of the other two. I am curious about a 3-body problem with one large body, ...
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### When two objects collide, which directions do they go in after the collision?

I am watching this video here: https://www.youtube.com/watch?v=KPhMMnIocR0&list=PL4VRaRxeNj-1F-JukcRVAzbiIi0ibrHqy. At 11:02, the author shows a simulation and says that for a Galton Board (https:...
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1 vote
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### Drawing a free-body diagram for a body residing on Lagrange point $L_4$ and $L_5$

I am aware of the fact that Lagrange points $L_4$ and $L_5$ are stable equilibrium points however I cannot understand why they are equilibrium points in the first place. Consider a Earth-Moon system ...
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### When is it appropriate to say Newtonian gravity is a force? When is it not appropiate? [closed]

Please help me understand the notion of force when it is applied to Newtonian gravity. From my understanding forces in physics involve interactions with at least 2 objects and can cause an ...
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### Confusions on The Gravitational Energy of a Point P in a Cube

I have been working, quite tirelessly, to try and find an answer to a question that has been bothering me for some time now. I have been working over some proofs, in the Newtonian Mechanics world, to ...
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### What changes the velocity perpendicular to radius in an elliptical orbit?

I'm working currently on a problem that asks to justify that angular momentum and kinetic energy conserves for a planet in an elliptical orbit. Although I've been taught that angular momentum should ...
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### Can the $r^2$ in gravity be seen as signifying the distance of object 1 to 2 times distance from object 2 to 1 [closed]

If the formula for gravity multiplies the mass in the numerator and the distance in the denominator is this akin to saying the multiplied masses over the multiplied distances times the gravitational ...
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### Is there a deeper relationship between symmetry and gravitational potential comparing Newton's and Einstein's gravity?

In this question, see Why is general relativity in (2+1) dimensions different from cylindrical systems in (3+1) dimensional GR?, it is mentioned "The gravitational potential Φ of an infinite rod ...
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