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

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Thought Experiment: An object falling indefinitely under gravity?

So the other day, I devised this thought experiment: Consider an infinite tunnel, and you drop a coin of mass $m$ into it. Considering the effect of gravity to be applicable and neglecting air ...
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2answers
35 views

Gravitational Force units

In the formula $F=mg$, does the mass $m$ needs to be in $kg$? And what is exact value of $g$? I've tried with both and didn't worked.
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1answer
69 views

Why do planets not stop revolving around the Sun? [duplicate]

Why do planets revolving around the Sun not stop revolving? Note I am not asking why planets do not collapse with Sun.
3
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2answers
73 views

What would be the photon's effective mass in Newton's Law of Gravitation?

If we equalize the force from the Newton's Law of Gravitation to Force on a photon in a gravitional field (I don't know if there is an equation for it). What would be the photon's effective mass? (I ...
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5answers
146 views

If you are on Earth, are your mass and weight the same?

I was wondering how mass and weight are different so I Googled it. I found that mass is constant no matter where you are because it is the amount of matter in an object and weight changes because it's ...
14
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6answers
4k views

Throwing an object around the Earth. Is it possible?

Some of you might be familiar with the cartoon called Astérix. In one of the episodes, Obelix (the really strong fat guy in the stylish striped pants) enters a spear throwing competition. He beats the ...
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2answers
52 views

Traveling to Mars in 24 Days

I am trying to calculate an electromagnetic rail launch from the Moon but I need to know if humans can survive the launch. Please see the specs below. 150 miles of track Proposed final launch speed ...
12
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2answers
724 views

What is a reasonably accurate but simple model of the Milky Way's gravitational field?

I am putting together a toy program which shows how stars move around in the galaxy. To run the simulation I need to know strength of the Milky Way's gravitational field at any location in it. I'm ...
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0answers
31 views

Why is there a difference when we measure the change of gravitational acceleration? [duplicate]

When we go up from the surface of the earth to measure the change of $g$, we use only height. But when we go down from the surface of the Earth, we use the density equation of the Earth. Why?
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1answer
126 views

How the Moon causes the tides?

I am considering the following question but I con't quite figure it out... I have looked up differential gravity but I cannot derive the equation for the effect on earth, and I haven't found any ...
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2answers
45 views

Linear Gravity Inside Solid Sphere Derivation

I am currently taking a dynamics module at university and am revising for a midterm exam. One of the problems on a past paper was to consider a mass falling through a hole in the Earth (pole to ...
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1answer
98 views

Can we make a “Gravity Mill” based on principle(s) of Newton's cradle?

DISCLAIMER: Newbie/Wannabe Physicist. What's to stop us from making a "Gravity Mill" which is a bar/rod attached with weights on both ends and pivoted at the ...
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1answer
57 views

newton's cradle with 2 balls dropped from same height at same time

I'm creating a logo and want it to be as exact as possible. The logo idea is a "Newtonian cradle", but with only 2 balls and on straight steel wire (can't be bend). What would happen if I dropped ...
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5answers
2k views

Evidence that stationary masses in space actually attract each other

I'm finding it rather difficult to find experimental evidence that two stationary masses in space (unaffected by external massive bodies or gravities) actually attract one another. For moving masses, ...
13
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1answer
204 views

Are “echo” explosions near Earth realistic?

Watch the simulation below for better visualization. Suppose we create an explosion, for example at $2R_{Earth}$ from the center of Earth. Just before the explosion was set off, the explosive was at ...
2
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2answers
68 views

Why does General Relativity predict more light deflection than Netwonian Physics?

If one looks at the limit as light's mass approaches zero, Newtonian Physics predicts a deflection of light (this can be seen by the fact that all objects are accelerate the same due to gravity.) ...
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2answers
139 views

What would happen if you dropped iPhone from Outer Space? [closed]

I read this article where the title suggests the iPhone was dropped from space, but it looks more like it was dropped from an airplane I would think that if astronauts dropped an iPhone from space ...
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3answers
1k views

How hard would I hit the ground on Mars?

How do you calculate with how much force a mass would hit the ground when falling from a certain height? In particular, I'm curious to know how the force of impact on Earth compares with Mars, so ...
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1answer
39 views

What is in the gravitational field that is making the work done independent of path taken?

I could not understand the same path independence in electric field. So I am trying to understand it first in gravitational field and then apply it to electric field.
3
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1answer
137 views

Is this xkcd comic feasible? [closed]

I was reading through some xkcd comics, and I came across this one (http://xkcd.com/620/). In the comic, one of the main characters harnesses 91% his weight in order to be able to 'fly' with the lift ...
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2answers
120 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 ...
3
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3answers
596 views

Is the shell theorem only an approximation?

I've read the shell theorem during gravitation lectures, i.e. I know it states that the net gravitational field inside a 3D spherical shell or a uniform 2D ring is zero. Now, assume a thin spherical ...
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1answer
145 views

Is gravitation really a conservative force?

A spinning gyroscope has a rigidity in space. That means it maintains its axis in relation to space – and not to the surface of the earth: Imagine a spinning gyroscope that is freely mounted in ...
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2answers
73 views

Orbital mechanics problem

Here is a problem from the 2009 $F=ma$ contest on orbital mechanics: Two stars, one of mass $M$, the other of mass $3M$, orbit their common center of mass. When the stars are collinear with the ...
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3answers
65 views

Reason for motion after terminal velocity

Let some object hang by a string. So no motion since weight was counter balanced no net force. Now string is cut. Object is falling under gravity and experiences air resistance. Terminal velocity is ...
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1answer
111 views

Can we manipulate gravitation? [closed]

When I studied general relativity, my teacher said that what we knew today about gravitation was no better than what we had already known since Newton's time, because so far we could only propose ...
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0answers
69 views

Reduced Three-body problem

I know that three-body problem in celestial mechanics can't be solved analytically in general. But suppose that the 3rd body is much smaller than the others, so it does not perturb their orbits. This ...
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2answers
88 views

Newton's Principia: Astronomical prerequisites?

I am planning to read the Principia but I currently have no knowledge about Astronomy. What astronomy prerequisites do I need to understand the Principia? I would prefer older books (or "treatises"), ...
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2answers
40 views

Gravitational potential energy of a system [closed]

I don't know how the gravitational potential energy of a system of particles is defined. For example, how would one calculate the potential energy of a system of two stars, one mass $M$, the other ...
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1answer
94 views

Do objects launched from Earth retain the Earth's lateral inertia (67,000mph)

We've probably all heard of the gun on a train theory. Fire a bullet that would normally travel 500mph on a train that's moving 200mph (in the same direction the train is moving) and you've now got ...
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1answer
41 views

Will the bouncing particle exert greater force on the surface?

Imagine elastic collision and no energy is lost from the system. A particle is emitted from the bottom of a box. The box is in inertial motion. The particle hits the top of the box and travels in ...
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3answers
158 views

Mountain and a feather dropped from same height. Which hits Earth first?

Let's assume: the feather has a mass of $0.006$kg the mountain has a mass of $6 \times 10^{15}$kg the earth has a mass of $6 \times 10^{24}$kg no air resistance both objects released at the same ...
2
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2answers
375 views

Dropping Objects of Different Masses

It's a commonly taught principle that dropping two objects of different mass at the same height, given that there is no air resistance, will result in the two objects making contact with the ground at ...
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1answer
25 views

Work done by a homogeneous gravitational field on a system of particles

Perhaps this is trivial: how does one prove that the work done by a homogeneous gravitational field on a system of particles is equal to the work done on a point mass with the total mass of the ...
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2answers
83 views

Why do things here on Earth fall down? [closed]

I want to have an answer with that question above for my physics lesson. I really don't have an idea about it, so, I ask help from you guys and hope that someone can help me with it.
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1answer
54 views

What is the ultimate “goal” of two particles in a gravitational field? [closed]

What is the ultimate "goal" of two particles in a gravitational field? Will the particles eventually occupy the same point at the same time if nothing stops them? What stops them from achieving the ...
2
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0answers
79 views

If the field concept was invented by Faraday, then how did Newton interpret the $g$?

This is Newton's law of universal gravitation. $F=G\frac{m_1.m_2}{r^2}$ Gravitational field $g$ is derived from this formula $g=G\frac{m_1}{r^2}$ This is named gravitational "field" strength. If ...
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1answer
51 views

Gravitation between a homogeneous ring and a mass [closed]

I got a problem with describing the gravitational attraction between a homogeneous ring and a point mass. Let $m$ be the point mass as showed in the picture, lying a $x$-distance away on the ...
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0answers
90 views

Will relativistic glider cause lower gravitational force in Newtonian gravitation?

In the articles The relativistic glider and The relativistic glider revisited, the relativistic glider is proposed where a quasi-rigid body can slow down its fall under gravity without any reaction ...
2
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2answers
99 views

Potential energy $= mgh$, what is $h$?

NOTE: when I say potential energy I mean gravitational PE The formula for potential energy is P.E = mgh. What is h referring to? Height, obviously. Consider the example: What is the potential ...
12
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2answers
2k views

Which ball falls faster, the cool one or the hot one?

Suppose we're on the top of the Tower of Pisa (or a larger version of it) with two identical cannonballs. We heat one up (say, to 200 degrees Celsius, or some other high temperature before it starts ...
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2answers
66 views

Heating of two identical balls [closed]

You are given two identical steel balls of radius (say) 5 cm. One ball is resting on a table, the other ball is hanging from a string. Both balls are heated (with a blow torch) until their radii have ...
0
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2answers
83 views

If earth stopped spinning how much heavier would 1kg feel [closed]

My girlfriend seems to think if the earth stops spinning we will all fall off However, she did spark my imagination, due to centripetal force things must feel lighter then they otherwise would on an ...
0
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3answers
320 views

What is gravity and what causes objects to act against it?

So I understand the concept of gravity, in that it's not actually a force, but more of a displacement in the spacetime grid. An object with a big enough mass will bend the spacetime, causing smaller ...
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3answers
75 views

About Kepler's Law

When Earth goes near the Sun, its velocity increases and when it goes far from the Sun its velocity decreases. Then why according to Kepler's law is the time period $T^2 \propto a^3$, where $a$ is the ...
4
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1answer
78 views

Work performed by a stationary object in a gravitational field “on Earth”

I was thinking about this problem: How much work is required to hold an object stationary in a gravitational field? or: How much energy is required to keep an object stationary in a ...
3
votes
3answers
66 views

Does uniform distribution of background matter affect the dynamics of a gravitational system?

Situation 1: A test particle of mass m moves around a big mass M in a Keplerian orbit. The orbital period is easily found, given certain initial conditions. Situation 2: The same system of two bodies ...
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0answers
66 views

What did Feynman mean by “gravity itself is a pseudo force”? [duplicate]

Feynman writes: One very important feature of pseudo forces is that they are always proportional to the masses; the same is true of gravity. The possibility exists, therefore that gravity itself ...
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2answers
60 views

Can gravitational constant be changed?

In my book(Principles of Physics by Resnick,Halliday,Walker) , the authors write: If $G$ - by some miracle - were suddenly increased by a factor of 10, you would be crushed to the floor by Earth's ...
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When we catch a falling object, where does its kinetic energy go?

Some of it is transformed into heat and noise, but where does the rest go? Further details: The object does not bounce off (e.g. a sack of rice marble ball). The hand remains completely static ...